diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionSplitTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionSplitTransitionProvider.java
deleted file mode 100644
index d374a75..0000000
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionSplitTransitionProvider.java
+++ /dev/null
@@ -1,373 +0,0 @@
-// Copyright 2018 The Bazel Authors. All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//    http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package com.google.devtools.build.lib.analysis.skylark;
-
-import static com.google.devtools.build.lib.analysis.skylark.SkylarkAttributesCollection.ERROR_MESSAGE_FOR_NO_ATTR;
-import static java.nio.charset.StandardCharsets.US_ASCII;
-
-import com.google.common.base.Joiner;
-import com.google.common.base.Preconditions;
-import com.google.common.collect.ImmutableList;
-import com.google.common.collect.ImmutableMap;
-import com.google.common.collect.ImmutableSet;
-import com.google.common.collect.Sets;
-import com.google.common.io.BaseEncoding;
-import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
-import com.google.devtools.build.lib.analysis.config.BuildOptions;
-import com.google.devtools.build.lib.analysis.config.FragmentOptions;
-import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
-import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
-import com.google.devtools.build.lib.packages.Attribute;
-import com.google.devtools.build.lib.packages.Attribute.SplitTransitionProvider;
-import com.google.devtools.build.lib.packages.AttributeMap;
-import com.google.devtools.build.lib.packages.ConfiguredAttributeMapper;
-import com.google.devtools.build.lib.packages.StructImpl;
-import com.google.devtools.build.lib.packages.StructProvider;
-import com.google.devtools.build.lib.syntax.Environment;
-import com.google.devtools.build.lib.syntax.EvalException;
-import com.google.devtools.build.lib.syntax.Mutability;
-import com.google.devtools.build.lib.syntax.Runtime;
-import com.google.devtools.build.lib.syntax.Runtime.NoneType;
-import com.google.devtools.build.lib.syntax.SkylarkDict;
-import com.google.devtools.build.lib.syntax.SkylarkType;
-import com.google.devtools.common.options.OptionDefinition;
-import com.google.devtools.common.options.OptionsParser;
-import com.google.devtools.common.options.OptionsParsingException;
-import java.lang.reflect.Field;
-import java.security.MessageDigest;
-import java.security.NoSuchAlgorithmException;
-import java.util.LinkedHashMap;
-import java.util.LinkedHashSet;
-import java.util.List;
-import java.util.Map;
-
-/**
- * This class implements a split transition provider that takes a Skylark transition function as
- * input. The transition function takes a settings argument, which is a dictionary containing the
- * current option values. It either returns a dictionary mapping option name to new option value
- * (for a patch transition), or a dictionary of such dictionaries (for a split transition).
- *
- * <p>TODO(bazel-team): Consider allowing dependency-typed attributes to actually return providers
- * instead of just labels (see {@link SkylarkAttributesCollection#addAttribute}).
- */
-public class FunctionSplitTransitionProvider implements SplitTransitionProvider {
-
-  private static final String COMMAND_LINE_OPTION_PREFIX = "//command_line_option:";
-
-  private final StarlarkDefinedConfigTransition starlarkDefinedConfigTransition;
-
-  public FunctionSplitTransitionProvider(
-      StarlarkDefinedConfigTransition starlarkDefinedConfigTransition) {
-    this.starlarkDefinedConfigTransition = starlarkDefinedConfigTransition;
-  }
-
-  @Override
-  public SplitTransition apply(AttributeMap attributeMap) {
-    return new FunctionSplitTransition(starlarkDefinedConfigTransition, attributeMap);
-  }
-
-  private static class FunctionSplitTransition implements SplitTransition {
-    private final StarlarkDefinedConfigTransition starlarkDefinedConfigTransition;
-    private final StructImpl attrObject;
-
-    FunctionSplitTransition(
-        StarlarkDefinedConfigTransition starlarkDefinedConfigTransition,
-        AttributeMap attributeMap) {
-      Preconditions.checkArgument(attributeMap instanceof ConfiguredAttributeMapper);
-      this.starlarkDefinedConfigTransition = starlarkDefinedConfigTransition;
-
-      ConfiguredAttributeMapper configuredAttributeMapper =
-          (ConfiguredAttributeMapper) attributeMap;
-      LinkedHashMap<String, Object> attributes = new LinkedHashMap<>();
-      for (String attribute : attributeMap.getAttributeNames()) {
-        Object val =
-            configuredAttributeMapper.get(attribute, attributeMap.getAttributeType(attribute));
-        attributes.put(
-            Attribute.getSkylarkName(attribute),
-            val == null ? Runtime.NONE : SkylarkType.convertToSkylark(val, (Environment) null));
-      }
-      attrObject = StructProvider.STRUCT.create(attributes, ERROR_MESSAGE_FOR_NO_ATTR);
-    }
-
-    @Override
-    public final List<BuildOptions> split(BuildOptions buildOptions) {
-      // TODO(waltl): we should be able to build this once and use it across different split
-      // transitions.
-      try {
-        Map<String, OptionInfo> optionInfoMap = buildOptionInfo(buildOptions);
-        SkylarkDict<String, Object> settings =
-            buildSettings(buildOptions, optionInfoMap, starlarkDefinedConfigTransition.getInputs());
-
-        ImmutableList.Builder<BuildOptions> splitBuildOptions = ImmutableList.builder();
-
-        ImmutableList<Map<String, Object>> transitions =
-            starlarkDefinedConfigTransition.getChangedSettings(settings, attrObject);
-        // TODO(juliexxia): Validate that the output values correctly match the output types.
-        validateFunctionOutputs(transitions, starlarkDefinedConfigTransition.getOutputs());
-
-        for (Map<String, Object> transition : transitions) {
-          BuildOptions options = buildOptions.clone();
-          applyTransition(options, transition, optionInfoMap);
-          splitBuildOptions.add(options);
-        }
-        return splitBuildOptions.build();
-
-      } catch (InterruptedException | EvalException e) {
-        // TODO(juliexxia): Throw an exception better than RuntimeException.
-        throw new RuntimeException(e);
-      }
-    }
-
-    private void validateFunctionOutputs(
-        ImmutableList<Map<String, Object>> transitions,
-        List<String> expectedOutputs) throws EvalException {
-      for (Map<String, Object> transition : transitions) {
-        LinkedHashSet<String> remainingOutputs = Sets.newLinkedHashSet(expectedOutputs);
-        for (String outputKey : transition.keySet()) {
-          if (!remainingOutputs.remove(outputKey)) {
-            throw new EvalException(
-                starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-                String.format("transition function returned undeclared output '%s'", outputKey));
-          }
-        }
-
-        if (!remainingOutputs.isEmpty()) {
-          throw new EvalException(
-              starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-              String.format(
-                  "transition outputs [%s] were not defined by transition function",
-                  Joiner.on(", ").join(remainingOutputs)));
-        }
-      }
-    }
-
-    /**
-     * Given a label-like string representing a command line option, returns the command line
-     * option string that it represents. This is a temporary measure to support command line
-     * options with strings that look "label-like", so that migrating users using this
-     * experimental syntax is easier later.
