Add alias_label struct field to all targets

src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java

@@ -39,6 +39,9 @@ public interface ConfiguredTarget extends TransitiveInfoCollection, Structure {
   /** All <code>ConfiguredTarget</code>s have a "label" field. */
   String LABEL_FIELD = "label";
 
+  /** All <code>ConfiguredTarget</code>s have an "alias_label" field. */
+  String ALIAS_LABEL_FIELD = "alias_label";
+
   /** All <code>ConfiguredTarget</code>s have a "files" field. */
   String FILES_FIELD = "files";
 

src/main/java/com/google/devtools/build/lib/analysis/configuredtargets/AbstractConfiguredTarget.java

@@ -66,6 +66,7 @@ public abstract class AbstractConfiguredTarget implements ConfiguredTarget, Visi
   // attributed to normal user-specified providers).
   private static final ImmutableSet<String> SPECIAL_FIELD_NAMES =
       ImmutableSet.of(
+          ALIAS_LABEL_FIELD,
           LABEL_FIELD,
           FILES_FIELD,
           DEFAULT_RUNFILES_FIELD,
@@ -141,6 +142,9 @@ public Object getValue(String name) {
     switch (name) {
       case LABEL_FIELD:
         return getLabel();
+      case ALIAS_LABEL_FIELD:
+        // Overridden in AliasConfiguredTarget.
+        return Starlark.NONE;
       case ACTIONS_FIELD_NAME:
         // Depending on subclass, the 'actions' field will either be unsupported or of type
         // java.util.List, which needs to be converted to Sequence before being returned.

src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java

@@ -163,15 +163,19 @@ public BuildConfigurationKey getConfigurationKey() {
 
   @Override
   public Object getValue(String name) {
-    if (name.equals(LABEL_FIELD)) {
-      return getLabel();
-    } else if (name.equals(FILES_FIELD)) {
-      // A shortcut for files to build in Starlark. FileConfiguredTarget and RuleConfiguredTarget
-      // always has FileProvider and Error- and PackageGroupConfiguredTarget-s shouldn't be
-      // accessible in Starlark.
-      return Depset.of(Artifact.class, getProvider(FileProvider.class).getFilesToBuild());
+    switch (name) {
+      case LABEL_FIELD:
+        return getLabel();
+      case ALIAS_LABEL_FIELD:
+        return getOriginalLabel();
+      case FILES_FIELD:
+        // A shortcut for files to build in Starlark. FileConfiguredTarget and RuleConfiguredTarget
+        // always has FileProvider and Error- and PackageGroupConfiguredTarget-s shouldn't be
+        // accessible in Starlark.
+        return Depset.of(Artifact.class, getProvider(FileProvider.class).getFilesToBuild());
+      default:
+        return actual.getValue(name);
     }
-    return actual.getValue(name);
   }
 
   @Override

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/core/TransitiveInfoCollectionApi.java

@@ -33,7 +33,14 @@
             + "<ul>\n" //
             + "<li><h3 id='modules.Target.label'>label</h3>\n" //
             + "<code><a href='../builtins/Label.html'>Label</a> Target.label</code><br/>\n" //
-            + "The identifier of the target.</li>\n" //
+            + "The label of the target after following all <code><a " //
+            + "href='general.html#alias>alias</a></code>es.</li>\n" //
+            //
+            + "<ul>\n" //
+            + "<li><h3 id='modules.Target.alias_label'>alias_label</h3>\n" //
+            + "<code><a href='../builtins/Label.html'>Label</a> Target.alias_label</code><br/>\n" //
+            + "The label of the target if it is an <code><a" //
+            + "href='general.html#alias>alias</a></code>, otherwise <code>None</code>.</li>\n" //
             //
             + "<li><h3 id='modules.Target.files'>files</h3>\n" //
             + "<code><a href='../builtins/depset.html'>depset</a> Target.files </code><br/>\n" //

src/test/java/com/google/devtools/build/lib/starlark/StarlarkRuleContextTest.java

@@ -4062,4 +4062,37 @@ public void testTemplateExpansionComputedSubstitutionMapEachMustBeTopLevel() thr
 
     checkError("//test:testing", "must be declared by a top-level def statement");
   }
+
+  @Test
+  public void testTargetAliasLabel() throws Exception {
+    scratch.file(
+        "test/rules.bzl",
+        "def _my_rule_impl(ctx):",
+        "  direct_dep = ctx.attr.deps[0]",
+        "  direct_dep.label == Label('//test:target') or fail()",
+        "  direct_dep.alias_label == None or fail()",
+        "  alias_dep = ctx.attr.deps[1]",
+        "  alias_dep.label == Label('//test:target') or fail()",
+        "  alias_dep.alias_label == Label('//test:alias') or fail()",
+        "my_rule = rule(",
+        "  implementation = _my_rule_impl,",
+        "  attrs = {'deps': attr.label_list()},",
+        ")");
+    scratch.file(
+        "test/BUILD",
+        "load(':rules.bzl', 'my_rule')",
+        "my_rule(",
+        "    name = 'my_rule',",
+        "    deps = [",
+        "        ':target',",
+        "        ':alias',",
+        "    ],",
+        ")",
+        "",
+        "cc_binary(name = 'target')",
+        "alias(name = 'alias', actual = ':further_alias')",
+        "alias(name = 'further_alias', actual = ':target')");
+
+    getConfiguredTarget("//test:my_rule");
+  }
 }