Add Label.to_display_form()

Fixes bazelbuild#20486

Also use `to_display_form()` to generate Clang module names in the
correct form for external repositories. `java_*` rules require more
delicate handling and will be migrated in a follow-up change.

RELNOTES: The `to_display_form()` method on `Label` returns a string
representation of a label optimized for readability by humans.

src/main/java/com/google/devtools/build/lib/actions/AbstractAction.java

@@ -368,13 +368,9 @@ private String getProgressMessageChecked(@Nullable RepositoryMapping mainReposit
   private String replaceProgressMessagePlaceholders(
       String progressMessage, @Nullable RepositoryMapping mainRepositoryMapping) {
     if (progressMessage.contains("%{label}") && owner.getLabel() != null) {
-      String labelString;
-      if (mainRepositoryMapping != null) {
-        labelString = owner.getLabel().getDisplayForm(mainRepositoryMapping);
-      } else {
-        labelString = owner.getLabel().toString();
-      }
-      progressMessage = progressMessage.replace("%{label}", labelString);
+      progressMessage =
+          progressMessage.replace(
+              "%{label}", owner.getLabel().getDisplayForm(mainRepositoryMapping));
     }
     if (progressMessage.contains("%{output}") && getPrimaryOutput() != null) {
       progressMessage =

src/main/java/com/google/devtools/build/lib/cmdline/BazelModuleContext.java

@@ -44,6 +44,13 @@ public abstract class BazelModuleContext {
   /** The repository mapping applicable to the repo where the .bzl file is located in. */
   public abstract RepositoryMapping repoMapping();
 
+  /**
+   * The repository mapping applicable to the main repository, possibly without WORKSPACE repos or
+   * null. This is purely meant to support {@link Label#getDisplayFormForStarlark(StarlarkThread)}.
+   */
+  @Nullable
+  public abstract RepositoryMapping bestEffortMainRepoMapping();
+
   /** Returns the name of the module's .bzl file, as provided to the parser. */
   public abstract String filename();
 
@@ -160,11 +167,12 @@ public static BazelModuleContext ofInnermostBzlOrFail(StarlarkThread thread, Str
   public static BazelModuleContext create(
       Label label,
       RepositoryMapping repoMapping,
+      @Nullable RepositoryMapping bestEffortMainRepoMapping,
       String filename,
       ImmutableList<Module> loads,
       byte[] bzlTransitiveDigest) {
     return new AutoValue_BazelModuleContext(
-        label, repoMapping, filename, loads, bzlTransitiveDigest);
+        label, repoMapping, bestEffortMainRepoMapping, filename, loads, bzlTransitiveDigest);
   }
 
   public final Label.PackageContext packageContext() {

src/main/java/com/google/devtools/build/lib/cmdline/Label.java

@@ -438,10 +438,29 @@ public String getUnambiguousCanonicalForm() {
    * @param mainRepositoryMapping the {@link RepositoryMapping} of the main repository
    * @return analogous to {@link PackageIdentifier#getDisplayForm(RepositoryMapping)}
    */
-  public String getDisplayForm(RepositoryMapping mainRepositoryMapping) {
+  public String getDisplayForm(@Nullable RepositoryMapping mainRepositoryMapping) {
     return packageIdentifier.getDisplayForm(mainRepositoryMapping) + ":" + name;
   }
 
+  @StarlarkMethod(
+      name = "to_display_form",
+      useStarlarkThread = true,
+      doc =
+          "Returns a string representation of this label that is optimized for human readability."
+              + " Use this to format a <code>Label</code> for use in BUILD files."
+              + " <p>The exact form of the return value is explicitly unspecified and subject to"
+              + " change."
+              + " The following properties are guaranteed for a <code>Label</code> <code>l</code>:"
+              + "<ul>"
+              + "  <li><code>l.to_display_form()</code> has no repository part if and only if <code>l</code> references the main repository;</li>"
+              + "  <li><code>Label(l.to_display_form()) == l</code> if the call to <code>Label</code> occurs in the main repository.</li>"
+              + "</ul>")
+  public String getDisplayFormForStarlark(StarlarkThread starlarkThread) throws EvalException {
+    checkRepoVisibilityForStarlark("to_display_form");
+    return getDisplayForm(
+        BazelModuleContext.ofInnermostBzlOrThrow(starlarkThread).bestEffortMainRepoMapping());
+  }
+
   /**
    * Returns a shorthand label string that is suitable for display, i.e. in addition to simplifying
    * the repository part, labels of the form {@code [@repo]//foo/bar:bar} are simplified to the