-     *
-     * @throws EvalException if the given string is not a valid format to represent to
-     *     a command line option
-     */
-    private String commandLineOptionLabelToOption(String label) throws EvalException {
-      if (label.startsWith(COMMAND_LINE_OPTION_PREFIX)) {
-        return label.substring(COMMAND_LINE_OPTION_PREFIX.length());
-      } else {
-        throw new EvalException(
-            starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-            String.format(
-                "Option key '%s' is of invalid form. "
-                    + "Expected command line option to begin with %s",
-                label, COMMAND_LINE_OPTION_PREFIX));
-      }
-    }
-
-    /**
-     * For all the options in the BuildOptions, build a map from option name to its information.
-     */
-    private Map<String, OptionInfo> buildOptionInfo(BuildOptions buildOptions) {
-      ImmutableMap.Builder<String, OptionInfo> builder = new ImmutableMap.Builder<>();
-
-      ImmutableSet<Class<? extends FragmentOptions>> optionClasses =
-          buildOptions
-          .getNativeOptions()
-          .stream()
-          .map(FragmentOptions::getClass)
-          .collect(ImmutableSet.toImmutableSet());
-
-      for (Class<? extends FragmentOptions> optionClass : optionClasses) {
-        ImmutableList<OptionDefinition> optionDefinitions =
-            OptionsParser.getOptionDefinitions(optionClass);
-        for (OptionDefinition def : optionDefinitions) {
-          String optionName = def.getOptionName();
-          builder.put(optionName, new OptionInfo(optionClass, def));
-        }
-      }
-
-      return builder.build();
-    }
-
-    /**
-     * Enter the options in buildOptions into a skylark dictionary, and return the dictionary.
-     *
-     * @throws IllegalArgumentException If the method is unable to look up the value in buildOptions
-     *     corresponding to an entry in optionInfoMap.
-     * @throws RuntimeException If the field corresponding to an option value in buildOptions is
-     *     inaccessible due to Java language access control, or if an option name is an invalid key
-     *     to the Skylark dictionary.
-     * @throws EvalException if any of the specified transition inputs do not correspond to a valid
-     *     build setting
-     */
-    private SkylarkDict<String, Object> buildSettings(BuildOptions buildOptions,
-        Map<String, OptionInfo> optionInfoMap, List<String> inputs) throws EvalException {
-      LinkedHashSet<String> remainingInputs = Sets.newLinkedHashSet(inputs);
-
-      try (Mutability mutability = Mutability.create("build_settings")) {
-        SkylarkDict<String, Object> dict = SkylarkDict.withMutability(mutability);
-
-        for (Map.Entry<String, OptionInfo> entry : optionInfoMap.entrySet()) {
-          String optionName = entry.getKey();
-          String optionKey = COMMAND_LINE_OPTION_PREFIX + optionName;
-
-          if (!remainingInputs.remove(optionKey)) {
-            // This option was not present in inputs. Skip it.
-            continue;
-          }
-          OptionInfo optionInfo = entry.getValue();
-
-          try {
-            Field field = optionInfo.getDefinition().getField();
-            FragmentOptions options = buildOptions.get(optionInfo.getOptionClass());
-            Object optionValue = field.get(options);
-
-            dict.put(optionKey, optionValue, null, mutability);
-          } catch (IllegalAccessException e) {
-            // These exceptions should not happen, but if they do, throw a RuntimeException.
-            throw new RuntimeException(e);
-          }
-        }
-
-        if (!remainingInputs.isEmpty()) {
-          throw new EvalException(
-              starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-              String.format(
-                  "transition inputs [%s] do not correspond to valid settings",
-                  Joiner.on(", ").join(remainingInputs)));
-        }
-
-        return dict;
-      }
-    }
-
-    /**
-     * Apply the transition dictionary to the build option, using optionInfoMap to look up the
-     * option info.
-     *
-     * @throws RuntimeException If a requested option field is inaccessible.
-     */
-    private void applyTransition(BuildOptions buildOptions, Map<String, Object> transition,
-        Map<String, OptionInfo> optionInfoMap)
-        throws EvalException {
-      for (Map.Entry<String, Object> entry : transition.entrySet()) {
-        String optionKey = entry.getKey();
-
-        // TODO(juliexxia): Handle keys which correspond to build_setting target labels instead
-        // of assuming every key is for a command line option.
-        String optionName = commandLineOptionLabelToOption(optionKey);
-        Object optionValue = entry.getValue();
-
-        // Convert NoneType to null.
-        if (optionValue instanceof NoneType) {
-          optionValue = null;
-        }
-
-        try {
-          if (!optionInfoMap.containsKey(optionName)) {
-            throw new EvalException(
-                starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-                String.format(
-                    "transition output '%s' does not correspond to a valid setting", optionKey));
-          }
-
-          OptionInfo optionInfo = optionInfoMap.get(optionName);
-          OptionDefinition def = optionInfo.getDefinition();
-          Field field = def.getField();
-          FragmentOptions options = buildOptions.get(optionInfo.getOptionClass());
-          if (optionValue == null || def.getType().isInstance(optionValue)) {
-            field.set(options, optionValue);
-          } else if (optionValue instanceof String) {
-            field.set(options, def.getConverter().convert((String) optionValue));
-          } else {
-            throw new EvalException(
-                starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-                "Invalid value type for option '" + optionName + "'");
-          }
-        } catch (IllegalAccessException e) {
-          throw new RuntimeException(
-              "IllegalAccess for option " + optionName + ": " + e.getMessage());
-        } catch (OptionsParsingException e) {
-          throw new EvalException(
-              starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-              "OptionsParsingError for option '" + optionName + "': " + e.getMessage());
-        }
-      }
-
-      BuildConfiguration.Options buildConfigOptions;
-      buildConfigOptions = buildOptions.get(BuildConfiguration.Options.class);
-
-      if (starlarkDefinedConfigTransition.isForAnalysisTesting()) {
-        buildConfigOptions.evaluatingForAnalysisTest = true;
-      }
-      updateOutputDirectoryNameFragment(buildConfigOptions, transition);
-    }
-
-    /**
-     * Compute the output directory name fragment corresponding to the transition, and append it to
-     * the existing name fragment in buildConfigOptions.
-     *
-     * @throws IllegalStateException If MD5 support is not available.
