diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index a455087..4f46206 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -606,6 +606,11 @@
 
     @Override
     public PathFragment getRootRelativePath() {
+      // flag-less way of checking of the root is <execroot>/.., or sibling of __main__.
+      if (getExecPath().startsWith(LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX)) {
+        return LabelConstants.EXTERNAL_PATH_PREFIX.getRelative(getExecPath().subFragment(1));
+      }
+
       return getExecPath();
     }
 
diff --git a/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java b/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java
index 8eeeb4f..e56f4ae 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java
@@ -44,6 +44,7 @@
   private final Path execRootParent;
   private final PathFragment derivedPathPrefix;
   private ImmutableMap<Root, ArtifactRoot> sourceArtifactRoots;
+  private boolean siblingRepositoryLayout = false;
 
   /**
    * Cache of source artifacts.
@@ -152,6 +153,10 @@
     this.sourceArtifactRoots = sourceArtifactRoots;
   }
 
+  public void setSiblingRepositoryLayout(boolean siblingRepositoryLayout) {
+    this.siblingRepositoryLayout = siblingRepositoryLayout;
+  }
+
   /**
    * Set the set of known packages and their corresponding source artifact roots. Must be called
    * exactly once after construction or clear().
@@ -397,10 +402,18 @@
         !relativePath.isEmpty(), "%s %s %s", relativePath, baseExecPath, baseRoot);
     PathFragment execPath =
         baseExecPath != null ? baseExecPath.getRelative(relativePath) : relativePath;
-    if (execPath.containsUplevelReferences()) {
-      // Source exec paths cannot escape the source root.
+
+    // Source exec paths cannot escape the source root.
+    if (siblingRepositoryLayout) {
+      // The exec path may start with .. if using --experimental_sibling_repository_layout, so test
+      // the subfragment from index 1 onwards.
+      if (execPath.subFragment(1).containsUplevelReferences()) {
+        return null;
+      }
+    } else if (execPath.containsUplevelReferences()) {
       return null;
     }
+
     // Don't create an artifact if it's derived.
     if (isDerivedArtifact(execPath)) {
       return null;
@@ -430,7 +443,8 @@
       return null;
     }
 
-    Pair<RepositoryName, PathFragment> repo = RepositoryName.fromPathFragment(dir);
+    Pair<RepositoryName, PathFragment> repo =
+        RepositoryName.fromPathFragment(dir, siblingRepositoryLayout);
     if (repo != null) {
       repositoryName = repo.getFirst();
       dir = repo.getSecond();
diff --git a/src/main/java/com/google/devtools/build/lib/actions/ArtifactRoot.java b/src/main/java/com/google/devtools/build/lib/actions/ArtifactRoot.java
index 22abaf9..3316026 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ArtifactRoot.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ArtifactRoot.java
@@ -19,6 +19,7 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Interner;
 import com.google.common.collect.Interners;
+import com.google.devtools.build.lib.cmdline.LabelConstants;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
 import com.google.devtools.build.lib.skylarkbuildapi.FileRootApi;
@@ -60,6 +61,11 @@
     return new ArtifactRoot(root, PathFragment.EMPTY_FRAGMENT, RootType.Source);
   }
 
+  public static ArtifactRoot asExternalSourceRoot(Root root) {
+    return new ArtifactRoot(
+        root, LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX, RootType.Source);
+  }
+
   /**
    * Constructs an ArtifactRoot given the output prefixes. (eg, "bin"), and (eg, "testlogs")
    * relative to the execRoot.
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTargetFactory.java b/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTargetFactory.java
index bba38e4..4e6c4eb 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTargetFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTargetFactory.java
@@ -238,7 +238,8 @@
               visibility);
       SourceArtifact artifact =
           artifactFactory.getSourceArtifact(
-              inputFile.getExecPath(),
+              inputFile.getExecPath(
+                  analysisEnvironment.getSkylarkSemantics().experimentalSiblingRepositoryLayout()),
               inputFile.getPackage().getSourceRoot(),
               ConfiguredTargetKey.of(target.getLabel(), config));
       return new InputFileConfiguredTarget(targetContext, inputFile, artifact);
diff --git a/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java b/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java
index ad49c9c..e4ad3c1 100644
--- a/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java
+++ b/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java
@@ -66,6 +66,7 @@
 import com.google.devtools.build.lib.exec.ExecutorLifecycleListener;
 import com.google.devtools.build.lib.exec.SpawnActionContextMaps;
 import com.google.devtools.build.lib.exec.SymlinkTreeStrategy;
+import com.google.devtools.build.lib.packages.StarlarkSemanticsOptions;
 import com.google.devtools.build.lib.profiler.AutoProfiler;
 import com.google.devtools.build.lib.profiler.ProfilePhase;
 import com.google.devtools.build.lib.profiler.Profiler;
@@ -432,7 +433,9 @@
                 packageRootMap.get(),
                 getExecRoot(),
                 runtime.getProductName(),
-                nonSymlinkedDirectoriesUnderExecRoot);
+                nonSymlinkedDirectoriesUnderExecRoot,
+                request.getOptions(StarlarkSemanticsOptions.class)
+                    .experimentalSiblingRepositoryLayout);
         symlinkForest.plantSymlinkForest();
       } catch (IOException e) {
         throw new ExecutorInitException("Source forest creation failed", e);
diff --git a/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java b/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java
index eda2aaa..5a52959 100644
--- a/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java
+++ b/src/main/java/com/google/devtools/build/lib/buildtool/SymlinkForest.java
@@ -51,11 +51,12 @@
   private final String productName;
   private final String prefix;
   private final ImmutableSortedSet<String> notSymlinkedInExecrootDirectories;
+  private final boolean siblingRepositoryLayout;
 
   /** Constructor for a symlink forest creator without non-symlinked directories parameter. */
   public SymlinkForest(
       ImmutableMap<PackageIdentifier, Root> packageRoots, Path execroot, String productName) {
-    this(packageRoots, execroot, productName, ImmutableSortedSet.of());
+    this(packageRoots, execroot, productName, ImmutableSortedSet.of(), false);
   }
 
   /**
@@ -74,12 +75,14 @@
       ImmutableMap<PackageIdentifier, Root> packageRoots,
       Path execroot,
       String productName,
-      ImmutableSortedSet<String> notSymlinkedInExecrootDirectories) {
+      ImmutableSortedSet<String> notSymlinkedInExecrootDirectories,
+      boolean siblingRepositoryLayout) {
     this.packageRoots = packageRoots;
     this.execroot = execroot;
     this.productName = productName;
     this.prefix = productName + "-";
     this.notSymlinkedInExecrootDirectories = notSymlinkedInExecrootDirectories;
+    this.siblingRepositoryLayout = siblingRepositoryLayout;
   }
 
   /**
@@ -123,8 +126,13 @@
     // directory.
     // From <output_base>/execroot/<main repo name>/external/<external repo name>
     // to   <output_base>/external/<external repo name>
-    Path execrootLink = execroot.getRelative(repository.getPathUnderExecRoot());
-    if (externalRepoLinks.isEmpty()) {
+    //
+    // However, if --experimental_sibling_repository_layout is true, symlink:
+    // From <output_base>/execroot/<external repo name>
+    // to   <output_base>/external/<external repo name>
+    Path execrootLink = execroot.getRelative(repository.getExecPath(siblingRepositoryLayout));
+
+    if (!siblingRepositoryLayout && externalRepoLinks.isEmpty()) {
       execroot.getRelative(LabelConstants.EXTERNAL_PACKAGE_NAME).createDirectoryAndParents();
     }
     if (!externalRepoLinks.add(execrootLink)) {
@@ -136,6 +144,9 @@
   private void plantSymlinkForestWithFullMainRepository(Path mainRepoRoot) throws IOException {
     // For the main repo top-level directory, generate symlinks to everything in the directory
     // instead of the directory itself.
+    if (siblingRepositoryLayout) {
+      execroot.createDirectory();
+    }
     for (Path target : mainRepoRoot.getDirectoryEntries()) {
       String baseName = target.getBaseName();
       if (this.notSymlinkedInExecrootDirectories.contains(baseName)) {
@@ -145,14 +156,19 @@
       // Create any links that don't start with bazel-, and ignore external/ directory if
       // user has it in the source tree because it conflicts with external repository location.
       if (!baseName.startsWith(prefix)
-          && !baseName.equals(LabelConstants.EXTERNAL_PATH_PREFIX.getBaseName())) {
+          && (siblingRepositoryLayout
+              || !baseName.equals(LabelConstants.EXTERNAL_PATH_PREFIX.getBaseName()))) {
         execPath.createSymbolicLink(target);
+        // TODO(jingwen-external): is this creating execroot/io_bazel/external?
       }
     }
   }
 
   private void plantSymlinkForestWithPartialMainRepository(Map<Path, Path> mainRepoLinks)
       throws IOException, AbruptExitException {
+    if (siblingRepositoryLayout) {
+      execroot.createDirectory();
+    }
     for (Map.Entry<Path, Path> entry : mainRepoLinks.entrySet()) {
       Path link = entry.getKey();
       Path target = entry.getValue();
@@ -205,14 +221,14 @@
     for (PackageIdentifier dir : dirsParentsFirst) {
       if (!dir.getRepository().isMain()) {
         execroot
-            .getRelative(dir.getRepository().getPathUnderExecRoot())
+            .getRelative(dir.getRepository().getExecPath(siblingRepositoryLayout))
             .createDirectoryAndParents();
       }
       if (dirRootsMap.get(dir).size() > 1) {
         if (LOG_FINER) {
-          logger.finer("mkdir " + execroot.getRelative(dir.getPathUnderExecRoot()));
+          logger.finer("mkdir " + execroot.getRelative(dir.getExecPath(siblingRepositoryLayout)));
         }
-        execroot.getRelative(dir.getPathUnderExecRoot()).createDirectoryAndParents();
+        execroot.getRelative(dir.getExecPath(siblingRepositoryLayout)).createDirectoryAndParents();
       }
     }
 
@@ -232,9 +248,10 @@
               "ln -s "
                   + root.getRelative(dir.getSourceRoot())
                   + " "
-                  + execroot.getRelative(dir.getPathUnderExecRoot()));
+                  + execroot.getRelative(dir.getExecPath(siblingRepositoryLayout)));
         }
-        execroot.getRelative(dir.getPathUnderExecRoot())
+        execroot
+            .getRelative(dir.getExecPath(siblingRepositoryLayout))
             .createSymbolicLink(root.getRelative(dir.getSourceRoot()));
       }
     }
@@ -275,7 +292,7 @@
       if (!pkgId.getPackageFragment().equals(PathFragment.EMPTY_FRAGMENT)) {
         continue;
       }
-      Path execrootDirectory = execroot.getRelative(pkgId.getPathUnderExecRoot());
+      Path execrootDirectory = execroot.getRelative(pkgId.getExecPath(siblingRepositoryLayout));
       // If there were no subpackages, this directory might not exist yet.
       if (!execrootDirectory.exists()) {
         execrootDirectory.createDirectoryAndParents();
@@ -298,6 +315,15 @@
   public void plantSymlinkForest() throws IOException, AbruptExitException {
     deleteTreesBelowNotPrefixed(execroot, prefix);
 
+    if (siblingRepositoryLayout) {
+      // Delete execroot/../<symlinks> to directories representing external repositories.
+      for (Path p : execroot.getParentDirectory().getDirectoryEntries()) {
+        if (p.isSymbolicLink()) {
+          p.deleteTree();
+        }
+      }
+    }
+
     boolean shouldLinkAllTopLevelItems = false;
     Map<Path, Path> mainRepoLinks = Maps.newLinkedHashMap();
     Set<Root> mainRepoRoots = Sets.newLinkedHashSet();
@@ -306,7 +332,7 @@
 
     for (Map.Entry<PackageIdentifier, Root> entry : packageRoots.entrySet()) {
       PackageIdentifier pkgId = entry.getKey();
-      if (pkgId.equals(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER)) {
+      if (!siblingRepositoryLayout && pkgId.equals(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER)) {
         // This isn't a "real" package, don't add it to the symlink tree.
         continue;
       }
@@ -327,9 +353,10 @@
           shouldLinkAllTopLevelItems = true;
         } else {
           String baseName = pkgId.getPackageFragment().getSegment(0);
-          // ignore external/ directory if user has it in the source tree
-          // because it conflicts with external repository location.
-          if (baseName.equals(LabelConstants.EXTERNAL_PATH_PREFIX.getBaseName())) {
+          if (!siblingRepositoryLayout
+              && baseName.equals(LabelConstants.EXTERNAL_PATH_PREFIX.getBaseName())) {
+            // ignore external/ directory if user has it in the source tree
+            // because it conflicts with external repository location.
             continue;
           }
           Path execrootLink = execroot.getRelative(baseName);
diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/Label.java b/src/main/java/com/google/devtools/build/lib/cmdline/Label.java
index 41374dd..e4154be 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/Label.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/Label.java
@@ -30,6 +30,7 @@
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
 import com.google.devtools.build.lib.syntax.Printer;
 import com.google.devtools.build.lib.syntax.SkylarkType;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.syntax.StarlarkThread;
 import com.google.devtools.build.lib.syntax.StarlarkValue;
 import com.google.devtools.build.lib.util.StringUtilities;
@@ -368,16 +369,20 @@
    * it will returns an empty string.
    */
   @SkylarkCallable(
-    name = "workspace_root",
-    structField = true,
-    doc =
-        "Returns the execution root for the workspace of this label, relative to the execroot. "
-            + "For instance:<br>"
-            + "<pre class=language-python>Label(\"@repo//pkg/foo:abc\").workspace_root =="
-            + " \"external/repo\"</pre>"
-  )
-  public String getWorkspaceRoot() {
-    return packageIdentifier.getRepository().getSourceRoot().toString();
+      name = "workspace_root",
+      structField = true,
+      doc =
+          "Returns the execution root for the workspace of this label, relative to the execroot. "
+              + "For instance:<br>"
+              + "<pre class=language-python>Label(\"@repo//pkg/foo:abc\").workspace_root =="
+              + " \"external/repo\"</pre>",
+      useStarlarkSemantics = true)
+  public String getWorkspaceRoot(StarlarkSemantics semantics) {
+    if (semantics.experimentalSiblingRepositoryLayout()) {
+      return packageIdentifier.getRepository().getExecPath(true).toString();
+    } else {
+      return packageIdentifier.getRepository().getSourceRoot().toString();
+    }
   }
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/LabelConstants.java b/src/main/java/com/google/devtools/build/lib/cmdline/LabelConstants.java
index 042b049..1608ed0 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/LabelConstants.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/LabelConstants.java
@@ -20,9 +20,15 @@
   public static final PathFragment EXTERNAL_PACKAGE_NAME = PathFragment.create("external");
   public static final PackageIdentifier EXTERNAL_PACKAGE_IDENTIFIER =
       PackageIdentifier.createInMainRepo(EXTERNAL_PACKAGE_NAME);
-  public static final PathFragment EXTERNAL_PATH_PREFIX = PathFragment.create("external");
   public static final PathFragment WORKSPACE_FILE_NAME = PathFragment.create("WORKSPACE");
   public static final PathFragment WORKSPACE_DOT_BAZEL_FILE_NAME =
       PathFragment.create("WORKSPACE.bazel");
   public static final String DEFAULT_REPOSITORY_DIRECTORY = "__main__";
+
+  // With this prefix, non-main repositories are symlinked under
+  // $output_base/execution_root/__main__/external
+  public static final PathFragment EXTERNAL_PATH_PREFIX = PathFragment.create("external");
+  // With this prefix, non-main repositories are sibling symlinks of
+  // $output_base/execution_root/__main__
+  public static final PathFragment EXPERIMENTAL_EXTERNAL_PATH_PREFIX = PathFragment.create("..");
 }
diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java b/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java
index e4f285d..eecec56 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java
@@ -76,19 +76,21 @@
    * @throws LabelSyntaxException if the exec path seems to be for an external repository that does
    *     not have a valid repository name (see {@link RepositoryName#create})
    */
-  public static PackageIdentifier discoverFromExecPath(PathFragment execPath, boolean forFiles)
-      throws LabelSyntaxException {
+  public static PackageIdentifier discoverFromExecPath(
+      PathFragment execPath, boolean forFiles, boolean siblingRepositoryLayout) {
     Preconditions.checkArgument(!execPath.isAbsolute(), execPath);
     PathFragment tofind = forFiles
         ? Preconditions.checkNotNull(
             execPath.getParentDirectory(), "Must pass in files, not root directory")
         : execPath;
-    if (tofind.startsWith(LabelConstants.EXTERNAL_PATH_PREFIX)) {
-      // TODO(ulfjack): Remove this when kchodorow@'s exec root rearrangement has been rolled out.
-      RepositoryName repository = RepositoryName.create("@" + tofind.getSegment(1));
-      return PackageIdentifier.create(repository, tofind.subFragment(2));
-    } else if (tofind.containsUplevelReferences()) {
-      RepositoryName repository = RepositoryName.create("@" + tofind.getSegment(1));
+    PathFragment prefix =
+        siblingRepositoryLayout
+            ? LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX
+            : LabelConstants.EXTERNAL_PATH_PREFIX;
+    if (tofind.startsWith(prefix)) {
+      // Using the path prefix can be either "external" or "..", depending on whether the sibling
+      // repository layout is used.
+      RepositoryName repository = RepositoryName.createFromValidStrippedName(tofind.getSegment(1));
       return PackageIdentifier.create(repository, tofind.subFragment(2));
     } else {
       return PackageIdentifier.createInMainRepo(tofind);
@@ -175,8 +177,8 @@
     return repository.getSourceRoot().getRelative(pkgName);
   }
 
