Add --host_swiftcopt

This allows you to pass arguments to swiftc host targets.

Closes #6921.

PiperOrigin-RevId: 228162636

src/main/java/com/google/devtools/build/lib/rules/apple/swift/SwiftCommandLineOptions.java

diff --git a/src/main/java/com/google/devtools/build/lib/rules/apple/swift/SwiftCommandLineOptions.java b/src/main/java/com/google/devtools/build/lib/rules/apple/swift/SwiftCommandLineOptions.java
index 8c4e28d..bf81f91 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/apple/swift/SwiftCommandLineOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/apple/swift/SwiftCommandLineOptions.java
@@ -31,4 +31,20 @@
     help = "Additional options to pass to Swift compilation."
   )
   public List<String> copts;
+
+  @Option(
+      name = "host_swiftcopt",
+      allowMultiple = true,
+      defaultValue = "",
+      documentationCategory = OptionDocumentationCategory.OUTPUT_PARAMETERS,
+      effectTags = {OptionEffectTag.ACTION_COMMAND_LINES, OptionEffectTag.AFFECTS_OUTPUTS},
+      help = "Additional options to pass to swiftc for host tools.")
+  public List<String> hostSwiftcoptList;
+
+  @Override
+  public FragmentOptions getHost() {
+    SwiftCommandLineOptions host = (SwiftCommandLineOptions) super.getHost();
+    host.copts = this.hostSwiftcoptList;
+    return host;
+  }
 }

src/test/java/com/google/devtools/build/lib/rules/apple/SwiftConfigurationTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/rules/apple/SwiftConfigurationTest.java b/src/test/java/com/google/devtools/build/lib/rules/apple/SwiftConfigurationTest.java
index 0a5f807..c0d31e0 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/apple/SwiftConfigurationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/apple/SwiftConfigurationTest.java
@@ -58,4 +58,37 @@
 
     assertThat(copts).containsAllOf("foo", "bar");
   }
+
+  @Test
+  public void testHostSwiftcopt() throws Exception {
+    scratch.file("examples/rule/BUILD");
+    scratch.file(
+        "examples/rule/apple_rules.bzl",
+        "def swift_binary_impl(ctx):",
+        "   copts = ctx.fragments.swift.copts()",
+        "   return struct(",
+        "      copts=copts,",
+        "   )",
+        "swift_binary = rule(",
+        "   implementation = swift_binary_impl,",
+        "   fragments = ['swift']",
+        ")");
+
+    scratch.file(
+        "examples/swift_skylark/BUILD",
+        "load('//examples/rule:apple_rules.bzl', 'swift_binary')",
+        "swift_binary(",
+        "   name='my_target',",
+        ")");
+
+    useConfiguration("--swiftcopt=foo", "--host_swiftcopt=bar", "--host_swiftcopt=baz");
+    ConfiguredTarget target =
+        getConfiguredTarget("//examples/swift_skylark:my_target", getHostConfiguration());
+
+    @SuppressWarnings("unchecked")
+    List<String> copts = (List<String>) target.get("copts");
+
+    assertThat(copts).doesNotContain("foo");
+    assertThat(copts).containsAllOf("bar", "baz");
+  }
 }