-     */
-    private void updateOutputDirectoryNameFragment(BuildConfiguration.Options buildConfigOptions,
-        Map<String, Object> transition) {
-      String transitionString = new String();
-      for (Map.Entry<String, Object> entry : transition.entrySet()) {
-        transitionString += entry.getKey() + ":";
-        if (entry.getValue() != null) {
-          transitionString += entry.getValue().toString() + "@";
-        }
-      }
-
-      // TODO(waltl): for transitions that don't read settings, it is possible to precompute and
-      // reuse the MD5 digest and even the transition itself.
-      try {
-        byte[] bytes = transitionString.getBytes(US_ASCII);
-        MessageDigest md = MessageDigest.getInstance("MD5");
-        byte[] digest = md.digest(bytes);
-        String hexDigest = BaseEncoding.base16().lowerCase().encode(digest);
-
-        if (buildConfigOptions.transitionDirectoryNameFragment == null) {
-          buildConfigOptions.transitionDirectoryNameFragment = hexDigest;
-        } else {
-          buildConfigOptions.transitionDirectoryNameFragment += "-" + hexDigest;
-        }
-      } catch (NoSuchAlgorithmException e) {
-        throw new IllegalStateException("MD5 not available", e);
-      }
-    }
-
-    /**
-     * Stores option info useful to a FunctionSplitTransition.
-     */
-    private static class OptionInfo {
-      private final Class<? extends FragmentOptions> optionClass;
-      private final OptionDefinition definition;
-
-      public OptionInfo(Class<? extends FragmentOptions> optionClass,
-          OptionDefinition definition) {
-        this.optionClass = optionClass;
-        this.definition = definition;
-      }
-
-      Class<? extends FragmentOptions> getOptionClass() {
-        return optionClass;
-      }
-
-      OptionDefinition getDefinition() {
-        return definition;
-      }
-    }
-  }
-}
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java
new file mode 100644
index 0000000..d0ee650
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java
@@ -0,0 +1,343 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.devtools.build.lib.analysis.skylark;
+
+import static java.nio.charset.StandardCharsets.US_ASCII;
+
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Sets;
+import com.google.common.io.BaseEncoding;
+import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.FragmentOptions;
+import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
+import com.google.devtools.build.lib.packages.StructImpl;
+import com.google.devtools.build.lib.syntax.EvalException;
+import com.google.devtools.build.lib.syntax.Mutability;
+import com.google.devtools.build.lib.syntax.Runtime.NoneType;
+import com.google.devtools.build.lib.syntax.SkylarkDict;
+import com.google.devtools.common.options.OptionDefinition;
+import com.google.devtools.common.options.OptionsParser;
+import com.google.devtools.common.options.OptionsParsingException;
+import java.lang.reflect.Field;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Utility class for common work done across {@link StarlarkAttributeTransitionProvider} and {@link
+ * StarlarkRuleTransitionProvider}.
+ */
+public class FunctionTransitionUtil {
+
+  private static final String COMMAND_LINE_OPTION_PREFIX = "//command_line_option:";
+
+  /**
+   * Figure out what build settings the given transition changes and apply those changes to the
+   * incoming {@link BuildOptions}. For native options, this involves a preprocess step of
+   * converting options to their "command line form".
+   *
+   * Also Validate that transitions output sensical results.
+   *
+   * @param buildOptions the pre-transition build options
+   * @param starlarkTransition the transition to apply
+   * @param attrObject the attributes of the rule to which this transition is attached
+   * @return the post-transition build options
+   */
+  static List<BuildOptions> applyAndValidate(
+      BuildOptions buildOptions,
+      StarlarkDefinedConfigTransition starlarkTransition,
+      StructImpl attrObject) {
+    // TODO(waltl): consider building this once and use it across different split
+    // transitions.
+    try {
+      Map<String, OptionInfo> optionInfoMap = buildOptionInfo(buildOptions);
+      SkylarkDict<String, Object> settings =
+          buildSettings(buildOptions, optionInfoMap, starlarkTransition);
+
+      ImmutableList.Builder<BuildOptions> splitBuildOptions = ImmutableList.builder();
+
+      ImmutableList<Map<String, Object>> transitions =
+          starlarkTransition.getChangedSettings(settings, attrObject);
+      // TODO(juliexxia): Validate that the output values correctly match the output types.
+      validateFunctionOutputs(transitions, starlarkTransition);
+
+      for (Map<String, Object> transition : transitions) {
+        BuildOptions options = buildOptions.clone();
+        applyTransition(options, transition, optionInfoMap, starlarkTransition);
+        splitBuildOptions.add(options);
+      }
+      return splitBuildOptions.build();
+
+    } catch (InterruptedException | EvalException e) {
+      // TODO(juliexxia): Throw an exception better than RuntimeException.
+      throw new RuntimeException(e);
+    }
+  }
+
+  private static void validateFunctionOutputs(
+      ImmutableList<Map<String, Object>> transitions,
+      StarlarkDefinedConfigTransition starlarkTransition)
+      throws EvalException {
+    for (Map<String, Object> transition : transitions) {
+      LinkedHashSet<String> remainingOutputs =
+          Sets.newLinkedHashSet(starlarkTransition.getOutputs());
+      for (String outputKey : transition.keySet()) {
+        if (!remainingOutputs.remove(outputKey)) {
+          throw new EvalException(
+              starlarkTransition.getLocationForErrorReporting(),
+              String.format("transition function returned undeclared output '%s'", outputKey));
+        }
+      }
+
+      if (!remainingOutputs.isEmpty()) {
+        throw new EvalException(
+            starlarkTransition.getLocationForErrorReporting(),
+            String.format(
+                "transition outputs [%s] were not defined by transition function",
+                Joiner.on(", ").join(remainingOutputs)));
+      }
+    }
+  }
+
+  /**
+   * Given a label-like string representing a command line option, returns the command line option
+   * string that it represents. This is a temporary measure to support command line options with
+   * strings that look "label-like", so that migrating users using this experimental syntax is
+   * easier later.
+   *
+   * @throws EvalException if the given string is not a valid format to represent to a command line
+   *     option
+   */
+  private static String commandLineOptionLabelToOption(
+      String label, StarlarkDefinedConfigTransition starlarkTransition) throws EvalException {
+    if (label.startsWith(COMMAND_LINE_OPTION_PREFIX)) {
+      return label.substring(COMMAND_LINE_OPTION_PREFIX.length());
+    } else {
+      throw new EvalException(
+          starlarkTransition.getLocationForErrorReporting(),
+          String.format(
+              "Option key '%s' is of invalid form. "
+                  + "Expected command line option to begin with %s",
+              label, COMMAND_LINE_OPTION_PREFIX));
+    }
+  }
+
+  /** For all the options in the BuildOptions, build a map from option name to its information. */
+  private static Map<String, OptionInfo> buildOptionInfo(BuildOptions buildOptions) {
+    ImmutableMap.Builder<String, OptionInfo> builder = new ImmutableMap.Builder<>();
+
+    ImmutableSet<Class<? extends FragmentOptions>> optionClasses =
+        buildOptions.getNativeOptions().stream()
+            .map(FragmentOptions::getClass)
+            .collect(ImmutableSet.toImmutableSet());
+
+    for (Class<? extends FragmentOptions> optionClass : optionClasses) {
+      ImmutableList<OptionDefinition> optionDefinitions =
+          OptionsParser.getOptionDefinitions(optionClass);
+      for (OptionDefinition def : optionDefinitions) {
+        String optionName = def.getOptionName();
+        builder.put(optionName, new OptionInfo(optionClass, def));
+      }
+    }
+
+    return builder.build();
+  }
+
+  /**
+   * Enter the options in buildOptions into a skylark dictionary, and return the dictionary.