-  public PathFragment getPathUnderExecRoot() {
-    return repository.getPathUnderExecRoot().getRelative(pkgName);
+  public PathFragment getExecPath(boolean siblingRepositoryLayout) {
+    return repository.getExecPath(siblingRepositoryLayout).getRelative(pkgName);
   }
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java b/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java
index 4e133bb..5f4d455 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java
@@ -132,17 +132,27 @@
   }
 
   /**
-   * Extracts the repository name from a PathFragment that was created with
-   * {@code PackageIdentifier.getSourceRoot}.
+   * Extracts the repository name from a PathFragment that was created with {@code
+   * PackageIdentifier.getSourceRoot}.
    *
-   * @return a {@code Pair} of the extracted repository name and the path fragment with stripped
-   * of "external/"-prefix and repository name, or null if none was found or the repository name
-   * was invalid.
+   * @return a {@code Pair} of the extracted repository name and the path fragment with stripped of
+   *     "external/"-prefix and repository name, or null if none was found or the repository name
+   *     was invalid.
    */
-  public static Pair<RepositoryName, PathFragment> fromPathFragment(PathFragment path) {
-    if (path.segmentCount() < 2 || !path.startsWith(LabelConstants.EXTERNAL_PATH_PREFIX)) {
+  public static Pair<RepositoryName, PathFragment> fromPathFragment(
+      PathFragment path, boolean siblingRepositoryLayout) {
+    if (path.segmentCount() < 2) {
       return null;
     }
+
+    PathFragment prefix =
+        siblingRepositoryLayout
+            ? LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX
+            : LabelConstants.EXTERNAL_PATH_PREFIX;
+    if (!path.startsWith(prefix)) {
+      return null;
+    }
+
     try {
       RepositoryName repoName = RepositoryName.create("@" + path.getSegment(1));
       PathFragment subPath = path.subFragment(2);
@@ -244,13 +254,30 @@
   }
 
   /**
+   * Returns the relative path to the repository's source for derived artifacts. This behavior is
+   * currently the same for source artifacts, but we create a new method name to keep call sites
+   * readable and not misleading.
+   */
+  public PathFragment getDerivedArtifactSourceRoot() {
+    return getSourceRoot();
+  }
+
+  /**
    * Returns the runfiles/execRoot path for this repository. If we don't know the name of this repo
    * (i.e., it is in the main repository), return an empty path fragment.
+   *
+   * <p>If --experimental_sibling_repository_layout is true, return "$execroot/../repo" (sibling of
+   * __main__), instead of "$execroot/external/repo".
    */
-  public PathFragment getPathUnderExecRoot() {
-    return isDefault() || isMain()
-        ? PathFragment.EMPTY_FRAGMENT
-        : LabelConstants.EXTERNAL_PATH_PREFIX.getRelative(strippedName());
+  public PathFragment getExecPath(boolean siblingRepositoryLayout) {
+    if (isDefault() || isMain()) {
+      return PathFragment.EMPTY_FRAGMENT;
+    }
+    PathFragment prefix =
+        siblingRepositoryLayout
+            ? LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX
+            : LabelConstants.EXTERNAL_PATH_PREFIX;
+    return prefix.getRelative(strippedName());
   }
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/packages/InputFile.java b/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
index 3667643..d2e24c1 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
@@ -101,10 +101,16 @@
   }
 
   /**
-   * Returns the exec path of the file, i.e. the path relative to the package source root.
+   * Returns the exec path of the file, i.e. the path relative to the execution root working
+   * directory.
    */
-  public PathFragment getExecPath() {
-    return label.getPackageIdentifier().getSourceRoot().getRelative(label.getName());
+  public PathFragment getExecPath(boolean siblingRepositoryLayout) {
+    return label
+        .getPackageIdentifier()
+        .getRepository()
+        .getExecPath(siblingRepositoryLayout)
+        .getRelative(label.getPackageName())
+        .getRelative(label.getName());
   }
 
   @Override
diff --git a/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java
index ec60aa2..31c3218 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java
@@ -215,6 +215,27 @@
   public boolean experimentalRepoRemoteExec;
 
   @Option(
+      name = "experimental_sibling_repository_layout",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
+      effectTags = {
+        OptionEffectTag.ACTION_COMMAND_LINES,
+        OptionEffectTag.BAZEL_INTERNAL_CONFIGURATION,
+        OptionEffectTag.LOADING_AND_ANALYSIS,
+        OptionEffectTag.LOSES_INCREMENTAL_STATE
+      },
+      metadataTags = {
+        OptionMetadataTag.EXPERIMENTAL,
+      },
+      help =
+          "If set to true, non-main repositories are planted as symlinks to the main repository in"
+              + " the execution root. That is, all repositories are direct children of the"
+              + " $output_base/execution_root directory. This has the side effect of freeing up"
+              + " $output_base/execution_root/__main__/external for the real top-level 'external' "
+              + "directory.")
+  public boolean experimentalSiblingRepositoryLayout;
+
+  @Option(
       name = "incompatible_bzl_disallow_load_after_statement",
       defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
@@ -633,6 +654,7 @@
             .experimentalStarlarkUnusedInputsList(experimentalStarlarkUnusedInputsList)
             .experimentalCcSharedLibrary(experimentalCcSharedLibrary)
             .experimentalRepoRemoteExec(experimentalRepoRemoteExec)
+            .experimentalSiblingRepositoryLayout(experimentalSiblingRepositoryLayout)
             .incompatibleApplicableLicenses(incompatibleApplicableLicenses)
             .incompatibleBzlDisallowLoadAfterStatement(incompatibleBzlDisallowLoadAfterStatement)
             .incompatibleDepsetUnion(incompatibleDepsetUnion)
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCommon.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCommon.java
index f962a98..b8d67f7 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCommon.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCommon.java
@@ -589,12 +589,19 @@
    * Determines a list of loose include directories that are only allowed to be referenced when
    * headers checking is {@link HeadersCheckingMode#LOOSE}.
    */
-  Set<PathFragment> getLooseIncludeDirs() {
+  Set<PathFragment> getLooseIncludeDirs() throws InterruptedException {
     ImmutableSet.Builder<PathFragment> result = ImmutableSet.builder();
     // The package directory of the rule contributes includes. Note that this also covers all
     // non-subpackage sub-directories.
-    PathFragment rulePackage = ruleContext.getLabel().getPackageIdentifier()
-        .getPathUnderExecRoot();
+    PathFragment rulePackage =
+        ruleContext
+            .getLabel()
+            .getPackageIdentifier()
+            .getExecPath(
+                ruleContext
+                    .getAnalysisEnvironment()
+                    .getSkylarkSemantics()
+                    .experimentalSiblingRepositoryLayout());
     result.add(rulePackage);
 
     if (ruleContext
@@ -604,17 +611,29 @@
             .experimentalIncludesAttributeSubpackageTraversal
         && ruleContext.getRule().isAttributeValueExplicitlySpecified("includes")) {
       PathFragment packageFragment =
-          ruleContext.getLabel().getPackageIdentifier().getPathUnderExecRoot();
+          ruleContext
+              .getLabel()
+              .getPackageIdentifier()
+              .getExecPath(
+                  ruleContext
+                      .getAnalysisEnvironment()
+                      .getSkylarkSemantics()
+                      .experimentalSiblingRepositoryLayout());
       // For now, anything with an 'includes' needs a blanket declaration
       result.add(packageFragment.getRelative("**"));
     }
     return result.build();
   }
 
-  List<PathFragment> getSystemIncludeDirs() {
+  List<PathFragment> getSystemIncludeDirs() throws InterruptedException {
+    boolean siblingRepositoryLayout =
+        ruleContext
+            .getAnalysisEnvironment()
+            .getSkylarkSemantics()
+            .experimentalSiblingRepositoryLayout();
     List<PathFragment> result = new ArrayList<>();
     PackageIdentifier packageIdentifier = ruleContext.getLabel().getPackageIdentifier();
-    PathFragment packageFragment = packageIdentifier.getPathUnderExecRoot();
+    PathFragment packageFragment = packageIdentifier.getExecPath(siblingRepositoryLayout);
     for (String includesAttr : ruleContext.getExpander().list("includes")) {
       if (includesAttr.startsWith("/")) {
         ruleContext.attributeWarning("includes",
@@ -622,7 +641,7 @@
         continue;
       }
       PathFragment includesPath = packageFragment.getRelative(includesAttr);
-      if (includesPath.containsUplevelReferences()) {
+      if (!siblingRepositoryLayout && includesPath.containsUplevelReferences()) {
         ruleContext.attributeError("includes",
             "Path references a path above the execution root.");
       }
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCompilationHelper.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCompilationHelper.java
index 24011e6..079185f 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCompilationHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcCompilationHelper.java
@@ -335,7 +335,8 @@
   }
 
   /** Sets fields that overlap for cc_library and cc_binary rules. */
-  public CcCompilationHelper fromCommon(CcCommon common, ImmutableList<String> additionalCopts) {
+  public CcCompilationHelper fromCommon(CcCommon common, ImmutableList<String> additionalCopts)
+      throws InterruptedException {
     Preconditions.checkNotNull(additionalCopts);
 
     setCopts(ImmutableList.copyOf(Iterables.concat(common.getCopts(), additionalCopts)));
@@ -754,7 +755,7 @@
    *
    * @throws RuleErrorException
    */
-  public CompilationInfo compile() throws RuleErrorException {
+  public CompilationInfo compile() throws RuleErrorException, InterruptedException {
 
     if (!generatePicAction && !generateNoPicAction) {
       ruleErrorConsumer.ruleError("Either PIC or no PIC actions have to be created.");
@@ -835,7 +836,7 @@
     }
   }
 
-  private PublicHeaders computePublicHeaders() {
+  private PublicHeaders computePublicHeaders() throws InterruptedException {
     PathFragment prefix = null;
     if (includePrefix != null) {
       prefix = PathFragment.create(includePrefix);
@@ -944,10 +945,8 @@
         virtualToOriginalHeaders.build());
   }
 
-  /**
-   * Create {@code CcCompilationContext} for cc compile action from generated inputs.
-   */
-  private CcCompilationContext initializeCcCompilationContext() {
+  /** Create {@code CcCompilationContext} for cc compile action from generated inputs. */
+  private CcCompilationContext initializeCcCompilationContext() throws InterruptedException {
     CcCompilationContext.Builder ccCompilationContextBuilder =
         CcCompilationContext.builder(actionConstructionContext, configuration, label);
 
@@ -959,8 +958,13 @@
     // generated files. It is important that the execRoot (EMPTY_FRAGMENT) comes
     // before the genfilesFragment to preferably pick up source files. Otherwise
     // we might pick up stale generated files.
+    boolean siblingRepositoryLayout =
+        actionConstructionContext
+            .getAnalysisEnvironment()
+            .getSkylarkSemantics()
+            .experimentalSiblingRepositoryLayout();
     PathFragment repositoryPath =
-        label.getPackageIdentifier().getRepository().getPathUnderExecRoot();
+        label.getPackageIdentifier().getRepository().getExecPath(siblingRepositoryLayout);
     ccCompilationContextBuilder.addQuoteIncludeDir(repositoryPath);
     ccCompilationContextBuilder.addQuoteIncludeDir(
         configuration.getGenfilesFragment().getRelative(repositoryPath));
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcModule.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcModule.java
index 4527907..87a30f4 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcModule.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcModule.java
@@ -826,7 +826,10 @@
                   .getRuleContext()
                   .getLabel()
                   .getPackageIdentifier()
-                  .getPathUnderExecRoot()
+                  .getExecPath(
+                      skylarkRuleContext
+                          .getSkylarkSemantics()
+                          .experimentalSiblingRepositoryLayout())
                   .getRelative(PathFragment.create(tool.second))
                   .getPathString();
         }
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProviderHelper.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProviderHelper.java
index 3dcd11e..bdc62d4 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProviderHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProviderHelper.java
@@ -70,7 +70,13 @@
     CcToolchainConfigInfo toolchainConfigInfo = attributes.getCcToolchainConfigInfo();
     ImmutableMap<String, PathFragment> toolPaths;
     CcToolchainFeatures toolchainFeatures;
-    PathFragment toolsDirectory = getToolsDirectory(ruleContext.getLabel());
+    PathFragment toolsDirectory =
+        getToolsDirectory(
+            ruleContext.getLabel(),
+            ruleContext
+                .getAnalysisEnvironment()
+                .getSkylarkSemantics()
+                .experimentalSiblingRepositoryLayout());
     try {
       toolPaths = computeToolPaths(toolchainConfigInfo, toolsDirectory);
       toolchainFeatures = new CcToolchainFeatures(toolchainConfigInfo, toolsDirectory);
@@ -463,8 +469,8 @@
     return toolPaths.get(tool.getNamePart());
   }
 