src/main/java/com/google/devtools/build/lib/cmdline/PackageIdentifier.java

@@ -24,6 +24,8 @@
 import com.google.devtools.build.skyframe.SkyFunctionName;
 import com.google.devtools.build.skyframe.SkyKey;
 import com.google.devtools.build.skyframe.SkyKey.SkyKeyInterner;
+
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -222,7 +224,7 @@ public String getUnambiguousCanonicalForm() {
    *       <dd>only with Bzlmod if the current package belongs to a repository that is not visible
    *           from the main module
    */
-  public String getDisplayForm(RepositoryMapping mainRepositoryMapping) {
+  public String getDisplayForm(@Nullable RepositoryMapping mainRepositoryMapping) {
     return repository.getDisplayForm(mainRepositoryMapping) + "//" + pkgName;
   }
 

src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java

@@ -247,20 +247,25 @@ public String getCanonicalForm() {
    *       <dt><code>@protobuf</code>
    *       <dd>if this repository is a WORKSPACE dependency and its <code>name</code> is "protobuf",
    *           or if this repository is a Bzlmod dependency of the main module and its apparent name
-   *           is "protobuf"
+   *           is "protobuf" (in both cases only if mainRepositoryMapping is not null)
    *       <dt><code>@@protobuf~3.19.2</code>
    *       <dd>only with Bzlmod, if this a repository that is not visible from the main module
    */
-  public String getDisplayForm(RepositoryMapping mainRepositoryMapping) {
+  public String getDisplayForm(@Nullable RepositoryMapping mainRepositoryMapping) {
     Preconditions.checkArgument(
-        mainRepositoryMapping.ownerRepo() == null || mainRepositoryMapping.ownerRepo().isMain());
+        mainRepositoryMapping == null
+            || mainRepositoryMapping.ownerRepo() == null
+            || mainRepositoryMapping.ownerRepo().isMain());
     if (!isVisible()) {
       return getNameWithAt();
     }
     if (isMain()) {
       // Packages in the main repository can always use repo-relative form.
       return "";
     }
+    if (mainRepositoryMapping == null) {
+      return getNameWithAt();
+    }
     if (!mainRepositoryMapping.usesStrictDeps()) {
       // If the main repository mapping is not using strict visibility, then Bzlmod is certainly
       // disabled, which means that canonical and apparent names can be used interchangeably from

src/main/java/com/google/devtools/build/lib/skyframe/BzlLoadFunction.java

@@ -66,6 +66,7 @@
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Consumer;
 import javax.annotation.Nullable;
@@ -762,6 +763,11 @@ private BzlLoadValue computeInternalWithCompiledBzl(
     if (repoMapping == null) {
       return null;
     }
+    Optional<RepositoryMapping> mainRepoMapping =
+        getMainRepositoryMapping(key, builtins.starlarkSemantics, env);
+    if (mainRepoMapping == null) {
+      return null;
+    }
     Label.RepoMappingRecorder repoMappingRecorder = new Label.RepoMappingRecorder();
     ImmutableList<Pair<String, Location>> programLoads = getLoadsFromProgram(prog);
     ImmutableList<Label> loadLabels =
@@ -840,6 +846,7 @@ private BzlLoadValue computeInternalWithCompiledBzl(
         BazelModuleContext.create(
             label,
             repoMapping,
+            mainRepoMapping.orElse(null),
             prog.getFilename(),
             ImmutableList.copyOf(loadMap.values()),
             transitiveDigest);
@@ -974,6 +981,34 @@ private static RepositoryMapping getRepositoryMapping(
     return repositoryMappingValue.getRepositoryMapping();
   }
 