+   *
+   * @throws IllegalArgumentException If the method is unable to look up the value in buildOptions
+   *     corresponding to an entry in optionInfoMap
+   * @throws RuntimeException If the field corresponding to an option value in buildOptions is
+   *     inaccessible due to Java language access control, or if an option name is an invalid key to
+   *     the Skylark dictionary
+   * @throws EvalException if any of the specified transition inputs do not correspond to a valid
+   *     build setting
+   */
+  static SkylarkDict<String, Object> buildSettings(
+      BuildOptions buildOptions,
+      Map<String, OptionInfo> optionInfoMap,
+      StarlarkDefinedConfigTransition starlarkTransition)
+      throws EvalException {
+    LinkedHashSet<String> remainingInputs = Sets.newLinkedHashSet(starlarkTransition.getInputs());
+
+    try (Mutability mutability = Mutability.create("build_settings")) {
+      SkylarkDict<String, Object> dict = SkylarkDict.withMutability(mutability);
+
+      for (Map.Entry<String, OptionInfo> entry : optionInfoMap.entrySet()) {
+        String optionName = entry.getKey();
+        String optionKey = COMMAND_LINE_OPTION_PREFIX + optionName;
+
+        if (!remainingInputs.remove(optionKey)) {
+          // This option was not present in inputs. Skip it.
+          continue;
+        }
+        OptionInfo optionInfo = entry.getValue();
+
+        try {
+          Field field = optionInfo.getDefinition().getField();
+          FragmentOptions options = buildOptions.get(optionInfo.getOptionClass());
+          Object optionValue = field.get(options);
+
+          dict.put(optionKey, optionValue, null, mutability);
+        } catch (IllegalAccessException e) {
+          // These exceptions should not happen, but if they do, throw a RuntimeException.
+          throw new RuntimeException(e);
+        }
+      }
+
+      if (!remainingInputs.isEmpty()) {
+        throw new EvalException(
+            starlarkTransition.getLocationForErrorReporting(),
+            String.format(
+                "transition inputs [%s] do not correspond to valid settings",
+                Joiner.on(", ").join(remainingInputs)));
+      }
+
+      return dict;
+    }
+  }
+
+  /**
+   * Apply the transition dictionary to the build option, using optionInfoMap to look up the option
+   * info.
+   *
+   * @param buildOptions the pre-transition build options
+   * @param newValues a map of option name: option value entries to override current option
+   * values in the buildOptions param
+   * @param optionInfoMap a map of option name: option info for all native options that may
+   * be accessed in this transition
+   * @param starlarkTransition transition object that is being applied. Used for error reporting
+   * and checking for analysis testing
+   * @throws EvalException If a requested option field is inaccessible
+   */
+  static void applyTransition(
+      BuildOptions buildOptions,
+      Map<String, Object> newValues,
+      Map<String, OptionInfo> optionInfoMap,
+      StarlarkDefinedConfigTransition starlarkTransition)
+      throws EvalException {
+    for (Map.Entry<String, Object> entry : newValues.entrySet()) {
+      String optionKey = entry.getKey();
+
+      // TODO(juliexxia): Handle keys which correspond to build_setting target labels instead
+      // of assuming every key is for a command line option.
+      String optionName = commandLineOptionLabelToOption(optionKey, starlarkTransition);
+      Object optionValue = entry.getValue();
+
+      // Convert NoneType to null.
+      if (optionValue instanceof NoneType) {
+        optionValue = null;
+      }
+
+      try {
+        if (!optionInfoMap.containsKey(optionName)) {
+          throw new EvalException(
+              starlarkTransition.getLocationForErrorReporting(),
+              String.format(
+                  "transition output '%s' does not correspond to a valid setting", optionKey));
+        }
+
+        OptionInfo optionInfo = optionInfoMap.get(optionName);
+        OptionDefinition def = optionInfo.getDefinition();
+        Field field = def.getField();
+        FragmentOptions options = buildOptions.get(optionInfo.getOptionClass());
+        if (optionValue == null || def.getType().isInstance(optionValue)) {
+          field.set(options, optionValue);
+        } else if (optionValue instanceof String) {
+          field.set(options, def.getConverter().convert((String) optionValue));
+        } else {
+          throw new EvalException(
+              starlarkTransition.getLocationForErrorReporting(),
+              "Invalid value type for option '" + optionName + "'");
+        }
+      } catch (IllegalAccessException e) {
+        throw new RuntimeException(
+            "IllegalAccess for option " + optionName + ": " + e.getMessage());
+      } catch (OptionsParsingException e) {
+        throw new EvalException(
+            starlarkTransition.getLocationForErrorReporting(),
+            "OptionsParsingError for option '" + optionName + "': " + e.getMessage());
+      }
+    }
+
+    BuildConfiguration.Options buildConfigOptions;
+    buildConfigOptions = buildOptions.get(BuildConfiguration.Options.class);
+
+    if (starlarkTransition.isForAnalysisTesting()) {
+      buildConfigOptions.evaluatingForAnalysisTest = true;
+    }
+    updateOutputDirectoryNameFragment(buildConfigOptions, newValues);
+  }
+
+  /**
+   * Compute the output directory name fragment corresponding to the transition, and append it to
+   * the existing name fragment in buildConfigOptions.
+   *
+   * @throws IllegalStateException If MD5 support is not available
+   */
+  private static void updateOutputDirectoryNameFragment(
+      BuildConfiguration.Options buildConfigOptions, Map<String, Object> transition) {
+    String transitionString = "";
+    for (Map.Entry<String, Object> entry : transition.entrySet()) {
+      transitionString += entry.getKey() + ":";
+      if (entry.getValue() != null) {
+        transitionString += entry.getValue() + "@";
+      }
+    }
+
+    // TODO(waltl): for transitions that don't read settings, it is possible to precompute and
+    // reuse the MD5 digest and even the transition itself.