-  static PathFragment getToolsDirectory(Label ccToolchainLabel) {
-    return ccToolchainLabel.getPackageIdentifier().getPathUnderExecRoot();
+  static PathFragment getToolsDirectory(Label ccToolchainLabel, boolean siblingRepositoryLayout) {
+    return ccToolchainLabel.getPackageIdentifier().getExecPath(siblingRepositoryLayout);
   }
 
   private static ImmutableMap<String, String> computeAdditionalMakeVariables(
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileAction.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileAction.java
index 3c92fe2..59dab83 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileAction.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileAction.java
@@ -61,6 +61,7 @@
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadCompatible;
+import com.google.devtools.build.lib.packages.StarlarkSemanticsOptions;
 import com.google.devtools.build.lib.profiler.AutoProfiler;
 import com.google.devtools.build.lib.profiler.Profiler;
 import com.google.devtools.build.lib.profiler.ProfilerTask;
@@ -594,7 +595,12 @@
                   .setCmdlineIncludes(getCmdlineIncludes(options))
                   .build());
       if (needsIncludeValidation) {
-        verifyActionIncludePaths(systemIncludeDirs);
+        verifyActionIncludePaths(
+            systemIncludeDirs,
+            actionExecutionContext
+                .getOptions()
+                .getOptions(StarlarkSemanticsOptions.class)
+                .experimentalSiblingRepositoryLayout);
       }
 
       if (!shouldScanIncludes) {
@@ -1035,7 +1041,8 @@
   }
 
   @VisibleForTesting
-  void verifyActionIncludePaths(List<PathFragment> systemIncludeDirs)
+  void verifyActionIncludePaths(
+      List<PathFragment> systemIncludeDirs, boolean siblingRepositoryLayout)
       throws ActionExecutionException {
     ImmutableSet<PathFragment> ignoredDirs = ImmutableSet.copyOf(getValidationIgnoredDirs());
     // We currently do not check the output of:
@@ -1052,9 +1059,16 @@
           || FileSystemUtils.startsWithAny(includePath, ignoredDirs)) {
         continue;
       }
-      // One starting ../ is okay for getting to a sibling repository.
-      if (includePath.startsWith(LabelConstants.EXTERNAL_PATH_PREFIX)) {
-        includePath = includePath.relativeTo(LabelConstants.EXTERNAL_PATH_PREFIX);
+
+      // Two conditions:
+      // 1. Paths cannot be absolute (e.g. multiple uplevels to /etc/passwd)
+      // 2. For relative paths, one starting ../ is okay for getting to a sibling repository.
+      PathFragment prefix =
+          siblingRepositoryLayout
+              ? LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX
+              : LabelConstants.EXTERNAL_PATH_PREFIX;
+      if (includePath.startsWith(prefix)) {
+        includePath = includePath.relativeTo(prefix);
       }
       if (includePath.isAbsolute() || includePath.containsUplevelReferences()) {
         throw new ActionExecutionException(
@@ -1465,7 +1479,8 @@
       Path execRoot,
       ArtifactResolver artifactResolver,
       ShowIncludesFilter showIncludesFilterForStdout,
-      ShowIncludesFilter showIncludesFilterForStderr)
+      ShowIncludesFilter showIncludesFilterForStderr,
+      boolean siblingRepositoryLayout)
       throws ActionExecutionException {
     if (!needsDotdInputPruning) {
       return NestedSetBuilder.emptySet(Order.STABLE_ORDER);
@@ -1485,7 +1500,9 @@
       discoveryBuilder.shouldValidateInclusions();
     }
 
-    return discoveryBuilder.build().discoverInputsFromDependencies(execRoot, artifactResolver);
+    return discoveryBuilder
+        .build()
+        .discoverInputsFromDependencies(execRoot, artifactResolver, siblingRepositoryLayout);
   }
 
   @VisibleForTesting
@@ -1493,7 +1510,8 @@
       ActionExecutionContext actionExecutionContext,
       Path execRoot,
       ArtifactResolver artifactResolver,
-      byte[] dotDContents)
+      byte[] dotDContents,
+      boolean siblingRepositoryLayout)
       throws ActionExecutionException {
     if (!needsDotdInputPruning || getDotdFile() == null) {
       return NestedSetBuilder.emptySet(Order.STABLE_ORDER);
@@ -1511,7 +1529,9 @@
       discoveryBuilder.shouldValidateInclusions();
     }
 
-    return discoveryBuilder.build().discoverInputsFromDependencies(execRoot, artifactResolver);
+    return discoveryBuilder
+        .build()
+        .discoverInputsFromDependencies(execRoot, artifactResolver, siblingRepositoryLayout);
   }
 
   public DependencySet processDepset(
@@ -1786,6 +1806,11 @@
       CppIncludeExtractionContext scanningContext =
           actionExecutionContext.getContext(CppIncludeExtractionContext.class);
       Path execRoot = actionExecutionContext.getExecRoot();
+      boolean siblingRepositoryLayout =
+          actionExecutionContext
+              .getOptions()
+              .getOptions(StarlarkSemanticsOptions.class)
+              .experimentalSiblingRepositoryLayout;
 
       NestedSet<Artifact> discoveredInputs;
       if (featureConfiguration.isEnabled(CppRuleClasses.PARSE_SHOWINCLUDES)) {
@@ -1794,14 +1819,16 @@
                 execRoot,
                 scanningContext.getArtifactResolver(),
                 showIncludesFilterForStdout,
-                showIncludesFilterForStderr);
+                showIncludesFilterForStderr,
+                siblingRepositoryLayout);
       } else {
         discoveredInputs =
             discoverInputsFromDotdFiles(
                 actionExecutionContext,
                 execRoot,
                 scanningContext.getArtifactResolver(),
-                dotDContents);
+                dotDContents,
+                siblingRepositoryLayout);
       }
       dotDContents = null; // Garbage collect in-memory .d contents.
 
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/FakeCppCompileAction.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/FakeCppCompileAction.java
index 683f24a..076fbdb 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/FakeCppCompileAction.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/FakeCppCompileAction.java
@@ -37,6 +37,7 @@
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadCompatible;
 import com.google.devtools.build.lib.events.Event;
+import com.google.devtools.build.lib.packages.StarlarkSemanticsOptions;
 import com.google.devtools.build.lib.rules.cpp.CcCommon.CoptsFilter;
 import com.google.devtools.build.lib.rules.cpp.CcToolchainFeatures.FeatureConfiguration;
 import com.google.devtools.build.lib.util.ShellEscaper;
@@ -170,10 +171,17 @@
         discoveryBuilder.shouldValidateInclusions();
       }
 
+      boolean siblingRepositoryLayout =
+          actionExecutionContext
+              .getOptions()
+              .getOptions(StarlarkSemanticsOptions.class)
+              .experimentalSiblingRepositoryLayout;
+
       discoveredInputs =
           discoveryBuilder
               .build()
-              .discoverInputsFromDependencies(execRoot, scanningContext.getArtifactResolver());
+              .discoverInputsFromDependencies(
+                  execRoot, scanningContext.getArtifactResolver(), siblingRepositoryLayout);
     }
 
     dotDContents = null; // Garbage collect in-memory .d contents.
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/FdoHelper.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/FdoHelper.java
index e2107eb..b54b5f0 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/FdoHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/FdoHelper.java
@@ -442,7 +442,8 @@
   }
 
   private static FdoInputFile fdoInputFileFromArtifacts(
-      RuleContext ruleContext, CcToolchainAttributesProvider attributes) {
+      RuleContext ruleContext, CcToolchainAttributesProvider attributes)
+      throws InterruptedException {
     ImmutableList<Artifact> fdoArtifacts = attributes.getFdoOptimizeArtifacts();
     if (fdoArtifacts.size() != 1) {
       ruleContext.ruleError("--fdo_optimize does not point to a single target");
@@ -458,7 +459,11 @@
     Label fdoLabel = attributes.getFdoOptimize().getLabel();
     if (!fdoLabel
         .getPackageIdentifier()
-        .getPathUnderExecRoot()
+        .getExecPath(
+            ruleContext
+                .getAnalysisEnvironment()
+                .getSkylarkSemantics()
+                .experimentalSiblingRepositoryLayout())
         .getRelative(fdoLabel.getName())
         .equals(fdoArtifact.getExecPath())) {
       ruleContext.ruleError("--fdo_optimize points to a target that is not an input file");
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscovery.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscovery.java
index b72b0d7..013e47d 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscovery.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscovery.java
@@ -21,7 +21,7 @@
 import com.google.devtools.build.lib.actions.ActionExecutionException;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.actions.ArtifactResolver;
-import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
+import com.google.devtools.build.lib.cmdline.LabelConstants;
 import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 import com.google.devtools.build.lib.cmdline.RepositoryName;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
@@ -126,7 +126,8 @@
    */
   @ThreadCompatible
   NestedSet<Artifact> discoverInputsFromDependencies(
-      Path execRoot, ArtifactResolver artifactResolver) throws ActionExecutionException {
+      Path execRoot, ArtifactResolver artifactResolver, boolean siblingRepositoryLayout)
+      throws ActionExecutionException {
     NestedSetBuilder<Artifact> inputs = NestedSetBuilder.stableOrder();
     if (dependencies == null) {
       return inputs.build();
@@ -147,6 +148,11 @@
         // the build with an error.
         if (execPath.startsWith(execRoot)) {
           execPathFragment = execPath.relativeTo(execRoot); // funky but tolerable path
+        } else if (siblingRepositoryLayout && execPath.startsWith(execRoot.getParentDirectory())) {
+          // for --experimental_sibling_repository_layout
+          execPathFragment =
+              LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX.getRelative(
+                  execPath.relativeTo(execRoot.getParentDirectory()));
         } else {
           problems.add(execPathFragment.getPathString());
           continue;
@@ -154,19 +160,10 @@
       }
       Artifact artifact = allowedDerivedInputsMap.get(execPathFragment);
       if (artifact == null) {
-        try {
-          RepositoryName repository =
-              PackageIdentifier.discoverFromExecPath(execPathFragment, false).getRepository();
-          artifact = artifactResolver.resolveSourceArtifact(execPathFragment, repository);
-        } catch (LabelSyntaxException e) {
-          throw new ActionExecutionException(
-              String.format(
-                  "Could not find the external repository for %s: " + e.getMessage(),
-                  execPathFragment),
-              e,
-              action,
-              false);
-        }
+        RepositoryName repository =
+            PackageIdentifier.discoverFromExecPath(execPathFragment, false, siblingRepositoryLayout)
+                .getRepository();
+        artifact = artifactResolver.resolveSourceArtifact(execPathFragment, repository);
       }
       if (artifact != null) {
         // We don't need to add the sourceFile itself as it is a mandatory input.
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/proto/CcProtoAspect.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/proto/CcProtoAspect.java
index 1014c39..6005815 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/proto/CcProtoAspect.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/proto/CcProtoAspect.java
@@ -337,7 +337,15 @@
         protoRootFragment = protoRootFragment.relativeTo(binOrGenfiles);
       }
       PathFragment repositoryPath =
-          ruleContext.getLabel().getPackageIdentifier().getRepository().getPathUnderExecRoot();
+          ruleContext
+              .getLabel()
+              .getPackageIdentifier()
+              .getRepository()
+              .getExecPath(
+                  ruleContext
+                      .getAnalysisEnvironment()
+                      .getSkylarkSemantics()
+                      .experimentalSiblingRepositoryLayout());
       if (protoRootFragment.startsWith(repositoryPath)) {
         protoRootFragment = protoRootFragment.relativeTo(repositoryPath);
       }
@@ -420,7 +428,8 @@
       helper.addAdditionalExportedHeaders(headers.build());
     }
 
-    private void createProtoCompileAction(Collection<Artifact> outputs) {
+    private void createProtoCompileAction(Collection<Artifact> outputs)
+        throws InterruptedException {
       PathFragment protoRootFragment = PathFragment.create(protoInfo.getDirectProtoSourceRoot());
       String genfilesPath;
       PathFragment genfilesFragment = ruleContext.getConfiguration().getGenfilesFragment();
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/JavaCommon.java b/src/main/java/com/google/devtools/build/lib/rules/java/JavaCommon.java
index 1ddb0bb..9ebd86a 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/JavaCommon.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/JavaCommon.java
@@ -592,14 +592,10 @@
     }
   }
 
-  public JavaTargetAttributes.Builder initCommon() {
+  public JavaTargetAttributes.Builder initCommon() throws InterruptedException {
     return initCommon(ImmutableList.of(), getCompatibleJavacOptions());
   }
 
-  private ImmutableList<String> getCompatibleJavacOptions() {
-    return semantics.getCompatibleJavacOptions(ruleContext, javaToolchain);
-  }
-
   /**
    * Initialize the common actions and build various collections of artifacts for the
    * initializationHook() methods of the subclasses.
@@ -609,7 +605,7 @@
    * @return the processed attributes
    */
   public JavaTargetAttributes.Builder initCommon(
-      Collection<Artifact> extraSrcs, Iterable<String> extraJavacOpts) {
+      Collection<Artifact> extraSrcs, Iterable<String> extraJavacOpts) throws InterruptedException {
     Preconditions.checkState(javacOpts == null);
     javacOpts = computeJavacOpts(ImmutableList.copyOf(extraJavacOpts));
     activePlugins = collectPlugins();
@@ -652,6 +648,10 @@
     return javaTargetAttributes;
   }
 
+  private ImmutableList<String> getCompatibleJavacOptions() {
+    return semantics.getCompatibleJavacOptions(ruleContext, javaToolchain);
+  }
+
   private boolean disallowDepsWithoutSrcs(String ruleClass) {
     return ruleClass.equals("java_library")
         || ruleClass.equals("java_binary")
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/JavaHelper.java b/src/main/java/com/google/devtools/build/lib/rules/java/JavaHelper.java
index 6e2e7d8..0d440dc 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/JavaHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/JavaHelper.java
@@ -23,6 +23,7 @@
 import com.google.devtools.build.lib.packages.BuildType;
 import com.google.devtools.build.lib.packages.Type;
 import com.google.devtools.build.lib.shell.ShellUtils;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import java.util.ArrayList;
 import java.util.List;
@@ -108,11 +109,15 @@
   }
 
   public static PathFragment getJavaResourcePath(
-      JavaSemantics semantics, RuleContext ruleContext, Artifact resource) {
+      JavaSemantics semantics, RuleContext ruleContext, Artifact resource)
+      throws InterruptedException {
     PathFragment rootRelativePath = resource.getRootRelativePath();
+    StarlarkSemantics starlarkSemantics =
+        ruleContext.getAnalysisEnvironment().getSkylarkSemantics();
 