+  @Nullable
+  private static Optional<RepositoryMapping> getMainRepositoryMapping(
+      BzlLoadValue.Key key, StarlarkSemantics starlarkSemantics, Environment env)
+      throws InterruptedException {
+    if (!starlarkSemantics.getBool(BuildLanguageOptions.ENABLE_BZLMOD)) {
+      return Optional.empty();
+    }
+    RepositoryMappingValue.Key repoMappingKey;
+    // When adding cases for other key types such as WORKSPACE or Bzlmod, make sure to track the
+    // usages of the repo mapping in persistent caches, such as repository marker files and the
+    // MODULE.bazel.lock file.
+    if (key instanceof BzlLoadValue.KeyForBuild) {
+      repoMappingKey = RepositoryMappingValue.key(RepositoryName.MAIN);
+    } else if (key instanceof BzlLoadValue.KeyForBuiltins) {
+      // Using the full main repo mapping here results in a cycle as it depends on WORKSPACE, but
+      // builtins are injected into WORKSPACE. Fixing this fully would require adding a new key type
+      // for builtins (transitively) loaded from WORKSPACE.
+      repoMappingKey = RepositoryMappingValue.KEY_FOR_ROOT_MODULE_WITHOUT_WORKSPACE_REPOS;
+    } else {
+      return Optional.empty();
+    }
+    var mainRepositoryMappingValue = (RepositoryMappingValue) env.getValue(repoMappingKey);
+    if (mainRepositoryMappingValue == null) {
+      return null;
+    }
+    return Optional.of(mainRepositoryMappingValue.getRepositoryMapping());
+  }
+
   /**
    * Validates a label appearing in a {@code load()} statement, throwing {@link
    * LabelSyntaxException} on failure.

src/main/starlark/builtins_bzl/common/cc/cc_compilation_helper.bzl

@@ -297,7 +297,7 @@ def _init_cc_compilation_context(
         if not module_map:
             module_map = cc_common.create_module_map(
                 file = actions.declare_file(label.name + ".cppmap"),
-                name = label.workspace_name + "//" + label.package + ":" + label.name,
+                name = label.to_display_form(),
             )
 
         # There are different modes for module compilation:

src/test/java/com/google/devtools/build/lib/analysis/actions/BuildInfoFileWriteActionTest.java

@@ -78,6 +78,7 @@ private static Object exec(String... lines) throws Exception {
               BazelModuleContext.create(
                   Label.parseCanonicalUnchecked("//test:label"),
                   RepositoryMapping.ALWAYS_FALLBACK,
+                  /* bestEffortMainRepoMapping= */ null,
                   "test/label.bzl",
                   /* loads= */ ImmutableList.of(),
                   /* bzlTransitiveDigest= */ new byte[0])),

src/test/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleExtensionResolutionTest.java

@@ -2479,4 +2479,37 @@ public void innate_noSuchValue() throws Exception {
             "//:repo.bzl does not export a repository_rule called data_repo, yet its use is"
                 + " requested at /ws/MODULE.bazel");
   }
+
+  @Test
+  public void labelToDisplayForm() throws Exception {
+    scratch.file(
+        workspaceRoot.getRelative("MODULE.bazel").getPathString(),
+        "bazel_dep(name='data_repo', version='1.0')",
+        "ext = use_extension('//:defs.bzl', 'ext')",
+        "use_repo(ext, 'foo', 'bar', 'baz')");
+    scratch.file(
+        workspaceRoot.getRelative("defs.bzl").getPathString(),
+        "load('@data_repo//:defs.bzl','data_repo')",
+        "def _ext_impl(ctx):",
+        "  data_repo(name='foo',data=Label('//:foo').to_display_form())",
+        "  data_repo(name='bar',data=Label('@data_repo//:bar').to_display_form())",
+        "  data_repo(name='baz',data=Label('@@canonical_name//:baz').to_display_form())",
+        "ext = module_extension(implementation=_ext_impl)");
+    scratch.file(workspaceRoot.getRelative("BUILD").getPathString());
+    scratch.file(
+        workspaceRoot.getRelative("data.bzl").getPathString(),
+        "load('@foo//:data.bzl', foo_data='data')",
+        "load('@bar//:data.bzl', bar_data='data')",
+        "load('@baz//:data.bzl', baz_data='data')",
+        "data = 'foo:'+foo_data+' bar:'+bar_data+' baz:'+baz_data");
+
+    SkyKey skyKey = BzlLoadValue.keyForBuild(Label.parseCanonical("//:data.bzl"));
+    EvaluationResult<BzlLoadValue> result =
+        evaluator.evaluate(ImmutableList.of(skyKey), evaluationContext);
+    if (result.hasError()) {
+      throw result.getError().getException();
+    }
+    assertThat(result.get(skyKey).getModule().getGlobal("data"))
+        .isEqualTo("foo://:foo bar:@@data_repo~1.0//:bar baz:@@canonical_name//:baz");
+  }
 }