+    try {
+      byte[] bytes = transitionString.getBytes(US_ASCII);
+      MessageDigest md = MessageDigest.getInstance("MD5");
+      byte[] digest = md.digest(bytes);
+      String hexDigest = BaseEncoding.base16().lowerCase().encode(digest);
+
+      if (buildConfigOptions.transitionDirectoryNameFragment == null) {
+        buildConfigOptions.transitionDirectoryNameFragment = hexDigest;
+      } else {
+        buildConfigOptions.transitionDirectoryNameFragment += "-" + hexDigest;
+      }
+    } catch (NoSuchAlgorithmException e) {
+      throw new IllegalStateException("MD5 not available", e);
+    }
+  }
+
+  /** Stores option info useful to a FunctionSplitTransition. */
+  static class OptionInfo {
+    private final Class<? extends FragmentOptions> optionClass;
+    private final OptionDefinition definition;
+
+    public OptionInfo(Class<? extends FragmentOptions> optionClass, OptionDefinition definition) {
+      this.optionClass = optionClass;
+      this.definition = definition;
+    }
+
+    Class<? extends FragmentOptions> getOptionClass() {
+      return optionClass;
+    }
+
+    OptionDefinition getDefinition() {
+      return definition;
+    }
+  }
+}
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkAttr.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkAttr.java
index 225b803..68c43bd 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkAttr.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkAttr.java
@@ -273,10 +273,11 @@
         } else {
           builder.hasStarlarkDefinedTransition();
         }
-        builder.cfg(new FunctionSplitTransitionProvider(starlarkDefinedTransition));
+        builder.cfg(new StarlarkAttributeTransitionProvider(starlarkDefinedTransition));
       } else if (!trans.equals("target")) {
-        throw new EvalException(ast.getLocation(),
-            "cfg must be either 'data', 'host', or 'target'.");
+        // TODO(b/121134880): update error message when starlark build configurations is ready.
+        throw new EvalException(
+            ast.getLocation(), "cfg must be either 'data', 'host', or 'target'.");
       }
     }
 
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkRuleClassFunctions.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkRuleClassFunctions.java
index c9931dc8..d1bd717 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkRuleClassFunctions.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkRuleClassFunctions.java
@@ -37,6 +37,7 @@
 import com.google.devtools.build.lib.analysis.TemplateVariableInfo;
 import com.google.devtools.build.lib.analysis.config.ConfigAwareRuleClassBuilder;
 import com.google.devtools.build.lib.analysis.config.HostTransition;
+import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
 import com.google.devtools.build.lib.analysis.skylark.SkylarkAttr.Descriptor;
 import com.google.devtools.build.lib.analysis.test.TestConfiguration;
 import com.google.devtools.build.lib.cmdline.Label;
@@ -276,6 +277,7 @@
       SkylarkList<?> execCompatibleWith,
       Object analysisTest,
       Object buildSetting,
+      Object cfg,
       FuncallExpression ast,
       Environment funcallEnv,
       StarlarkContext context)
@@ -349,6 +351,13 @@
     builder.addRequiredToolchains(
         collectToolchainLabels(
             toolchains.getContents(String.class, "toolchains"), ast.getLocation()));
+
+    if (!buildSetting.equals(Runtime.NONE) && !cfg.equals(Runtime.NONE)) {
+      throw new EvalException(
+          ast.getLocation(),
+          "Build setting rules cannot use the `cfg` param to apply transitions to themselves.");
+    }
+    // TODO(juliexxia): use @Param.enableOnlyWithFlag for this.
     if (!buildSetting.equals(Runtime.NONE)) {
       if (funcallEnv.getSemantics().experimentalBuildSettingApi()) {
         builder.setBuildSetting((BuildSetting) buildSetting);
@@ -360,6 +369,16 @@
                 + "specifying --experimental_build_setting_api");
       }
     }
+    if (!cfg.equals(Runtime.NONE)) {
+      if (!(cfg instanceof StarlarkDefinedConfigTransition)) {
+        throw new EvalException(
+            ast.getLocation(),
+            "`cfg` must be set to a transition object initialized by the transition() function.");
+      }
+      StarlarkDefinedConfigTransition starlarkDefinedConfigTransition =
+          (StarlarkDefinedConfigTransition) cfg;
+      builder.cfg(new StarlarkRuleTransitionProvider(starlarkDefinedConfigTransition));
+    }
 
     for (Object o : providesArg) {
       if (!SkylarkAttr.isProvider(o)) {
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java
new file mode 100644
index 0000000..ae86e86
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java
@@ -0,0 +1,86 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.devtools.build.lib.analysis.skylark;
+
+import static com.google.devtools.build.lib.analysis.skylark.FunctionTransitionUtil.applyAndValidate;
+import static com.google.devtools.build.lib.analysis.skylark.SkylarkAttributesCollection.ERROR_MESSAGE_FOR_NO_ATTR;
+
+import com.google.common.base.Preconditions;
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
+import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
+import com.google.devtools.build.lib.packages.Attribute;
+import com.google.devtools.build.lib.packages.Attribute.SplitTransitionProvider;
+import com.google.devtools.build.lib.packages.AttributeMap;
+import com.google.devtools.build.lib.packages.ConfiguredAttributeMapper;
+import com.google.devtools.build.lib.packages.StructImpl;
+import com.google.devtools.build.lib.packages.StructProvider;
+import com.google.devtools.build.lib.syntax.Environment;
+import com.google.devtools.build.lib.syntax.Runtime;
+import com.google.devtools.build.lib.syntax.SkylarkType;
+import java.util.LinkedHashMap;
+import java.util.List;
+
+/**
+ * This class implements a {@link SplitTransitionProvider} to provide a starlark-defined transition
+ * that rules can apply to their dependencies' configurations. This transition has access to (1) the
+ * a map of the current configuration's build settings and (2) the configured attributes of the
+ * given rule (not its dependencies').
+ *
+ * <p>For starlark defined rule class transitions, see {@link StarlarkRuleTransitionProvider}.
+ *
+ * <p>TODO(bazel-team): Consider allowing dependency-typed attributes to actually return providers
+ * instead of just labels (see {@link SkylarkAttributesCollection#addAttribute}).