-    if (!ruleContext.getLabel().getWorkspaceRoot().isEmpty()) {
-      PathFragment workspace = PathFragment.create(ruleContext.getLabel().getWorkspaceRoot());
+    if (!ruleContext.getLabel().getWorkspaceRoot(starlarkSemantics).isEmpty()) {
+      PathFragment workspace =
+          PathFragment.create(ruleContext.getLabel().getWorkspaceRoot(starlarkSemantics));
       rootRelativePath = rootRelativePath.relativeTo(workspace);
     }
 
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/JavaRuntime.java b/src/main/java/com/google/devtools/build/lib/rules/java/JavaRuntime.java
index 6925ef7..6a15ac4 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/JavaRuntime.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/JavaRuntime.java
@@ -47,7 +47,12 @@
     JavaCommon.checkRuleLoadedThroughMacro(ruleContext);
     NestedSetBuilder<Artifact> filesBuilder = NestedSetBuilder.stableOrder();
     filesBuilder.addTransitive(PrerequisiteArtifacts.nestedSet(ruleContext, "srcs", Mode.TARGET));
-    PathFragment javaHome = defaultJavaHome(ruleContext.getLabel());
+    boolean siblingRepositoryLayout =
+        ruleContext
+            .getAnalysisEnvironment()
+            .getSkylarkSemantics()
+            .experimentalSiblingRepositoryLayout();
+    PathFragment javaHome = defaultJavaHome(ruleContext.getLabel(), siblingRepositoryLayout);
     if (ruleContext.attributes().isAttributeValueExplicitlySpecified("java_home")) {
       PathFragment javaHomeAttribute =
           PathFragment.create(ruleContext.getExpander().expand("java_home"));
@@ -124,11 +129,11 @@
         || path.endsWith(PathFragment.create("bin/java.exe"));
   }
 
-  static PathFragment defaultJavaHome(Label javabase) {
+  static PathFragment defaultJavaHome(Label javabase, boolean siblingRepositoryLayout) {
     if (javabase.getPackageIdentifier().getRepository().isDefault()) {
       return javabase.getPackageFragment();
     }
-    return javabase.getPackageIdentifier().getSourceRoot();
+    return javabase.getPackageIdentifier().getExecPath(siblingRepositoryLayout);
   }
 
   private static PathFragment getRunfilesJavaExecutable(PathFragment javaHome, Label javabase) {
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaLiteProtoAspect.java b/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaLiteProtoAspect.java
index bd866fa..8f917c3 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaLiteProtoAspect.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaLiteProtoAspect.java
@@ -243,7 +243,7 @@
                       aspectCommon.getProtoRuntimeDeps())));
     }
 
-    private void createProtoCompileAction(Artifact sourceJar) {
+    private void createProtoCompileAction(Artifact sourceJar) throws InterruptedException {
       ProtoCompileActionBuilder.registerActions(
           ruleContext,
           ImmutableList.of(
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoAspect.java b/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoAspect.java
index 5e633ed..2077149 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoAspect.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoAspect.java
@@ -281,7 +281,7 @@
       return protoBlackList.checkSrcs(protoInfo.getDirectProtoSources(), "java_proto_library");
     }
 
-    private void createProtoCompileAction(Artifact sourceJar) {
+    private void createProtoCompileAction(Artifact sourceJar) throws InterruptedException {
       ImmutableList.Builder<ToolchainInvocation> invocations = ImmutableList.builder();
       invocations.add(
           new ToolchainInvocation(
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoSkylarkCommon.java b/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoSkylarkCommon.java
index f74dea9..db1c4c8 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoSkylarkCommon.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/proto/JavaProtoSkylarkCommon.java
@@ -43,7 +43,7 @@
       Artifact sourceJar,
       String protoToolchainAttr,
       String flavour)
-      throws EvalException {
+      throws EvalException, InterruptedException {
     ProtoInfo protoInfo = target.get(ProtoInfo.PROVIDER);
     ProtoCompileActionBuilder.registerActions(
         skylarkRuleContext.getRuleContext(),
diff --git a/src/main/java/com/google/devtools/build/lib/rules/objc/J2ObjcAspect.java b/src/main/java/com/google/devtools/build/lib/rules/objc/J2ObjcAspect.java
index 7541582..777a2ad 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/objc/J2ObjcAspect.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/objc/J2ObjcAspect.java
@@ -616,7 +616,8 @@
       ProtoLangToolchainProvider protoToolchain,
       RuleContext ruleContext,
       ImmutableList<Artifact> filteredProtoSources,
-      J2ObjcSource j2ObjcSource) {
+      J2ObjcSource j2ObjcSource)
+      throws InterruptedException {
     ImmutableList<Artifact> outputHeaderMappingFiles =
         ProtoCommon.getGeneratedOutputs(ruleContext, filteredProtoSources, ".j2objc.mapping");
     ImmutableList<Artifact> outputClassMappingFiles =
@@ -753,7 +754,7 @@
   }
 
   private static J2ObjcSource protoJ2ObjcSource(
-      RuleContext ruleContext, ImmutableList<Artifact> protoSources) {
+      RuleContext ruleContext, ImmutableList<Artifact> protoSources) throws InterruptedException {
     PathFragment objcFileRootExecPath = getProtoOutputRoot(ruleContext);
 
     List<PathFragment> headerSearchPaths =
@@ -768,12 +769,21 @@
         headerSearchPaths);
   }
 
-  private static PathFragment getProtoOutputRoot(RuleContext ruleContext) {
+  private static PathFragment getProtoOutputRoot(RuleContext ruleContext)
+      throws InterruptedException {
     return ruleContext
         .getConfiguration()
         .getGenfilesFragment()
         .getRelative(
-            ruleContext.getLabel().getPackageIdentifier().getRepository().getPathUnderExecRoot());
+            ruleContext
+                .getLabel()
+                .getPackageIdentifier()
+                .getRepository()
+                .getExecPath(
+                    ruleContext
+                        .getAnalysisEnvironment()
+                        .getSkylarkSemantics()
+                        .experimentalSiblingRepositoryLayout()));
   }
 
   private static boolean isProtoRule(ConfiguredTarget base) {
diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibrary.java b/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibrary.java
index cf1318f..3aed38127 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibrary.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibrary.java
@@ -31,7 +31,7 @@
 
   @Override
   public ConfiguredTarget create(RuleContext ruleContext)
-      throws ActionConflictException, RuleErrorException {
+      throws ActionConflictException, RuleErrorException, InterruptedException {
     ProtoCommon.checkRuleHasValidMigrationTag(ruleContext);
     ProtoInfo protoInfo =
         ProtoCommon.createProtoInfo(
diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCommon.java b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCommon.java
index a0eb92a..81415b1 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCommon.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCommon.java
@@ -35,6 +35,7 @@
 import com.google.devtools.build.lib.packages.BuildType;
 import com.google.devtools.build.lib.packages.RuleClass.ConfiguredTargetFactory.RuleErrorException;
 import com.google.devtools.build.lib.packages.Type;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.util.Pair;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
 import com.google.devtools.build.lib.vfs.PathFragment;
@@ -214,13 +215,17 @@
   // ProtoInfo so it's not an easy change :(
   @Nullable
   private static Library createLibraryWithoutVirtualSourceRoot(
-      RuleContext ruleContext, ImmutableList<Artifact> directSources) {
+      RuleContext ruleContext, ImmutableList<Artifact> directSources) throws InterruptedException {
     String protoSourceRoot =
         ruleContext
             .getLabel()
             .getPackageIdentifier()
             .getRepository()
-            .getPathUnderExecRoot()
+            .getExecPath(
+                ruleContext
+                    .getAnalysisEnvironment()
+                    .getSkylarkSemantics()
+                    .experimentalSiblingRepositoryLayout())
             .getPathString();
 
     ImmutableList.Builder<Pair<Artifact, String>> builder = ImmutableList.builder();
@@ -258,7 +263,8 @@
   private static Library createLibraryWithVirtualSourceRootMaybe(
       RuleContext ruleContext,
       ImmutableList<Artifact> protoSources,
-      boolean generatedProtosInVirtualImports) {
+      boolean generatedProtosInVirtualImports)
+      throws InterruptedException {
     PathFragment importPrefixAttribute = getPathFragmentAttribute(ruleContext, "import_prefix");
     PathFragment stripImportPrefixAttribute =
         getPathFragmentAttribute(ruleContext, "strip_import_prefix");
@@ -283,20 +289,28 @@
     PathFragment stripImportPrefix;
     PathFragment importPrefix;
 
+    StarlarkSemantics starlarkSemantics =
+        ruleContext.getAnalysisEnvironment().getSkylarkSemantics();
+    boolean siblingRepositoryLayout = starlarkSemantics.experimentalSiblingRepositoryLayout();
     if (stripImportPrefixAttribute != null || importPrefixAttribute != null) {
       if (stripImportPrefixAttribute == null) {
-        stripImportPrefix = PathFragment.create(ruleContext.getLabel().getWorkspaceRoot());
+        stripImportPrefix =
+            PathFragment.create(ruleContext.getLabel().getWorkspaceRoot(starlarkSemantics));
       } else if (stripImportPrefixAttribute.isAbsolute()) {
         stripImportPrefix =
             ruleContext
                 .getLabel()
                 .getPackageIdentifier()
                 .getRepository()
-                .getSourceRoot()
+                .getExecPath(siblingRepositoryLayout)
                 .getRelative(stripImportPrefixAttribute.toRelative());
       } else {
         stripImportPrefix =
-            ruleContext.getPackageDirectory().getRelative(stripImportPrefixAttribute);
+            ruleContext
+                .getLabel()
+                .getPackageIdentifier()
+                .getExecPath(siblingRepositoryLayout)
+                .getRelative(stripImportPrefixAttribute);
       }
 
       if (importPrefixAttribute != null) {
@@ -312,7 +326,11 @@
     } else {
       // Has generated sources, but neither strip_import_prefix nor import_prefix
       stripImportPrefix =
-          ruleContext.getLabel().getPackageIdentifier().getRepository().getPathUnderExecRoot();
+          ruleContext
+              .getLabel()
+              .getPackageIdentifier()
+              .getRepository()
+              .getDerivedArtifactSourceRoot();
 
       importPrefix = PathFragment.EMPTY_FRAGMENT;
     }
@@ -323,7 +341,9 @@
     PathFragment sourceRootPath = ruleContext.getUniqueDirectory("_virtual_imports");
 
     for (Artifact realProtoSource : protoSources) {
-      if (!realProtoSource.getRootRelativePath().startsWith(stripImportPrefix)) {
+      if (siblingRepositoryLayout && realProtoSource.isSourceArtifact()
+          ? !realProtoSource.getExecPath().startsWith(stripImportPrefix)
+          : !realProtoSource.getRootRelativePath().startsWith(stripImportPrefix)) {
         ruleContext.ruleError(
             String.format(
                 ".proto file '%s' is not under the specified strip prefix '%s'",
@@ -332,7 +352,12 @@
       }
       Pair<PathFragment, Artifact> importsPair =
           computeImports(
-              ruleContext, realProtoSource, sourceRootPath, importPrefix, stripImportPrefix);
+              ruleContext,
+              realProtoSource,
+              sourceRootPath,
+              importPrefix,
+              stripImportPrefix,
+              starlarkSemantics.experimentalSiblingRepositoryLayout());
       protoSourceImportPair.add(new Pair<>(realProtoSource, importsPair.first.toString()));
       symlinks.add(importsPair.second);
     }
@@ -351,14 +376,22 @@
       Artifact realProtoSource,
       PathFragment sourceRootPath,
       PathFragment importPrefix,
-      PathFragment stripImportPrefix) {
-    PathFragment importPath =
-        importPrefix.getRelative(
-            realProtoSource.getRootRelativePath().relativeTo(stripImportPrefix));
+      PathFragment stripImportPrefix,
+      boolean siblingRepositoryLayout) {
+    PathFragment importPath;
 
-      Artifact virtualProtoSource =
-          ruleContext.getDerivedArtifact(
-              sourceRootPath.getRelative(importPath), ruleContext.getBinOrGenfilesDirectory());
+    if (siblingRepositoryLayout && realProtoSource.isSourceArtifact()) {
+      importPath =
+          importPrefix.getRelative(realProtoSource.getExecPath().relativeTo(stripImportPrefix));
+    } else {
+      importPath =
+          importPrefix.getRelative(
+              realProtoSource.getRootRelativePath().relativeTo(stripImportPrefix));
+    }
+
+    Artifact virtualProtoSource =
+        ruleContext.getDerivedArtifact(
+            sourceRootPath.getRelative(importPath), ruleContext.getBinOrGenfilesDirectory());
 
       ruleContext.registerAction(
           SymlinkAction.toArtifact(
@@ -458,7 +491,8 @@
    * ruleContext}.
    */
   public static ProtoInfo createProtoInfo(
-      RuleContext ruleContext, boolean generatedProtosInVirtualImports) {
+      RuleContext ruleContext, boolean generatedProtosInVirtualImports)
+      throws InterruptedException {
     checkSourceFilesAreInSamePackage(ruleContext);
     ImmutableList<Artifact> directProtoSources =
         ruleContext.getPrerequisiteArtifacts("srcs", Mode.TARGET).list();
diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
index b5d7881..f26499a 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
@@ -41,6 +41,7 @@
 import com.google.devtools.build.lib.analysis.stringtemplate.TemplateContext;
 import com.google.devtools.build.lib.analysis.stringtemplate.TemplateExpander;
 import com.google.devtools.build.lib.cmdline.Label;
+import com.google.devtools.build.lib.cmdline.LabelConstants;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
@@ -190,7 +191,7 @@
     }
   }
 
-  public Action[] build() {
+  public Action[] build() throws InterruptedException {
     if (isEmpty(outputs)) {
       return NO_ACTIONS;
     }
@@ -202,7 +203,8 @@
     }
   }
 
-  private SpawnAction.Builder createAction() throws MissingPrerequisiteException {
+  private SpawnAction.Builder createAction()
+      throws MissingPrerequisiteException, InterruptedException {
     SpawnAction.Builder result =
         new SpawnAction.Builder().addTransitiveInputs(protoInfo.getTransitiveProtoSources());
 
@@ -244,7 +246,7 @@
 
   /** Commandline generator for protoc invocations. */
   @VisibleForTesting
-  CustomCommandLine.Builder createProtoCompilerCommandLine() {
+  CustomCommandLine.Builder createProtoCompilerCommandLine() throws InterruptedException {
     CustomCommandLine.Builder result = CustomCommandLine.builder();
 
     if (langPlugin != null) {
@@ -262,13 +264,20 @@
 
     boolean areDepsStrict = areDepsStrict(ruleContext);
 