src/test/java/com/google/devtools/build/lib/cmdline/LabelTest.java

@@ -419,6 +419,12 @@ public void testDisplayForm() throws Exception {
         .isEqualTo("@unremapped//:unremapped");
   }
 
+  @Test
+  public void testDisplayFormNullMapping() throws Exception {
+    assertThat(displayFormFor("//foo/bar:bar", null)).isEqualTo("//foo/bar:bar");
+    assertThat(displayFormFor("@@foo//bar:bar", null)).isEqualTo("@@foo//bar:bar");
+  }
+
   private static String shorthandDisplayFormFor(
       String rawLabel, RepositoryMapping repositoryMapping) throws Exception {
     return Label.parseCanonical(rawLabel).getShorthandDisplayForm(repositoryMapping);

src/test/java/com/google/devtools/build/lib/cmdline/PackageIdentifierTest.java

@@ -120,6 +120,7 @@ public void testDisplayFormInMainRepository() throws Exception {
                 RepositoryMapping.create(
                     ImmutableMap.of("foo", RepositoryName.create("bar")), RepositoryName.MAIN)))
         .isEqualTo("//some/pkg");
+    assertThat(pkg.getDisplayForm(null)).isEqualTo("//some/pkg");
   }
 
   @Test
@@ -139,5 +140,6 @@ public void testDisplayFormInExternalRepository() throws Exception {
                     ImmutableMap.of("local", RepositoryName.create("other_repo")),
                     RepositoryName.MAIN)))
         .isEqualTo("@@canonical//some/pkg");
+    assertThat(pkg.getDisplayForm(null)).isEqualTo("@@canonical//some/pkg");
   }
 }

src/test/java/com/google/devtools/build/lib/cmdline/RepositoryNameTest.java

@@ -105,4 +105,22 @@ public void testGetDisplayForm() throws Exception {
                 .getDisplayForm(repositoryMapping))
         .isEqualTo("@@[unknown repo 'local' requested from @@owner]");
   }
+
+  @Test
+  public void testGetDisplayFormWithNullMapping() throws Exception {
+    assertThat(RepositoryName.create("").getDisplayForm(null)).isEmpty();
+    assertThat(RepositoryName.create("canonical").getDisplayForm(null))
+        .isEqualTo("@@canonical");
+
+    assertThat(
+        RepositoryName.create("")
+            .toNonVisible(RepositoryName.create("owner"))
+            .getDisplayForm(null))
+        .isEqualTo("@@[unknown repo '' requested from @@owner]");
+    assertThat(
+        RepositoryName.create("canonical")
+            .toNonVisible(RepositoryName.create("owner"))
+            .getDisplayForm(null))
+        .isEqualTo("@@[unknown repo 'canonical' requested from @@owner]");
+  }
 }

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

@@ -5047,6 +5047,7 @@ public void testLabelWithStrictVisibility() throws Exception {
             bzlLabel,
             RepositoryMapping.create(
                 ImmutableMap.of("my_module", currentRepo, "dep", otherRepo), currentRepo),
+            /* bestEffortMainRepoMapping= */ null,
             "lib/label.bzl",
             /* loads= */ ImmutableList.of(),
             /* bzlTransitiveDigest= */ new byte[0]);

src/test/java/com/google/devtools/build/lib/starlark/util/BazelEvaluationTestCase.java

@@ -175,6 +175,7 @@ private Object newModule(ImmutableMap.Builder<String, Object> predeclared) {
     return BazelModuleContext.create(
         label,
         RepositoryMapping.ALWAYS_FALLBACK,
+        /* bestEffortMainRepoMapping= */ null,
         "test/label.bzl",
         /* loads= */ ImmutableList.of(),
         /* bzlTransitiveDigest= */ new byte[0]);