+ */
+public class StarlarkAttributeTransitionProvider implements SplitTransitionProvider {
+  private final StarlarkDefinedConfigTransition starlarkDefinedConfigTransition;
+
+  StarlarkAttributeTransitionProvider(
+      StarlarkDefinedConfigTransition starlarkDefinedConfigTransition) {
+    this.starlarkDefinedConfigTransition = starlarkDefinedConfigTransition;
+  }
+
+  @Override
+  public SplitTransition apply(AttributeMap attributeMap) {
+    Preconditions.checkArgument(attributeMap instanceof ConfiguredAttributeMapper);
+    return new FunctionSplitTransition(
+        starlarkDefinedConfigTransition, (ConfiguredAttributeMapper) attributeMap);
+  }
+
+  private static class FunctionSplitTransition implements SplitTransition {
+    private final StarlarkDefinedConfigTransition starlarkDefinedConfigTransition;
+    private final StructImpl attrObject;
+
+    FunctionSplitTransition(
+        StarlarkDefinedConfigTransition starlarkDefinedConfigTransition,
+        ConfiguredAttributeMapper attributeMap) {
+      this.starlarkDefinedConfigTransition = starlarkDefinedConfigTransition;
+
+      LinkedHashMap<String, Object> attributes = new LinkedHashMap<>();
+      for (String attribute : attributeMap.getAttributeNames()) {
+        Object val = attributeMap.get(attribute, attributeMap.getAttributeType(attribute));
+        attributes.put(
+            Attribute.getSkylarkName(attribute),
+            val == null ? Runtime.NONE : SkylarkType.convertToSkylark(val, (Environment) null));
+      }
+      attrObject = StructProvider.STRUCT.create(attributes, ERROR_MESSAGE_FOR_NO_ATTR);
+    }
+
+    @Override
+    public final List<BuildOptions> split(BuildOptions buildOptions) {
+      return applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject);
+    }
+  }
+}
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java
new file mode 100644
index 0000000..cc86ba3
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java
@@ -0,0 +1,105 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.devtools.build.lib.analysis.skylark;
+
+import static com.google.devtools.build.lib.analysis.skylark.FunctionTransitionUtil.applyAndValidate;
+
+import com.google.common.collect.Iterables;
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
+import com.google.devtools.build.lib.analysis.config.transitions.PatchTransition;
+import com.google.devtools.build.lib.packages.Attribute;
+import com.google.devtools.build.lib.packages.BuildType;
+import com.google.devtools.build.lib.packages.RawAttributeMapper;
+import com.google.devtools.build.lib.packages.Rule;
+import com.google.devtools.build.lib.packages.RuleTransitionFactory;
+import com.google.devtools.build.lib.packages.StructImpl;
+import com.google.devtools.build.lib.packages.StructProvider;
+import com.google.devtools.build.lib.syntax.Environment;
+import com.google.devtools.build.lib.syntax.Runtime;
+import com.google.devtools.build.lib.syntax.SkylarkType;
+import java.util.LinkedHashMap;
+import java.util.List;
+
+/**
+ * This class implements {@link RuleTransitionFactory} to provide a starlark-defined transition that
+ * rules can apply to their own configuration. This transition has access to (1) the a map of the
+ * current configuration's build settings and (2) the configured* attributes of the given rule (not
+ * its dependencies').
+ *
+ * <p>*In some corner cases, we can't access the configured attributes the configuration of the
+ * child may be different than the configuration of the parent. For now, forbid all access to
+ * attributes that read selects.
+ *
+ * <p>For starlark-defined attribute transitions, see {@link StarlarkAttributeTransitionProvider}.
+ */
+public class StarlarkRuleTransitionProvider implements RuleTransitionFactory {
+
+  private final StarlarkDefinedConfigTransition starlarkDefinedConfigTransition;
+
+  StarlarkRuleTransitionProvider(StarlarkDefinedConfigTransition starlarkDefinedConfigTransition) {
+    this.starlarkDefinedConfigTransition = starlarkDefinedConfigTransition;
+  }
+
+  @Override
+  public PatchTransition buildTransitionFor(Rule rule) {
+    return new FunctionPatchTransition(starlarkDefinedConfigTransition, rule);
+  }
+
+  private static class FunctionPatchTransition implements PatchTransition {
+
+    private final StarlarkDefinedConfigTransition starlarkDefinedConfigTransition;
+    private final StructImpl attrObject;
+
+    FunctionPatchTransition(
+        StarlarkDefinedConfigTransition starlarkDefinedConfigTransition, Rule rule) {
+      this.starlarkDefinedConfigTransition = starlarkDefinedConfigTransition;
+
+      LinkedHashMap<String, Object> attributes = new LinkedHashMap<>();
+      RawAttributeMapper attributeMapper = RawAttributeMapper.of(rule);
+      for (Attribute attribute : rule.getAttributes()) {
+        Object val = attributeMapper.getRawAttributeValue(rule, attribute);
+        if (val instanceof BuildType.SelectorList) {
+          // For now, don't allow access to attributes that read selects.
+          // TODO(b/121134880): make this restriction more fine grained.
+          continue;
+        }
+        attributes.put(
+            Attribute.getSkylarkName(attribute.getPublicName()),
+            val == null ? Runtime.NONE : SkylarkType.convertToSkylark(val, (Environment) null));
+      }
+      attrObject =
+          StructProvider.STRUCT.create(
+              attributes,
+              "No attribute '%s'. Either this attribute does "
+                  + "not exist for this rule or is set by a select. Starlark rule transitions "
+                  + "currently cannot read attributes behind selects.");
+    }
+
+    // TODO(b/121134880): validate that the targets these transitions are applied on don't read any
+    // attributes that are then configured by the outputs of these transitions.
+    @Override
+    public BuildOptions patch(BuildOptions buildOptions) {
+      List<BuildOptions> result =
+          applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject);
+      if (result.size() != 1) {
+        // TODO(b/121134880): handle this with a better (checked) exception)
+        throw new RuntimeException(
+            "Rule transition only allowed to return a single transitioned configuration.");
+      }
+      return Iterables.getOnlyElement(result);
+    }
+  }
+}
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleTransitionFactory.java b/src/main/java/com/google/devtools/build/lib/packages/RuleTransitionFactory.java
index 2445ddb..f6019133 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleTransitionFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleTransitionFactory.java
@@ -23,15 +23,16 @@
   /**
    * Generates a transition to be used when entering the given rule.
    *
-   * <p>This cannot be a
-   * {@link com.google.devtools.build.lib.analysis.config.transitions.SplitTransition} because
-   * splits are conceptually a property of the <i>parent<i> rule. In other words, it makes sense for
-   * a parent to say "build my deps in configurations A and B". But it doesn't make sense for a dep
-   * to say "build myself in configurations A and B" if its parent doesn't know how to intelligently
-   * handle the results.
+   * <p>This cannot be a {@link
+   * com.google.devtools.build.lib.analysis.config.transitions.SplitTransition} because splits are
+   * conceptually a property of the <i>parent<i> rule. In other words, it makes sense for a parent
+   * to say "build my deps in configurations A and B". But it doesn't make sense for a dep to say
+   * "build myself in configurations A and B" if its parent doesn't know how to intelligently handle
+   * the results.
    *
-   * <p>If this class determines that no transition should be performed, it should return
-   * {@code NoTransition.INSTANCE}.
+   * <p>If this class determines that no transition should be performed, it should return {@code
+   * NoTransition.INSTANCE}.
    */
+  // TODO(bazel-team): refactor to only take an AttributeMap since that's how it's used anyway.