+    boolean siblingRepositoryLayout =
+        ruleContext
+            .getAnalysisEnvironment()
+            .getSkylarkSemantics()
+            .experimentalSiblingRepositoryLayout();
+
     // Add include maps
     addIncludeMapArguments(
         getOutputDirectory(ruleContext),
         result,
         areDepsStrict ? protoInfo.getStrictImportableProtoSourcesImportPaths() : null,
         protoInfo.getStrictImportableProtoSourceRoots(),
-        protoInfo.getTransitiveProtoSources());
+        protoInfo.getTransitiveProtoSources(),
+        siblingRepositoryLayout);
 
     if (areDepsStrict) {
       // Note: the %s in the line below is used by proto-compiler. That is, the string we create
@@ -297,7 +306,8 @@
                 .mapped(
                     new ExpandToPathFnWithImports(
                         getOutputDirectory(ruleContext),
-                        protoInfo.getTransitiveProtoSourceRoots())));
+                        protoInfo.getTransitiveProtoSourceRoots(),
+                        siblingRepositoryLayout)));
       }
     }
 
@@ -312,7 +322,8 @@
   private static class MissingPrerequisiteException extends Exception {}
 
   public static void writeDescriptorSet(
-      RuleContext ruleContext, ProtoInfo protoInfo, Services allowServices) {
+      RuleContext ruleContext, ProtoInfo protoInfo, Services allowServices)
+      throws InterruptedException {
     Artifact output = protoInfo.getDirectDescriptorSet();
     NestedSet<Artifact> dependenciesDescriptorSets =
         ProtoCommon.computeDependenciesDescriptorSets(ruleContext);
@@ -394,7 +405,8 @@
       Iterable<Artifact> outputs,
       String flavorName,
       Exports useExports,
-      Services allowServices) {
+      Services allowServices)
+      throws InterruptedException {
     SpawnAction.Builder actions =
         createActions(
             ruleContext,
@@ -419,7 +431,8 @@
       Iterable<Artifact> outputs,
       String flavorName,
       Exports useExports,
-      Services allowServices) {
+      Services allowServices)
+      throws InterruptedException {
 
     if (isEmpty(outputs)) {
       return null;
@@ -441,6 +454,12 @@
       return null;
     }
 
+    boolean siblingRepositoryLayout =
+        ruleContext
+            .getAnalysisEnvironment()
+            .getSkylarkSemantics()
+            .experimentalSiblingRepositoryLayout();
+
     result
         .addOutputs(outputs)
         .setResources(AbstractAction.DEFAULT_RESOURCE_SET)
@@ -455,7 +474,8 @@
                 areDepsStrict(ruleContext) ? Deps.STRICT : Deps.NON_STRICT,
                 arePublicImportsStrict(ruleContext) ? useExports : Exports.DO_NOT_USE,
                 allowServices,
-                ruleContext.getFragment(ProtoConfiguration.class).protocOpts()),
+                ruleContext.getFragment(ProtoConfiguration.class).protocOpts(),
+                siblingRepositoryLayout),
             ParamFileInfo.builder(ParameterFileType.UNQUOTED).build())
         .setProgressMessage("Generating %s proto_library %s", flavorName, ruleContext.getLabel())
         .setMnemonic(MNEMONIC);
@@ -496,7 +516,8 @@
       Deps strictDeps,
       Exports useExports,
       Services allowServices,
-      ImmutableList<String> protocOpts) {
+      ImmutableList<String> protocOpts,
+      boolean siblingRepositoryLayout) {
     CustomCommandLine.Builder cmdLine = CustomCommandLine.builder();
 
     cmdLine.addAll(
@@ -541,7 +562,8 @@
         cmdLine,
         strictDeps == Deps.STRICT ? protoInfo.getStrictImportableProtoSourcesImportPaths() : null,
         protoInfo.getStrictImportableProtoSourceRoots(),
-        protoInfo.getTransitiveProtoSources());
+        protoInfo.getTransitiveProtoSources(),
+        siblingRepositoryLayout);
 
     if (strictDeps == Deps.STRICT) {
       cmdLine.addFormatted(STRICT_DEPS_FLAG_TEMPLATE, ruleLabel);
@@ -559,7 +581,8 @@
                 .mapped(
                     new ExpandToPathFnWithImports(
                         outputDirectory,
-                        protoInfo.getExportedProtoSourceRoots())));
+                        protoInfo.getExportedProtoSourceRoots(),
+                        siblingRepositoryLayout)));
       }
     }
 
@@ -580,20 +603,25 @@
       CustomCommandLine.Builder commandLine,
       @Nullable NestedSet<Pair<Artifact, String>> protosInDirectDependencies,
       NestedSet<String> directProtoSourceRoots,
-      NestedSet<Artifact> transitiveImports) {
+      NestedSet<Artifact> transitiveImports,
+      boolean siblingRepositoryLayout) {
     // For each import, include both the import as well as the import relativized against its
     // protoSourceRoot. This ensures that protos can reference either the full path or the short
     // path when including other protos.
     commandLine.addAll(
         VectorArg.of(transitiveImports)
-            .mapped(new ExpandImportArgsFn(outputDirectory, directProtoSourceRoots)));
+            .mapped(
+                new ExpandImportArgsFn(
+                    outputDirectory, directProtoSourceRoots, siblingRepositoryLayout)));
     if (protosInDirectDependencies != null) {
       if (!protosInDirectDependencies.isEmpty()) {
         commandLine.addAll(
             "--direct_dependencies",
             VectorArg.join(":")
                 .each(protosInDirectDependencies)
-                .mapped(new ExpandToPathFnWithImports(outputDirectory, directProtoSourceRoots)));
+                .mapped(
+                    new ExpandToPathFnWithImports(
+                        outputDirectory, directProtoSourceRoots, siblingRepositoryLayout)));
 
       } else {
         // The proto compiler requires an empty list to turn on strict deps checking
@@ -603,7 +631,10 @@
   }
 
   private static String guessProtoPathUnderRoot(
-      String outputDirectory, PathFragment sourceRootPath, Artifact proto) {
+      String outputDirectory,
+      PathFragment sourceRootPath,
+      Artifact proto,
+      boolean siblingRepositoryLayout) {
     // TODO(lberki): Instead of guesswork like this, we should track which proto belongs to
     // which source root. Unfortunately, that's a non-trivial migration since
     // ProtoInfo is on the Starlark API. Therefore, we hack:
@@ -617,8 +648,15 @@
         return proto.getExecPath().relativeTo(sourceRootPath).getPathString();
       }
     } else {
+      PathFragment prefix =
+          siblingRepositoryLayout
+              ? LabelConstants.EXPERIMENTAL_EXTERNAL_PATH_PREFIX
+              : LabelConstants.EXTERNAL_PATH_PREFIX;
       if (proto.getRootRelativePath().startsWith(sourceRootPath)) {
         return proto.getRootRelativePath().relativeTo(sourceRootPath).getPathString();
+      } else if (proto.getExecPath().startsWith(prefix)
+          && proto.getExecPath().startsWith(sourceRootPath)) {
+        return proto.getExecPath().relativeTo(sourceRootPath).getPathString();
       }
     }
 
@@ -639,12 +677,15 @@
   static final class ExpandImportArgsFn implements CapturingMapFn<Artifact> {
     private final String outputDirectory;
     private final NestedSet<String> directProtoSourceRoots;
+    private final boolean siblingRepositoryLayout;
 
     public ExpandImportArgsFn(
         String outputDirectory,
-        NestedSet<String> directProtoSourceRoots) {
+        NestedSet<String> directProtoSourceRoots,
+        boolean siblingRepositoryLayout) {
       this.outputDirectory = outputDirectory;
       this.directProtoSourceRoots = directProtoSourceRoots;
+      this.siblingRepositoryLayout = siblingRepositoryLayout;
     }
 
     /**
@@ -656,7 +697,9 @@
     public void expandToCommandLine(Artifact proto, Consumer<String> args) {
       for (String directProtoSourceRoot : directProtoSourceRoots.toList()) {
         PathFragment sourceRootPath = PathFragment.create(directProtoSourceRoot);
-        String arg = guessProtoPathUnderRoot(outputDirectory, sourceRootPath, proto);
+        String arg =
+            guessProtoPathUnderRoot(
+                outputDirectory, sourceRootPath, proto, siblingRepositoryLayout);
         if (arg != null) {
           args.accept("-I" + arg + "=" + proto.getExecPathString());
         }
@@ -669,12 +712,15 @@
   static final class ExpandToPathFnWithImports implements CapturingMapFn<Pair<Artifact, String>> {
     private final String outputDirectory;
     private final NestedSet<String> directProtoSourceRoots;
+    private final boolean siblingRepositoryLayout;
 
     public ExpandToPathFnWithImports(
         String outputDirectory,
-        NestedSet<String> directProtoSourceRoots) {
+        NestedSet<String> directProtoSourceRoots,
+        boolean siblingRepositoryLayout) {
       this.outputDirectory = outputDirectory;
       this.directProtoSourceRoots = directProtoSourceRoots;
+      this.siblingRepositoryLayout = siblingRepositoryLayout;
     }
 
     @Override
@@ -684,7 +730,9 @@
       } else {
         for (String directProtoSourceRoot : directProtoSourceRoots.toList()) {
           PathFragment sourceRootPath = PathFragment.create(directProtoSourceRoot);
-          String arg = guessProtoPathUnderRoot(outputDirectory, sourceRootPath, proto.first);
+          String arg =
+              guessProtoPathUnderRoot(
+                  outputDirectory, sourceRootPath, proto.first, siblingRepositoryLayout);
           if (arg != null) {
             args.accept(arg);
           }
diff --git a/src/main/java/com/google/devtools/build/lib/sandbox/AbstractContainerizingSandboxedSpawn.java b/src/main/java/com/google/devtools/build/lib/sandbox/AbstractContainerizingSandboxedSpawn.java
index 0ed26fa..a886383 100644
--- a/src/main/java/com/google/devtools/build/lib/sandbox/AbstractContainerizingSandboxedSpawn.java
+++ b/src/main/java/com/google/devtools/build/lib/sandbox/AbstractContainerizingSandboxedSpawn.java
@@ -123,7 +123,14 @@
             outputs.files(),
             outputs.dirs())) {
       Preconditions.checkArgument(!path.isAbsolute());
-      Preconditions.checkArgument(!path.containsUplevelReferences());
+      if (path.segmentCount() > 1) {
+        // Allow a single up-level reference to allow inputs from the siblings of the main
+        // repository in the sandbox execution root.
+        Preconditions.checkArgument(
+            !path.subFragment(1).containsUplevelReferences(),
+            "%s escapes the sandbox exec root.",
+            path);
+      }
       for (int i = 0; i < path.segmentCount(); i++) {
         dirsToCreate.add(sandboxExecRoot.getRelative(path.subFragment(0, i)));
       }
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionFunction.java
index a51d334..11fe880 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionFunction.java
@@ -53,7 +53,6 @@
 import com.google.devtools.build.lib.causes.LabelCause;
 import com.google.devtools.build.lib.clock.BlazeClock;
 import com.google.devtools.build.lib.cmdline.Label;
-import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
 import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 import com.google.devtools.build.lib.collect.compacthashset.CompactHashSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
@@ -69,6 +68,7 @@
 import com.google.devtools.build.lib.skyframe.ActionRewindStrategy.RewindPlan;
 import com.google.devtools.build.lib.skyframe.ArtifactFunction.MissingFileArtifactValue;
 import com.google.devtools.build.lib.skyframe.SkyframeActionExecutor.ActionPostprocessing;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.util.io.TimestampGranularityMonitor;
 import com.google.devtools.build.lib.vfs.FileSystem;
@@ -667,6 +667,13 @@
           "resolver should only be called once: %s %s",
           keysRequested,
           execPaths);
+      StarlarkSemantics starlarkSemantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
+      if (starlarkSemantics == null) {
+        return null;
+      }
+
+      boolean siblingRepositoryLayout = starlarkSemantics.experimentalSiblingRepositoryLayout();
+
       // Create SkyKeys list based on execPaths.
       Map<PathFragment, SkyKey> depKeys = new HashMap<>();
       for (PathFragment path : execPaths) {
@@ -674,19 +681,11 @@
             Preconditions.checkNotNull(
                 path.getParentDirectory(), "Must pass in files, not root directory");
         Preconditions.checkArgument(!parent.isAbsolute(), path);
-        try {
-          SkyKey depKey =
-              ContainingPackageLookupValue.key(PackageIdentifier.discoverFromExecPath(path, true));
-          depKeys.put(path, depKey);
-          keysRequested.add(depKey);
-        } catch (LabelSyntaxException e) {
-          // This code is only used to do action cache checks. If one of the file names we got from
-          // the action cache is corrupted, or if the action cache is from a different Bazel
-          // binary, then the path may not be valid for this Bazel binary, and trigger this
-          // exception. In that case, it's acceptable for us to ignore the exception - we'll get an
-          // action cache miss and re-execute the action, which is what we should do.
-          continue;
-        }
+        SkyKey depKey =
+            ContainingPackageLookupValue.key(
+                PackageIdentifier.discoverFromExecPath(path, true, siblingRepositoryLayout));
+        depKeys.put(path, depKey);
+        keysRequested.add(depKey);
       }
 
       Map<SkyKey, SkyValue> values = env.getValues(depKeys.values());
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/PackageLookupFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/PackageLookupFunction.java
index 34708f9..c37f308 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/PackageLookupFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/PackageLookupFunction.java
@@ -28,6 +28,7 @@
 import com.google.devtools.build.lib.packages.RepositoryFetchException;
 import com.google.devtools.build.lib.pkgcache.PathPackageLocator;
 import com.google.devtools.build.lib.syntax.EvalException;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.Root;
 import com.google.devtools.build.lib.vfs.RootedPath;
@@ -252,12 +253,18 @@
             Transience.PERSISTENT);
       }
 
+      StarlarkSemantics starlarkSemantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
+      if (starlarkSemantics == null) {
+        return null;
+      }
+
       if (localRepository.exists()
           && !localRepository.getRepository().equals(packageIdentifier.getRepository())) {
         // There is a repository mismatch, this is an error.
         // The correct package path is the one originally given, minus the part that is the local
         // repository.
-        PathFragment pathToRequestedPackage = packageIdentifier.getPathUnderExecRoot();
+        PathFragment pathToRequestedPackage =
+            packageIdentifier.getExecPath(starlarkSemantics.experimentalSiblingRepositoryLayout());
         PathFragment localRepositoryPath = localRepository.getPath();
         if (localRepositoryPath.isAbsolute()) {
           // We need the package path to also be absolute.
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeExecutor.java b/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeExecutor.java
index 9d09908..33e34fb 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeExecutor.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeExecutor.java
@@ -93,7 +93,6 @@
 import com.google.devtools.build.lib.analysis.skylark.StarlarkTransition.TransitionException;
 import com.google.devtools.build.lib.buildtool.BuildRequestOptions;
 import com.google.devtools.build.lib.cmdline.Label;
-import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
 import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 import com.google.devtools.build.lib.cmdline.RepositoryName;
 import com.google.devtools.build.lib.cmdline.TargetParsingException;
@@ -335,6 +334,8 @@
   private final TrimmedConfigurationProgressReceiver trimmingListener =
       new TrimmedConfigurationProgressReceiver(trimmingCache);
 
+  private boolean siblingRepositoryLayout = false;
+
   /** An {@link ArtifactResolverSupplier} that supports setting of an {@link ArtifactFactory}. */
   public static class MutableArtifactFactorySupplier implements ArtifactResolverSupplier {
 
@@ -1169,13 +1170,9 @@
       throws InterruptedException {
     final Map<PathFragment, SkyKey> packageKeys = new HashMap<>();
     for (PathFragment execPath : execPaths) {
-      try {
-        PackageIdentifier pkgIdentifier =
-            PackageIdentifier.discoverFromExecPath(execPath, forFiles);
-        packageKeys.put(execPath, ContainingPackageLookupValue.key(pkgIdentifier));
-      } catch (LabelSyntaxException e) {
-        continue;
-      }
+      PackageIdentifier pkgIdentifier =
+          PackageIdentifier.discoverFromExecPath(execPath, forFiles, siblingRepositoryLayout);
+      packageKeys.put(execPath, ContainingPackageLookupValue.key(pkgIdentifier));
     }
 
     EvaluationResult<ContainingPackageLookupValue> result;
@@ -1185,6 +1182,7 @@
             .setNumThreads(1)
             .setEventHander(eventHandler)
             .build();
+
     synchronized (valueLookupLock) {
       result = buildDriver.evaluate(packageKeys.values(), evaluationContext);
     }
@@ -1371,7 +1369,10 @@
 
     setShowLoadingProgress(packageCacheOptions.showLoadingProgress);
     setDefaultVisibility(packageCacheOptions.defaultVisibility);
-    setSkylarkSemantics(getEffectiveStarlarkSemantics(starlarkSemanticsOptions));
+
+    StarlarkSemantics starlarkSemantics = getEffectiveStarlarkSemantics(starlarkSemanticsOptions);
+    setSkylarkSemantics(starlarkSemantics);
+    setSiblingDirectoryLayout(starlarkSemantics.experimentalSiblingRepositoryLayout());
     setPackageLocator(pkgLocator);
 
     syscalls.set(getPerBuildSyscallCache(packageCacheOptions.globbingThreads));
@@ -1392,6 +1393,11 @@
     cyclesReporter.set(createCyclesReporter());
   }
 
+  private void setSiblingDirectoryLayout(boolean experimentalSiblingRepositoryLayout) {
+    this.siblingRepositoryLayout = experimentalSiblingRepositoryLayout;
+    this.artifactFactory.get().setSiblingRepositoryLayout(experimentalSiblingRepositoryLayout);
+  }
+
   public StarlarkSemantics getEffectiveStarlarkSemantics(
       StarlarkSemanticsOptions starlarkSemanticsOptions) {
     return starlarkSemanticsOptions.toSkylarkSemantics();
diff --git a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/java/JavaProtoCommonApi.java b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/java/JavaProtoCommonApi.java
index af86838..0bf0b33 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/java/JavaProtoCommonApi.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/java/JavaProtoCommonApi.java
@@ -69,7 +69,7 @@
       FileT sourceJar,
       String protoToolchainAttr,
       String flavour)
-      throws EvalException;
+      throws EvalException, InterruptedException;
 
   @SkylarkCallable(
       name = "has_proto_sources",
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
index 89d44c1..26bb70c 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
@@ -53,6 +53,8 @@
     public static final String EXPERIMENTAL_ACTION_ARGS = "experimental_action_args";
     public static final String EXPERIMENTAL_ALLOW_INCREMENTAL_REPOSITORY_UPDATES =
         "experimental_allow_incremental_repository_updates";
+    public static final String EXPERIMENTAL_SIBLING_REPOSITORY_LAYOUT =
+        "experimental_sibling_repository_layout";
     public static final String EXPERIMENTAL_ASPECT_OUTPUT_PROPAGATION =
         "experimental_aspect_output_propagation";
     public static final String EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS =
@@ -94,6 +96,8 @@
         return experimentalActionArgs();
       case FlagIdentifier.EXPERIMENTAL_ALLOW_INCREMENTAL_REPOSITORY_UPDATES:
         return experimentalAllowIncrementalRepositoryUpdates();
+      case FlagIdentifier.EXPERIMENTAL_SIBLING_REPOSITORY_LAYOUT:
+        return experimentalSiblingRepositoryLayout();
       case FlagIdentifier.EXPERIMENTAL_ASPECT_OUTPUT_PROPAGATION:
         return experimentalAspectOutputPropagation();
       case FlagIdentifier.EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS:
@@ -199,6 +203,8 @@
 
   public abstract boolean experimentalRepoRemoteExec();
 
+  public abstract boolean experimentalSiblingRepositoryLayout();
+
   public abstract boolean incompatibleAlwaysCheckDepsetElements();
 
   public abstract boolean incompatibleApplicableLicenses();
@@ -301,6 +307,7 @@
           .experimentalStarlarkUnusedInputsList(true)
           .experimentalCcSharedLibrary(false)
           .experimentalRepoRemoteExec(false)
+          .experimentalSiblingRepositoryLayout(false)
           .incompatibleAlwaysCheckDepsetElements(false)
           .incompatibleApplicableLicenses(false)
           .incompatibleBzlDisallowLoadAfterStatement(true)
@@ -364,6 +371,8 @@
 
     public abstract Builder experimentalRepoRemoteExec(boolean value);
 
+    public abstract Builder experimentalSiblingRepositoryLayout(boolean value);
+
     public abstract Builder incompatibleAlwaysCheckDepsetElements(boolean value);
 
     public abstract Builder incompatibleApplicableLicenses(boolean value);
diff --git a/src/main/java/com/google/devtools/build/skydoc/SkydocMain.java b/src/main/java/com/google/devtools/build/skydoc/SkydocMain.java
index efb8ecc..c1f9f94 100644
--- a/src/main/java/com/google/devtools/build/skydoc/SkydocMain.java
+++ b/src/main/java/com/google/devtools/build/skydoc/SkydocMain.java
@@ -427,7 +427,7 @@
       List<AspectInfoWrapper> aspectInfoList,
       ImmutableMap.Builder<Label, String> moduleDocMap)
       throws InterruptedException, IOException, LabelSyntaxException, StarlarkEvaluationException {
-    Path path = pathOfLabel(label);
+    Path path = pathOfLabel(label, semantics);
 
     if (pending.contains(path)) {
       throw new StarlarkEvaluationException("cycle with " + path);
@@ -462,7 +462,7 @@
           throw new StarlarkEvaluationException(
               String.format(
                   "File %s imported '%s', yet %s was not found, even at roots %s.",
-                  path, module, pathOfLabel(relativeLabel), depRoots),
+                  path, module, pathOfLabel(relativeLabel, semantics), depRoots),
               noSuchFileException);
         }
       }
@@ -477,10 +477,11 @@
     return thread;
   }
 
-  private Path pathOfLabel(Label label) {
+  private Path pathOfLabel(Label label, StarlarkSemantics semantics) {
     String workspacePrefix = "";
-    if (!label.getWorkspaceRoot().isEmpty() && !label.getWorkspaceName().equals(workspaceName)) {
-      workspacePrefix = label.getWorkspaceRoot() + "/";
+    if (!label.getWorkspaceRoot(semantics).isEmpty()
+        && !label.getWorkspaceName().equals(workspaceName)) {
+      workspacePrefix = label.getWorkspaceRoot(semantics) + "/";
     }
 
     return Paths.get(workspacePrefix + label.toPathFragment());
diff --git a/src/test/java/com/google/devtools/build/lib/BUILD b/src/test/java/com/google/devtools/build/lib/BUILD
index 97365d9..697a66f 100644
--- a/src/test/java/com/google/devtools/build/lib/BUILD
+++ b/src/test/java/com/google/devtools/build/lib/BUILD
@@ -186,6 +186,7 @@
         ":test_runner",
         ":testutil",
         "//src/main/java/com/google/devtools/build/lib/cmdline",
+        "//src/main/java/com/google/devtools/build/lib/syntax:frontend",
         "//src/main/java/com/google/devtools/build/lib/vfs:pathfragment",
     ],
 )
diff --git a/src/test/java/com/google/devtools/build/lib/buildtool/SymlinkForestTest.java b/src/test/java/com/google/devtools/build/lib/buildtool/SymlinkForestTest.java
index 49278d9..ca54ed6 100644
--- a/src/test/java/com/google/devtools/build/lib/buildtool/SymlinkForestTest.java
+++ b/src/test/java/com/google/devtools/build/lib/buildtool/SymlinkForestTest.java
@@ -205,7 +205,8 @@
 
     Path linkRoot = fileSystem.getPath("/linkRoot");
     linkRoot.createDirectoryAndParents();
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, rootA, "pkgA");
@@ -233,7 +234,8 @@
             .put(createPkg(rootX, rootY, "foo"), rootX)
             .build();
 
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
     assertLinksTo(linkRoot, rootX, "file");
   }
@@ -261,7 +263,8 @@
             .put(createExternalPkg(outputBase, "Z", ""), outputBase)
             .build();
 
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir_main");
@@ -285,6 +288,62 @@
   }
 
   @Test
+  public void test_withSiblingRepoLayout_plantSymlinkForest() throws Exception {
+    Root outputBase = Root.fromPath(fileSystem.getPath("/ob"));
+    Root mainRepo = Root.fromPath(fileSystem.getPath("/my_repo"));
+    Path linkRoot = outputBase.getRelative("execroot/ws_name");
+
+    mainRepo.asPath().createDirectoryAndParents();
+    linkRoot.createDirectoryAndParents();
+
+    ImmutableMap<PackageIdentifier, Root> packageRootMap =
+        ImmutableMap.<PackageIdentifier, Root>builder()
+            .put(createMainPkg(mainRepo, "dir_main"), mainRepo)
+            .put(createMainPkg(mainRepo, "dir_lib/pkg"), mainRepo)
+            .put(createMainPkg(mainRepo, ""), mainRepo)
+            // Remote repo without top-level package.
+            .put(createExternalPkg(outputBase, "X", "dir_x/pkg"), outputBase)
+            // Remote repo with and without top-level package.
+            .put(createExternalPkg(outputBase, "Y", ""), outputBase)
+            .put(createExternalPkg(outputBase, "Y", "dir_y/pkg"), outputBase)
+            // Only top-level pkg.
+            .put(createExternalPkg(outputBase, "Z", ""), outputBase)
+            .build();
+
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), true)
+        .plantSymlinkForest();
+
+    // Expected sibling repository layout (X, Y and Z are siblings of ws_name):
+    //
+    // .
+    // ├── execroot
+    // │   ├── ws_name { ... }
+    // │   ├── X -> external/X
+    // │   ├── Y -> external/Y
+    // │   └── Z -> external/Z
+    // └── external
+    //     ├── X
+    //     ├── Y
+    //     └── Z
+
+    assertLinksTo(linkRoot, mainRepo, "dir_main");
+    assertLinksTo(linkRoot, mainRepo, "dir_lib");
+    assertLinksTo(linkRoot, mainRepo, "file");
+    assertLinksTo(
+        linkRoot.getParentDirectory().getRelative("X"),
+        outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX + "/X"));
+    assertLinksTo(
+        linkRoot.getParentDirectory().getRelative("Y"),
+        outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX + "/Y"));
+    assertLinksTo(
+        linkRoot.getParentDirectory().getRelative("Z"),
+        outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX + "/Z"));
+    assertThat(linkRoot.getParentDirectory().getRelative("Y/file").exists()).isTrue();
+    assertThat(linkRoot.getParentDirectory().getRelative("Z/file").exists()).isTrue();
+  }
+
+  @Test
   public void testPlantSymlinkForestForMainRepo() throws Exception {
     // For the main repo, plantSymlinkForest function should only link all files and dirs under
     // main repo root that're presented in packageRootMap.
@@ -305,7 +364,8 @@
             .put(createExternalPkg(outputBase, "X", "dir_x/pkg"), outputBase)
             .build();
 
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir1");
@@ -319,7 +379,7 @@
   }
 
   @Test
-  public void testTestExternalDirInMainRepoIsIgnored1() throws Exception {
+  public void test_withSubdirRepoLayout_TestExternalDirInMainRepoIsIgnored1() throws Exception {
     // Test external/ is ignored even when packages like "//external/foo" is specified.
     Root outputBase = Root.fromPath(fileSystem.getPath("/ob"));
     Root mainRepo = Root.fromPath(fileSystem.getPath("/my_repo"));
@@ -338,7 +398,8 @@
             .put(createExternalPkg(outputBase, "X", "dir_x/pkg"), outputBase)
             .build();
 
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir1");
@@ -349,7 +410,7 @@
   }
 
   @Test
-  public void testTestExternalDirInMainRepoIsIgnored2() throws Exception {
+  public void test_withSubDirRepoLayout_TestExternalDirInMainRepoIsIgnored2() throws Exception {
     // Test external/ is ignored when root package "//:" is specified.
     Root outputBase = Root.fromPath(fileSystem.getPath("/ob"));
     Root mainRepo = Root.fromPath(fileSystem.getPath("/my_repo"));
@@ -369,7 +430,8 @@
             .put(createExternalPkg(outputBase, "X", "dir_x/pkg"), outputBase)
             .build();
 
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir1");
@@ -380,6 +442,109 @@
   }
 