   PatchTransition buildTransitionFor(Rule rule);
 }
diff --git a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleFunctionsApi.java b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleFunctionsApi.java
index 18543da..c5f3f7b 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleFunctionsApi.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleFunctionsApi.java
@@ -31,6 +31,7 @@
 import com.google.devtools.build.lib.syntax.Runtime.UnboundMarker;
 import com.google.devtools.build.lib.syntax.SkylarkDict;
 import com.google.devtools.build.lib.syntax.SkylarkList;
+import com.google.devtools.build.lib.syntax.SkylarkSemantics.FlagIdentifier;
 
 /**
  * Interface for a global Skylark library containing rule-related helper and registration functions.
@@ -328,6 +329,18 @@
                     + "See the <a href='config.html'><code>config</code></a> module. If this is "
                     + "set, a mandatory attribute named \"build_setting_default\" is automatically"
                     + "added to this rule, with a type corresponding to the value passed in here."),
+        @Param(
+            name = "cfg",
+            type = Object.class,
+            noneable = true,
+            defaultValue = "None",
+            named = true,
+            positional = false,
+            enableOnlyWithFlag = FlagIdentifier.EXPERIMENTAL_STARLARK_CONFIG_TRANSITION,
+            valueWhenDisabled = "None",
+            doc =
+                "If set, points to the configuration transition the rule will "
+                    + "apply to its own configuration before analysis.")
       },
       useAst = true,
       useEnvironment = true,
@@ -349,6 +362,7 @@
       SkylarkList<?> execCompatibleWith,
       Object analysisTest,
       Object buildSetting,
+      Object cfg,
       FuncallExpression ast,
       Environment funcallEnv,
       StarlarkContext context)
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
index 492ceb8..413c818 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
@@ -43,6 +43,8 @@
         SkylarkSemantics::experimentalEnableAndroidMigrationApis),
     EXPERIMENTAL_BUILD_SETTING_API(SkylarkSemantics::experimentalBuildSettingApi),
     EXPERIMENTAL_PLATFORM_API(SkylarkSemantics::experimentalPlatformsApi),
+    EXPERIMENTAL_STARLARK_CONFIG_TRANSITION(
+        SkylarkSemantics::experimentalStarlarkConfigTransitions),
     INCOMPATIBLE_DISABLE_OBJC_PROVIDER_RESOURCES(
         SkylarkSemantics::incompatibleDisableObjcProviderResources),
     INCOMPATIBLE_NO_OUTPUT_ATTR_DEFAULT(SkylarkSemantics::incompatibleNoOutputAttrDefault),
diff --git a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
index 7e76957..90260c1 100644
--- a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
+++ b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
@@ -119,6 +119,7 @@
       SkylarkList<?> execCompatibleWith,
       Object analysisTest,
       Object buildSetting,
+      Object cfg,
       FuncallExpression ast,
       Environment funcallEnv,
       StarlarkContext context)
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/FunctionSplitTransitionProviderTest.java b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java
similarity index 91%
rename from src/test/java/com/google/devtools/build/lib/analysis/FunctionSplitTransitionProviderTest.java
rename to src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java
index 6aab61a..803d595 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/FunctionSplitTransitionProviderTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java
@@ -18,9 +18,7 @@
 import static org.junit.Assert.fail;
 
 import com.google.common.collect.ArrayListMultimap;
-import com.google.common.collect.Iterables;
 import com.google.common.collect.ListMultimap;
-import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
 import com.google.devtools.build.lib.analysis.util.BuildViewTestCase;
 import com.google.devtools.build.lib.packages.util.BazelMockAndroidSupport;
 import java.util.List;
@@ -29,11 +27,9 @@
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
-/**
- * Tests for FunctionSplitTransitionProvider.
- */
+/** Tests for StarlarkAttributeTransitionProvider. */
 @RunWith(JUnit4.class)
-public class FunctionSplitTransitionProviderTest extends BuildViewTestCase {
+public class StarlarkAttrTransitionProviderTest extends BuildViewTestCase {
 
   private void writeWhitelistFile() throws Exception {
     scratch.file(
@@ -85,63 +81,6 @@
   }
 
   @Test
-  @SuppressWarnings("unchecked")
-  public void testParameterizedTransition() throws Exception {
-    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
-
-    writeWhitelistFile();
-    scratch.file(
-        "test/skylark/my_rule.bzl",
-        "def _transition_impl(settings, attr):",
-        "  if (attr.big_foot_is_real):",
-        "    return {'t0': {'//command_line_option:cpu': 'x86_64'}}",
-        "  else:",
-        "    return {'t0': {'//command_line_option:cpu': 'armeabi-v7a'}}",
-        "",
-        "my_transition = transition(implementation = _transition_impl, inputs = [],",
-        "  outputs = ['//command_line_option:cpu'])",
-        "def _my_rule_impl(ctx): ",
-        "  return struct(",
-        "    split_attr_dep = ctx.split_attr.dep,",
-        "    attr_dep = ctx.attr.dep)",
-        "my_rule = rule(",
-        "  implementation = _my_rule_impl,",
-        "  attrs = {",
-        "    'big_foot_is_real': attr.bool(default = False),",
-        "    'dep':  attr.label(cfg = my_transition),",
-        "    '_whitelist_function_transition': attr.label(",
-        "        default = '//tools/whitelists/function_transition_whitelist',",
-        "    ),",
-        "  })",
-        "def _simple_impl(ctx):",
-        "  return []",
-        "simple_rule = rule(implementation = _simple_impl)");
-
-    scratch.file(
-        "test/skylark/BUILD",
-        "load('//test/skylark:my_rule.bzl', 'my_rule', 'simple_rule')",
-        "my_rule(name = 'lies', dep = ':dep')",
-        "my_rule(name = 'the-truth', dep = ':dep', big_foot_is_real = True)",
-        "simple_rule(name = 'dep')");
-
-    useConfiguration("--cpu=k8");
-
-    BuildConfiguration liesDepConfiguration =
-        getConfiguration(
-            Iterables.getOnlyElement(
-                (List<ConfiguredTarget>)
-                    getConfiguredTarget("//test/skylark:lies").get("attr_dep")));
-    assertThat(liesDepConfiguration.getCpu()).isEqualTo("armeabi-v7a");
-
-    BuildConfiguration theTruthDepConfiguration =
-        getConfiguration(
-            Iterables.getOnlyElement(
-                (List<ConfiguredTarget>)
-                    getConfiguredTarget("//test/skylark:the-truth").get("attr_dep")));
-    assertThat(theTruthDepConfiguration.getCpu()).isEqualTo("x86_64");
-  }
-
-  @Test
   public void testFunctionSplitTransitionCheckSplitAttrDeps() throws Exception {
     writeBasicTestFiles();
     testSplitTransitionCheckSplitAttrDeps(getConfiguredTarget("//test/skylark:test"));
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
new file mode 100644
index 0000000..b77875e
--- /dev/null
+++ b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
@@ -0,0 +1,273 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.analysis;
+
+import static com.google.common.truth.Truth.assertThat;
+import static junit.framework.TestCase.fail;
+
+import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
+import com.google.devtools.build.lib.analysis.test.TestConfiguration.TestOptions;
+import com.google.devtools.build.lib.analysis.util.BuildViewTestCase;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for StarlarkRuleTransitionProvider. */