   @Test
+  public void test_withSiblingRepoLayout_TestExternalDirInMainRepoExists() throws Exception {
+    // Test external/ is ignored even when packages like "//external/foo" is specified.
+    Root outputBase = Root.fromPath(fileSystem.getPath("/ob"));
+    Root mainRepo = Root.fromPath(fileSystem.getPath("/my_repo"));
+    Path linkRoot = outputBase.getRelative("execroot/ws_name");
+
+    linkRoot.createDirectoryAndParents();
+    mainRepo.asPath().createDirectoryAndParents();
+
+    ImmutableMap<PackageIdentifier, Root> packageRootMap =
+        ImmutableMap.<PackageIdentifier, Root>builder()
+            .put(createMainPkg(mainRepo, "dir1/pkg/foo"), mainRepo)
+            .put(createMainPkg(mainRepo, "dir2/pkg"), mainRepo)
+            .put(createMainPkg(mainRepo, "dir3"), mainRepo)
+            // external/ should not be linked even we have "//external/foo" package
+            .put(createMainPkg(mainRepo, "external/foo"), mainRepo)
+            .put(createExternalPkg(outputBase, "X", "dir_x/pkg"), outputBase)
+            .build();
+
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), true)
+        .plantSymlinkForest();
+
+    // Expected output base layout with sibling repositories in the execroot where
+    // ws_name and X are siblings:
+    //
+    // /ob
+    // ├── execroot
+    // │   ├── ws_name
+    // │   │   ├── dir1
+    // │   │   │   └── pkg
+    // │   │   │       └── foo -> /my_repo/dir1/pkg/foo
+    // │   │   ├── dir2
+    // │   │   │   └── pkg -> /my_repo/dir2/pkg
+    // │   │   ├── dir3 -> /my_repo/dir3
+    // │   │   └── external
+    // │   │       └── foo -> /my_repo/external/foo
+    // │   └── X -> /ob/external/X
+    // └── external
+    //     └── X
+
+    assertLinksTo(linkRoot, mainRepo, "dir1");
+    assertLinksTo(linkRoot, mainRepo, "dir2");
+    assertLinksTo(linkRoot, mainRepo, "dir3");
+
+    assertThat(
+            outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX).getRelative("X").exists())
+        .isTrue();
+    assertThat(outputBase.getRelative("execroot/X").exists()).isTrue();
+    assertLinksTo(
+        linkRoot.getParentDirectory().getRelative("X"), // Sibling of the main repo.
+        outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX).getRelative("X"));
+
+    assertThat(linkRoot.getRelative("external/foo").exists()).isTrue();
+  }
+
+  @Test
+  public void test_withSiblingRepoLayoutAndRootPackageInRoots_TestExternalDirInMainRepoExists()
+      throws Exception {
+    // Test external/ is ignored when root package "//:" is specified.
+    Root outputBase = Root.fromPath(fileSystem.getPath("/ob"));
+    Root mainRepo = Root.fromPath(fileSystem.getPath("/my_repo"));
+    Path linkRoot = outputBase.getRelative("execroot/ws_name");
+
+    linkRoot.createDirectoryAndParents();
+    mainRepo.asPath().createDirectoryAndParents();
+    mainRepo.getRelative("external/foo").createDirectoryAndParents();
+
+    ImmutableMap<PackageIdentifier, Root> packageRootMap =
+        ImmutableMap.<PackageIdentifier, Root>builder()
+            // Empty package will cause every top-level files to be linked, except external/
+            .put(createMainPkg(mainRepo, ""), mainRepo)
+            .put(createExternalPkg(outputBase, "X", "dir_x/pkg"), outputBase)
+            .build();
+
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), true)
+        .plantSymlinkForest();
+
+    // Expected output base layout with sibling repositories in the execroot where
+    // ws_name and X are siblings:
+    //
+    // /ob
+    // ├── execroot
+    // │   ├── ws_name
+    // │   │   └── external
+    // │   │       └── foo -> /my_repo/external/foo
+    // │   └── X -> /ob/external/X
+    // └── external
+    //     └── X
+
+    assertThat(
+            outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX).getRelative("X").exists())
+        .isTrue();
+    assertThat(outputBase.getRelative("execroot/X").exists()).isTrue();
+    assertLinksTo(
+        linkRoot.getParentDirectory().getRelative("X"), // Sibling of the main repo.
+        outputBase.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX).getRelative("X"));
+
+    assertThat(linkRoot.getRelative("external/foo").exists()).isTrue();
+  }
+
+  @Test
   public void testExternalPackage() throws Exception {
     Path linkRoot = fileSystem.getPath("/linkRoot");
     linkRoot.createDirectoryAndParents();
@@ -391,7 +556,8 @@
             .put(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER, root)
             .build();
 
-    new SymlinkForest(packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of())
+    new SymlinkForest(
+            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of(), false)
         .plantSymlinkForest();
     assertThat(linkRoot.getRelative(LabelConstants.EXTERNAL_PATH_PREFIX).exists()).isFalse();
   }
@@ -417,7 +583,11 @@
             .build();
 
     new SymlinkForest(
-            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of("build"))
+            packageRootMap,
+            linkRoot,
+            TestConstants.PRODUCT_NAME,
+            ImmutableSortedSet.of("build"),
+            false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir1");
@@ -446,7 +616,11 @@
 
     SymlinkForest symlinkForest =
         new SymlinkForest(
-            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of("build"));
+            packageRootMap,
+            linkRoot,
+            TestConstants.PRODUCT_NAME,
+            ImmutableSortedSet.of("build"),
+            false);
     symlinkForest.plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir1");
@@ -491,7 +665,11 @@
             .build();
 
     new SymlinkForest(
-            packageRootMap, linkRoot, TestConstants.PRODUCT_NAME, ImmutableSortedSet.of("build"))
+            packageRootMap,
+            linkRoot,
+            TestConstants.PRODUCT_NAME,
+            ImmutableSortedSet.of("build"),
+            false)
         .plantSymlinkForest();
 
     assertLinksTo(linkRoot, mainRepo, "dir1");
@@ -522,7 +700,8 @@
                         packageRootMap,
                         linkRoot,
                         TestConstants.PRODUCT_NAME,
-                        ImmutableSortedSet.of("build"))
+                        ImmutableSortedSet.of("build"),
+                        false)
                     .plantSymlinkForest());
     assertThat(exception)
         .hasMessageThat()
@@ -558,7 +737,8 @@
                         packageRootMap,
                         linkRoot,
                         TestConstants.PRODUCT_NAME,
-                        ImmutableSortedSet.of("build"))
+                        ImmutableSortedSet.of("build"),
+                        false)
                     .plantSymlinkForest());
     assertThat(exception)
         .hasMessageThat()
diff --git a/src/test/java/com/google/devtools/build/lib/cmdline/LabelTest.java b/src/test/java/com/google/devtools/build/lib/cmdline/LabelTest.java
index 7f9741f..bb50aa2 100644
--- a/src/test/java/com/google/devtools/build/lib/cmdline/LabelTest.java
+++ b/src/test/java/com/google/devtools/build/lib/cmdline/LabelTest.java
@@ -18,6 +18,7 @@
 
 import com.google.common.collect.ImmutableMap;
 import com.google.common.testing.EqualsTester;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.testutil.TestUtils;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import java.util.regex.Pattern;
@@ -465,9 +466,10 @@
   @Test
   public void testGetWorkspaceRoot() throws Exception {
     Label label = Label.parseAbsolute("//bar/baz", ImmutableMap.of());
-    assertThat(label.getWorkspaceRoot()).isEmpty();
+    assertThat(label.getWorkspaceRoot(StarlarkSemantics.DEFAULT_SEMANTICS)).isEmpty();
     label = Label.parseAbsolute("@repo//bar/baz", ImmutableMap.of());
-    assertThat(label.getWorkspaceRoot()).isEqualTo("external/repo");
+    assertThat(label.getWorkspaceRoot(StarlarkSemantics.DEFAULT_SEMANTICS))
+        .isEqualTo("external/repo");
   }
 
   @Test
diff --git a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
index af485de..b13f2dc 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
@@ -121,6 +121,7 @@
         // <== Add new options here in alphabetic order ==>
         "--debug_depset_depth=" + rand.nextBoolean(),
         "--experimental_action_args=" + rand.nextBoolean(),
+        "--experimental_sibling_repository_layout=" + rand.nextBoolean(),
         "--experimental_allow_incremental_repository_updates=" + rand.nextBoolean(),
         "--experimental_aspect_output_propagation=" + rand.nextBoolean(),
         "--experimental_build_setting_api=" + rand.nextBoolean(),
@@ -174,6 +175,7 @@
         // <== Add new options here in alphabetic order ==>
         .debugDepsetDepth(rand.nextBoolean())
         .experimentalActionArgs(rand.nextBoolean())
+        .experimentalSiblingRepositoryLayout(rand.nextBoolean())
         .experimentalAllowIncrementalRepositoryUpdates(rand.nextBoolean())
         .experimentalAspectOutputPropagation(rand.nextBoolean())
         .experimentalBuildSettingApi(rand.nextBoolean())
diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java
index 954e538..9c995b6 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java
@@ -1279,7 +1279,7 @@
         "cc_library(name='a', srcs=['a.cc'], copts=['-Id/../../somewhere'])");
     CppCompileAction compileAction = getCppCompileAction("//root:a");
     try {
-      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs());
+      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs(), false);
     } catch (ActionExecutionException exception) {
       assertThat(exception)
           .hasMessageThat()
@@ -1296,7 +1296,7 @@
         "cc_library(name='a', srcs=['a.cc'], copts=['-I/somewhere'])");
     CppCompileAction compileAction = getCppCompileAction("//root:a");
     try {
-      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs());
+      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs(), false);
     } catch (ActionExecutionException exception) {
       assertThat(exception)
           .hasMessageThat()
@@ -1313,7 +1313,7 @@
         "cc_library(name='a', srcs=['a.cc'], copts=['-isystem../system'])");
     CppCompileAction compileAction = getCppCompileAction("//root:a");
     try {
-      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs());
+      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs(), false);
     } catch (ActionExecutionException exception) {
       assertThat(exception)
           .hasMessageThat()
@@ -1330,7 +1330,7 @@
         "cc_library(name='a', srcs=['a.cc'], copts=['-isystem/system'])");
     CppCompileAction compileAction = getCppCompileAction("//root:a");
     try {
-      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs());
+      compileAction.verifyActionIncludePaths(compileAction.getSystemIncludeDirs(), false);
     } catch (ActionExecutionException exception) {
       assertThat(exception)
           .hasMessageThat()
diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscoveryTest.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscoveryTest.java
index de3b621..e79f974 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscoveryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/HeaderDiscoveryTest.java
@@ -73,7 +73,7 @@
         .setDependencies(dependencies)
         .setAllowedDerivedInputs(includedHeaders)
         .build()
-        .discoverInputsFromDependencies(execRoot, artifactResolver);
+        .discoverInputsFromDependencies(execRoot, artifactResolver, false);
   }
 
   private SpecialArtifact treeArtifact(Path path) {
diff --git a/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcLibraryTest.java b/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcLibraryTest.java
index 7b25ee7..095351f 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcLibraryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcLibraryTest.java
@@ -971,7 +971,7 @@
     assertThat(
             compileAction
                 .discoverInputsFromDotdFiles(
-                    new ActionExecutionContextBuilder().build(), null, null, null)
+                    new ActionExecutionContextBuilder().build(), null, null, null, false)
                 .toList())
         .isEmpty();
   }
@@ -1629,7 +1629,7 @@
             ActionExecutionException.class,
             () ->
                 compileAction.discoverInputsFromDotdFiles(
-                    new ActionExecutionContextBuilder().build(), null, null, null));
+                    new ActionExecutionContextBuilder().build(), null, null, null, false));
     assertThat(expected).hasMessageThat().contains("error while parsing .d file");
   }
 
diff --git a/src/test/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryTest.java b/src/test/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryTest.java
index 6b98c9d..a905862 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryTest.java
@@ -348,8 +348,7 @@
             ".");
   }
 
-  @Test
-  public void testExternalRepoWithGeneratedProto() throws Exception {
+  private void testExternalRepoWithGeneratedProto(boolean siblingRepoLayout) throws Exception {
     if (!isThisBazel()) {
       return;
     }
@@ -358,6 +357,10 @@
         scratch.resolve("WORKSPACE"), "local_repository(name = 'foo', path = '/foo')");
     invalidatePackages();
 
+    if (siblingRepoLayout) {
+      setSkylarkSemanticsOptions("--experimental_sibling_repository_layout");
+    }
+
     scratch.file("/foo/WORKSPACE");
     scratch.file(
         "/foo/x/BUILD",
@@ -383,6 +386,16 @@
   }
 
   @Test
+  public void testExternalRepoWithGeneratedProto_withSubdirRepoLayout() throws Exception {
+    testExternalRepoWithGeneratedProto(/*siblingRepoLayout=*/ false);
+  }
+
+  @Test
+  public void test_siblingRepoLayout_ExternalRepoWithGeneratedProto() throws Exception {
+    testExternalRepoWithGeneratedProto(/*siblingRepoLayout=*/ true);
+  }
+
+  @Test
   public void testExportedStrippedImportPrefixes() throws Exception {
     if (!isThisBazel()) {
       return;
@@ -441,8 +454,7 @@
         .containsExactly(genfiles + "/a/_virtual_imports/a", genfiles + "/c/_virtual_imports/c");
   }
 
-  @Test
-  public void testImportPrefixInExternalRepo() throws Exception {
+  private void testImportPrefixInExternalRepo(boolean siblingRepoLayout) throws Exception {
     if (!isThisBazel()) {
       return;
     }
@@ -451,6 +463,10 @@
         scratch.resolve("WORKSPACE"), "local_repository(name = 'yolo_repo', path = '/yolo_repo')");
     invalidatePackages();
 
+    if (siblingRepoLayout) {
+      setSkylarkSemanticsOptions("--experimental_sibling_repository_layout");
+    }
+
     scratch.file("/yolo_repo/WORKSPACE");
     scratch.file("/yolo_repo/yolo_pkg/yolo.proto");
     scratch.file(
@@ -483,7 +499,16 @@
   }
 
   @Test
-  public void testImportPrefixAndStripInExternalRepo() throws Exception {
+  public void testImportPrefixInExternalRepo_withSubdirRepoLayout() throws Exception {
+    testImportPrefixInExternalRepo(/*siblingRepoLayout=*/ false);
+  }
+
+  @Test
+  public void testImportPrefixInExternalRepo_withSiblingRepoLayout() throws Exception {
+    testImportPrefixInExternalRepo(/*siblingRepoLayout=*/ true);
+  }
+
+  private void testImportPrefixAndStripInExternalRepo(boolean siblingRepoLayout) throws Exception {
     if (!isThisBazel()) {
       return;
     }
@@ -492,6 +517,10 @@
         scratch.resolve("WORKSPACE"), "local_repository(name = 'yolo_repo', path = '/yolo_repo')");
     invalidatePackages();
 
+    if (siblingRepoLayout) {
+      setSkylarkSemanticsOptions("--experimental_sibling_repository_layout");
+    }
+
     scratch.file("/yolo_repo/WORKSPACE");
     scratch.file("/yolo_repo/yolo_pkg_to_be_stripped/yolo_pkg/yolo.proto");
     scratch.file(
@@ -525,7 +554,16 @@
   }
 
   @Test
-  public void testStripImportPrefixInExternalRepo() throws Exception {
+  public void testImportPrefixAndStripInExternalRepo_withSubdirRepoLayout() throws Exception {
+    testImportPrefixAndStripInExternalRepo(/*siblingRepoLayout=*/ false);
+  }
+
+  @Test
+  public void testImportPrefixAndStripInExternalRepo_withSiblingRepoLayout() throws Exception {
+    testImportPrefixAndStripInExternalRepo(/*siblingRepoLayout=*/ true);
+  }
+
+  private void testStripImportPrefixInExternalRepo(boolean siblingRepoLayout) throws Exception {
     if (!isThisBazel()) {
       return;
     }
@@ -534,6 +572,10 @@
         scratch.resolve("WORKSPACE"), "local_repository(name = 'yolo_repo', path = '/yolo_repo')");
     invalidatePackages();
 
+    if (siblingRepoLayout) {
+      setSkylarkSemanticsOptions("--experimental_sibling_repository_layout");
+    }
+
     scratch.file("/yolo_repo/WORKSPACE");
     scratch.file("/yolo_repo/yolo_pkg_to_be_stripped/yolo_pkg/yolo.proto");
     scratch.file(
@@ -565,7 +607,17 @@
   }
 
   @Test
-  public void testRelativeStripImportPrefixInExternalRepo() throws Exception {
+  public void testStripImportPrefixInExternalRepo_withSubdirRepoLayout() throws Exception {
+    testStripImportPrefixInExternalRepo(/*siblingRepoLayout=*/ false);
+  }
+
+  @Test
+  public void testStripImportPrefixInExternalRepo_withSiblingRepoLayout() throws Exception {
+    testStripImportPrefixInExternalRepo(/*siblingRepoLayout=*/ true);
+  }
+
+  private void testRelativeStripImportPrefixInExternalRepo(boolean siblingRepoLayout)
+      throws Exception {
     if (!isThisBazel()) {
       return;
     }
@@ -574,6 +626,10 @@
         scratch.resolve("WORKSPACE"), "local_repository(name = 'yolo_repo', path = '/yolo_repo')");
     invalidatePackages();
 
+    if (siblingRepoLayout) {
+      setSkylarkSemanticsOptions("--experimental_sibling_repository_layout");
+    }
+
     scratch.file("/yolo_repo/WORKSPACE");
     scratch.file("/yolo_repo/yolo_pkg_to_be_stripped/yolo_pkg/yolo.proto");
     scratch.file(
@@ -605,6 +661,16 @@
   }
 
   @Test
+  public void testRelativeStripImportPrefixInExternalRepo_withSubdirRepoLayout() throws Exception {
+    testRelativeStripImportPrefixInExternalRepo(/*siblingRepoLayout=*/ false);
+  }
+
+  @Test
+  public void testRelativeStripImportPrefixInExternalRepo_withSiblingRepoLayout() throws Exception {
+    testRelativeStripImportPrefixInExternalRepo(/*siblingRepoLayout=*/ true);
+  }
+
+  @Test
   public void testIllegalStripImportPrefix() throws Exception {
     scratch.file(
         "third_party/a/BUILD",
diff --git a/src/test/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilderTest.java b/src/test/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilderTest.java
index c37f408..8a465b5 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilderTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilderTest.java
@@ -126,7 +126,8 @@
             Deps.NON_STRICT,
             Exports.DO_NOT_USE,
             Services.ALLOW,
-            /* protocOpts= */ ImmutableList.of());
+            /* protocOpts= */ ImmutableList.of(),
+            false);
 
     assertThat(cmdLine.arguments())
         .containsExactly(
@@ -158,7 +159,8 @@
             Deps.NON_STRICT,
             Exports.DO_NOT_USE,
             Services.ALLOW,
-            /* protocOpts= */ ImmutableList.of());
+            /* protocOpts= */ ImmutableList.of(),
+            false);
 
     assertThat(cmdLine.arguments()).containsExactly("out/source_file.proto");
   }
@@ -193,7 +195,8 @@
             Deps.STRICT,
             Exports.DO_NOT_USE,
             Services.ALLOW,
-            /* protocOpts= */ ImmutableList.of());
+            /* protocOpts= */ ImmutableList.of(),
+            false);
 
     assertThat(cmdLine.arguments())
         .containsExactly(
@@ -237,7 +240,8 @@
             Deps.NON_STRICT,
             Exports.USE,
             Services.ALLOW,
-            /* protocOpts= */ ImmutableList.of());
+            /* protocOpts= */ ImmutableList.of(),
+            false);
 
     assertThat(cmdLine.arguments())
         .containsExactly(
@@ -267,7 +271,8 @@
             Deps.STRICT,
             Exports.DO_NOT_USE,
             Services.DISALLOW,
-            /* protocOpts= */ ImmutableList.of("--foo", "--bar"));
+            /* protocOpts= */ ImmutableList.of("--foo", "--bar"),
+            false);
 
     assertThat(cmdLine.arguments()).containsAtLeast("--disallow_services", "--foo", "--bar");
   }
@@ -308,7 +313,8 @@
             Deps.STRICT,
             Exports.DO_NOT_USE,
             Services.ALLOW,
-            /* protocOpts= */ ImmutableList.of());
+            /* protocOpts= */ ImmutableList.of(),
+            false);
 
     assertThat(hasBeenCalled[0]).isFalse();
     cmdLine.arguments();
@@ -356,7 +362,8 @@
                     Deps.STRICT,
                     Exports.DO_NOT_USE,
                     Services.ALLOW,
-                    /* protocOpts= */ ImmutableList.of()));
+                    /* protocOpts= */ ImmutableList.of(),
+                    false));
     assertThat(e)
         .hasMessageThat()
         .isEqualTo(
@@ -460,7 +467,8 @@
         commandLine,
         protosInDirectDependenciesBuilder,
         NestedSetBuilder.wrap(Order.STABLE_ORDER, protoSourceRoots),
-        transitiveImportsNestedSet);
+        transitiveImportsNestedSet,
+        false);
     return commandLine.build().arguments();
   }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java
index 5cf4a41..60fd338 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java
@@ -42,6 +42,7 @@
 import com.google.devtools.build.lib.pkgcache.PathPackageLocator;
 import com.google.devtools.build.lib.skyframe.ExternalFilesHelper.ExternalFileAction;
 import com.google.devtools.build.lib.skyframe.PackageLookupFunction.CrossRepositoryLabelViolationStrategy;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.testutil.FoundationTestCase;
 import com.google.devtools.build.lib.testutil.TestConstants;
 import com.google.devtools.build.lib.util.Fingerprint;
@@ -139,6 +140,7 @@
     driver = new SequentialBuildDriver(evaluator);
     PrecomputedValue.BUILD_ID.set(differencer, UUID.randomUUID());
     PrecomputedValue.PATH_PACKAGE_LOCATOR.set(differencer, pkgLocator.get());
+    PrecomputedValue.STARLARK_SEMANTICS.set(differencer, StarlarkSemantics.DEFAULT_SEMANTICS);
   }
 
   private Artifact getSourceArtifact(String path) throws Exception {
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunctionTest.java
index 953d051..bd749af 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunctionTest.java
@@ -58,6 +58,7 @@
 import com.google.devtools.build.lib.skyframe.RecursiveFilesystemTraversalFunction.FileOperationException;
 import com.google.devtools.build.lib.skyframe.RecursiveFilesystemTraversalValue.ResolvedFile;
 import com.google.devtools.build.lib.skyframe.RecursiveFilesystemTraversalValue.TraversalRequest;
+import com.google.devtools.build.lib.syntax.StarlarkSemantics;
 import com.google.devtools.build.lib.testutil.FoundationTestCase;
 import com.google.devtools.build.lib.testutil.TimestampGranularityUtils;
 import com.google.devtools.build.lib.util.io.OutErr;
@@ -194,6 +195,7 @@
     driver = new SequentialBuildDriver(evaluator);
     PrecomputedValue.BUILD_ID.set(differencer, UUID.randomUUID());
     PrecomputedValue.PATH_PACKAGE_LOCATOR.set(differencer, pkgLocator.get());
+    PrecomputedValue.STARLARK_SEMANTICS.set(differencer, StarlarkSemantics.DEFAULT_SEMANTICS);
   }
 
   private Artifact sourceArtifact(String path) {
diff --git a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkIntegrationTest.java b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkIntegrationTest.java
index d530a26..163e228 100644
--- a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkIntegrationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkIntegrationTest.java
@@ -133,7 +133,7 @@
   }
 
   @Test
-  public void testExternalRepoLabelWorkspaceRoot() throws Exception {
+  public void testExternalRepoLabelWorkspaceRoot_subdirRepoLayout() throws Exception {
     scratch.overwriteFile(
         "WORKSPACE",
         new ImmutableList.Builder<String>()
@@ -160,6 +160,35 @@
   }
 
   @Test
+  public void testExternalRepoLabelWorkspaceRoot_siblingRepoLayout() throws Exception {
+    scratch.overwriteFile(
+        "WORKSPACE",
+        new ImmutableList.Builder<String>()
+            .addAll(analysisMock.getWorkspaceContents(mockToolsConfig))
+            .add("local_repository(name='r', path='/r')")
+            .build());
+
+    scratch.file("/r/WORKSPACE");
+    scratch.file(
+        "/r/test/skylark/extension.bzl",
+        "load('@//myinfo:myinfo.bzl', 'MyInfo')",
+        "def _impl(ctx):",
+        "  return [MyInfo(result = ctx.label.workspace_root)]",
+        "my_rule = rule(implementation = _impl, attrs = { })");
+    scratch.file(
+        "/r/BUILD", "load('//:test/skylark/extension.bzl', 'my_rule')", "my_rule(name='t')");
+
+    // Required since we have a new WORKSPACE file.
+    invalidatePackages(true);
+
+    setSkylarkSemanticsOptions("--experimental_sibling_repository_layout");
+
+    ConfiguredTarget myTarget = getConfiguredTarget("@r//:t");
+    String result = (String) getMyInfoFromTarget(myTarget).getValue("result");
+    assertThat(result).isEqualTo("../r");
+  }
+
+  @Test
   public void testSameMethodNames() throws Exception {
     // The alias feature of load() may hide the fact that two methods in the stack trace have the
     // same name. This is perfectly legal as long as these two methods are actually distinct.
diff --git a/src/test/shell/bazel/cc_integration_test.sh b/src/test/shell/bazel/cc_integration_test.sh
index c703887..592b28a 100755
--- a/src/test/shell/bazel/cc_integration_test.sh
+++ b/src/test/shell/bazel/cc_integration_test.sh
@@ -932,4 +932,115 @@
   expect_log "@local_config_cc//:empty"
 }
 
+function setup_workspace_layout_with_external_directory() {
+  # Make the following layout to test builds in //external subpackages:
+  #
+  #├── baz
+  #│   ├── binary.cc
+  #│   └── BUILD
+  #└── external
+  #    └── foo
+  #        ├── BUILD
+  #        ├── lib.cc
+  #        └── lib.h
+  mkdir -p external/foo
+  cat > external/foo/BUILD <<EOF
+cc_library(
+    name = "lib",
+    srcs = ["lib.cc"],
+    hdrs = ["lib.h"],
+    visibility = ["//baz:__subpackages__"],
+)
+EOF
+  cat > external/foo/lib.cc <<EOF
+#include "external/foo/lib.h"
+#include <iostream>
+
+using std::cout;
+using std::endl;
+using std::string;
+
+HelloLib::HelloLib(const string& greeting) : greeting_(new string(greeting)) {
+}
+
+void HelloLib::greet(const string& thing) {
+  cout << *greeting_ << " " << thing << endl;
+}
+EOF
+
+  cat > external/foo/lib.h <<EOF
+#include <string>
+#include <memory>
+
+class HelloLib {
+ public:
+  explicit HelloLib(const std::string &greeting);
+  void greet(const std::string &thing);
+
+ private:
+  std::unique_ptr<const std::string> greeting_;
+};
+EOF
+
+  mkdir baz
+  cat > baz/BUILD <<EOF
+cc_binary(
+    name = "binary",
+    srcs = ["binary.cc"],
+    deps = ["//external/foo:lib"],
+)
+EOF
+  cat > baz/binary.cc <<EOF
+#include "external/foo/lib.h"
+#include <string>
+
+int main(int argc, char** argv) {
+  HelloLib lib("Hello");
+  std::string thing = "world";
+  if (argc > 1) {
+    thing = argv[1];
+  }
+  lib.greet(thing);
+  return 0;
+}
+EOF
+
+}
+
+function test_execroot_subdir_layout_fails_for_external_subpackages() {
+  setup_workspace_layout_with_external_directory
+
+  bazel build --experimental_sibling_repository_layout=false //baz:binary &> $TEST_log \
+    && fail "build should have failed with sources in the external directory" || true
+  expect_log "error:.*external/foo/lib.*"
+  expect_log "Target //baz:binary failed to build"
+}
+
+function test_execroot_sibling_layout_null_build_for_external_subpackages() {
+  setup_workspace_layout_with_external_directory
+  bazel build --experimental_sibling_repository_layout //baz:binary || fail "expected build success"
+
+  # Null build.
+  bazel build --experimental_sibling_repository_layout //baz:binary &> $TEST_log || fail "expected build success"
+  expect_log "INFO: 0 processes"
+}
+
+function test_execroot_sibling_layout_header_scanning_in_external_subpackage() {
+  setup_workspace_layout_with_external_directory
+  cat << 'EOF' > external/foo/BUILD
+cc_library(
+    name = "lib",
+    srcs = ["lib.cc"],
+    # missing header declaration
+    visibility = ["//baz:__subpackages__"],
+)
+EOF
+
+  bazel build --experimental_sibling_repository_layout --spawn_strategy=standalone //external/foo:lib &> $TEST_log \
+    && fail "build should not have succeeded with missing header file"
+
+  expect_log "undeclared inclusion(s) in rule '//external/foo:lib'" \
+     "could not find 'undeclared inclusion' error message in bazel output"
+}
+
 run_suite "cc_integration_test"
diff --git a/src/test/shell/bazel/execroot_test.sh b/src/test/shell/bazel/execroot_test.sh
index cd89aee..f4ad5fc 100755
--- a/src/test/shell/bazel/execroot_test.sh
+++ b/src/test/shell/bazel/execroot_test.sh
@@ -44,4 +44,54 @@
   assert_contains "$(dirname $execroot)/${ws_name}/bazel-out" out
 }
 
+function test_sibling_repository_layout() {
+    touch WORKSPACE
+
+    mkdir -p external/foo
+    cat > external/foo/BUILD <<'EOF'
+genrule(
+  name = "use-srcs",
+  srcs = ["BUILD"],
+  cmd = "cp $< $@",
+  outs = ["used-srcs"],
+)
+EOF
+
+    bazel build --experimental_sibling_repository_layout //external/foo:use-srcs \
+        || fail "expected success"
+
+    execroot="$(bazel info execution_root)"
+
+    test -e "$execroot/external/foo/BUILD"
+
+    test -e "$execroot/../bazel_tools/tools/genrule/genrule-setup.sh"
+    test ! -e "$execroot/external/bazel_tools/tools/genrule/genrule-setup.sh"
+}
+
+function test_no_sibling_repository_layout() {
+    touch WORKSPACE
+
+    mkdir -p external/foo
+    cat > external/foo/BUILD <<'EOF'
+genrule(
+  name = "use-srcs",
+  srcs = ["BUILD"],
+  cmd = "cp $< $@",
+  outs = ["used-srcs"],
+)
+EOF
+
+    bazel build //external/foo:use-srcs --experimental_sibling_repository_layout=false \
+        &> $TEST_log && fail "should have failed" || true
+    expect_log "external/foo/BUILD.*: No such file or directory"
+
+    execroot="$(bazel info execution_root)"
+
+    test ! -e "$execroot/external/foo/BUILD"
+
+    test ! -e "$execroot/../bazel_tools/tools/genrule/genrule-setup.sh"
+    test -e "$execroot/external/bazel_tools/tools/genrule/genrule-setup.sh"
+
+}
+
 run_suite "execution root tests"