+@RunWith(JUnit4.class)
+public class StarlarkRuleTransitionProviderTest extends BuildViewTestCase {
+
+  @Test
+  public void testOutputOnlyTransition() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  return {'//command_line_option:test_arg': ['post-transition']}",
+        "my_transition = transition(implementation = _impl, inputs = [],",
+        "  outputs = ['//command_line_option:test_arg'])");
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(implementation = _impl, cfg = my_transition)");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    useConfiguration("--test_arg=pre-transition");
+
+    BuildConfiguration configuration = getConfiguration(getConfiguredTarget("//test"));
+    assertThat(configuration.getOptions().get(TestOptions.class).testArguments)
+        .containsExactly("post-transition");
+  }
+
+  @Test
+  public void testInputAndOutputTransition() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  return {'//command_line_option:test_arg': ",
+        "    [settings['//command_line_option:test_arg'][0]+'->post-transition']}",
+        "my_transition = transition(",
+        "  implementation = _impl,",
+        "  inputs = ['//command_line_option:test_arg'],",
+        "  outputs = ['//command_line_option:test_arg'],",
+        ")");
+
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(implementation = _impl, cfg = my_transition)");
+
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    useConfiguration("--test_arg=pre-transition");
+
+    BuildConfiguration configuration = getConfiguration(getConfiguredTarget("//test"));
+    assertThat(configuration.getOptions().get(TestOptions.class).testArguments)
+        .containsExactly("pre-transition->post-transition");
+  }
+
+  @Test
+  public void testBuildSettingCannotTransition() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  return {'//command_line_option:test_arg': ['post-transition']}",
+        "my_transition = transition(implementation = _impl, inputs = [],",
+        "  outputs = ['//command_line_option:test_arg'])");
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _impl,",
+        "  cfg = my_transition,",
+        "  build_setting = config.string()",
+        ")");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    reporter.removeHandler(failFastHandler);
+    getConfiguredTarget("//test");
+    assertContainsEvent(
+        "Build setting rules cannot use the `cfg` param to apply transitions to themselves");
+  }
+
+  @Test
+  public void testBadCfgInput() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/rules.bzl",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _impl,",
+        "  cfg = 'my_transition',",
+        ")");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    reporter.removeHandler(failFastHandler);
+    getConfiguredTarget("//test");
+    assertContainsEvent(
+        "`cfg` must be set to a transition object initialized by the transition() function.");
+  }
+
+  @Test
+  public void testMultipleReturnConfigs() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  return {",
+        "      't0': {'//command_line_option:test_arg': ['split_one']},",
+        "      't1': {'//command_line_option:test_arg': ['split_two']},",
+        "  }",
+        "my_transition = transition(implementation = _impl, inputs = [],",
+        "  outputs = ['//command_line_option:test_arg'])");
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(implementation = _impl, cfg = my_transition)");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    try {
+      getConfiguredTarget("//test");
+      fail("Should have failed");
+    } catch (RuntimeException e) {
+      assertThat(e)
+          .hasMessageThat()
+          .contains("Rule transition only allowed to return a single transitioned configuration.");
+    }
+  }
+
+  @Test
+  public void testCanDoBadStuffWithParameterizedTransitionsAndSelects() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  if (attr.my_configurable_attr):",
+        "    return {'//command_line_option:test_arg': ['true']}",
+        "  else:",
+        "    return {'//command_line_option:test_arg': ['false']}",
+        "my_transition = transition(implementation = _impl, inputs = [],",
+        "  outputs = ['//command_line_option:test_arg'])");
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _impl,",
+        "  cfg = my_transition,",
+        "  attrs = {'my_configurable_attr': attr.bool(default = False)},",
+        ")");
+    scratch.file(
+        "test/BUILD",
+        "load('//test:rules.bzl', 'my_rule')",
+        "my_rule(",
+        "  name = 'test',",
+        "  my_configurable_attr = select({",
+        "    '//conditions:default': False,",
+        "    ':true-config': True,",
+        "  })",
+        ")",
+        "config_setting(",
+        "  name = 'true-config',",
+        "  values = {'test_arg': 'true'},",
+        ")");
+
+    try {
+      getConfiguredTarget("//test");
+      fail("Should have failed");
+    } catch (RuntimeException e) {
+      assertThat(e)
+          .hasMessageThat()
+          .contains(
+              "No attribute 'my_configurable_attr'. "
+                  + "Either this attribute does not exist for this rule or is set by a select. "
+                  + "Starlark rule transitions currently cannot read attributes behind selects.");
+    }
+  }
+
+  @Test
+  public void testLabelTypedAttrReturnsLabelNotDep() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=true");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  if attr.dict_attr[Label('//test:key')] == 'value':",
+        "    return {'//command_line_option:test_arg': ['post-transition']}",
+        "  else:",
+        "    return {'//command_line_option:test_arg': ['uh-oh']}",
+        "my_transition = transition(implementation = _impl, inputs = [],",
+        "  outputs = ['//command_line_option:test_arg'])");
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _impl,",
+        "  cfg = my_transition,",
+        "  attrs = {'dict_attr': attr.label_keyed_string_dict()},",
+        ")",
+        "simple_rule = rule(_impl)");
+    scratch.file(
+        "test/BUILD",
+        "load('//test:rules.bzl', 'my_rule', 'simple_rule')",
+        "my_rule(",
+        "  name = 'test',",
+        "  dict_attr = {':key': 'value'},",
+        ")",
+        "simple_rule(name = 'key')");
+
+    useConfiguration("--test_arg=pre-transition");
+
+    BuildConfiguration configuration = getConfiguration(getConfiguredTarget("//test"));
+    assertThat(configuration.getOptions().get(TestOptions.class).testArguments)
+        .containsExactly("post-transition");
+  }
+
+  @Test
+  public void testCannotTransitionWithoutFlag() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_starlark_config_transitions=false");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _impl(settings, attr):",
+        "  return {'//command_line_option:test_arg': ['post-transition']}",
+        "my_transition = transition(implementation = _impl, inputs = [],",
+        "  outputs = ['//command_line_option:test_arg'])");
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _impl(ctx):",
+        "  return []",
+        "my_rule = rule(implementation = _impl, cfg = my_transition)");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    reporter.removeHandler(failFastHandler);
+    getConfiguredTarget("//test");
+    assertContainsEvent(
+        "transition() is experimental and disabled by default. This API is in "
+            + "development and subject to change at any time. Use "
+            + "--experimental_starlark_config_transitions to use this experimental API.");
+  }
+}