Add support for loading .scl files

The Starlark Configuration Language (SCL) is a dialect of Starlark that resembles pure Starlark with just a few Bazel symbols (namely `visibility()` and `struct`), and a few restrictions on `load()` syntax and non-ASCII string data.

This CL adds support for loading .scl files anywhere a .bzl could be used. The restrictions on `load()` are implemented in this CL, but follow-up CLs will handle differentiating the top-level environment from .bzl, restricting non-ASCII data, and adding user documentation.

The dialect is gated on the flag `--experimental_enable_scl_dialect`.

- `BzlLoadValue` gains a new bool field on its abstract Key type to indicate whether it is for .scl. This was chosen instead of creating a new Key subclass so that .scl can work equally well in different contexts (loaded by BUILD, WORKSPACE, MODULE.bazel, or even @_builtins).

- `BzlLoadFunction.checkValidLoadLabel` and `.getLoadLabels` are both split into a public and private method. The public methods assume the load is coming from a .bzl file for validation purposes, and take a `StarlarkSemantics` to check whether the experimental flag is enabled.

- Eliminate `BzlLoadFunction.getBUILDLabel()` helper, which is obsolete.

- Modify existing tests to incidentally check that bzlmod .bzls can load .scl files and that .scl files can appear in transitive loads as reported by the Package (for `loadfiles()`).

RELNOTES: None
PiperOrigin-RevId: 528563228
Change-Id: I8493d1f33d35e1af8003dc61e5fdb626676d7e53

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/SingleExtensionEvalFunction.java

@@ -111,8 +111,7 @@ public SkyValue compute(SkyKey skyKey, Environment env)
 
     // Check that the .bzl label isn't crazy.
     try {
-      BzlLoadFunction.checkValidLoadLabel(
-          extensionId.getBzlFileLabel(), /*fromBuiltinsRepo=*/ false);
+      BzlLoadFunction.checkValidLoadLabel(extensionId.getBzlFileLabel(), starlarkSemantics);
     } catch (LabelSyntaxException e) {
       throw new SingleExtensionEvalFunctionException(
           ExternalDepsException.withCauseAndMessage(

src/main/java/com/google/devtools/build/lib/packages/semantics/BuildLanguageOptions.java

@@ -196,6 +196,15 @@ public final class BuildLanguageOptions extends OptionsBase {
       help = "If set to true, enables the APIs required to support the Android Starlark migration.")
   public boolean experimentalEnableAndroidMigrationApis;
 
+  @Option(
+      name = "experimental_enable_scl_dialect",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
+      effectTags = OptionEffectTag.BUILD_FILE_SEMANTICS,
+      // TODO(brandjon): point to more extensive user documentation somewhere
+      help = "If set to true, .scl files may be used in load() statements.")
+  public boolean experimentalEnableSclDialect;
+
   @Option(
       name = "enable_bzlmod",
       oldName = "experimental_enable_bzlmod",
@@ -677,6 +686,7 @@ public StarlarkSemantics toStarlarkSemantics() {
             .setBool(CHECK_BZL_VISIBILITY, checkBzlVisibility)
             .setBool(
                 EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS, experimentalEnableAndroidMigrationApis)
+            .setBool(EXPERIMENTAL_ENABLE_SCL_DIALECT, experimentalEnableSclDialect)
             .setBool(ENABLE_BZLMOD, enableBzlmod)
             .setBool(
                 EXPERIMENTAL_JAVA_PROTO_LIBRARY_DEFAULT_HAS_SERVICES,
@@ -770,6 +780,7 @@ public StarlarkSemantics toStarlarkSemantics() {
       "-experimental_disable_external_package";
   public static final String EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS =
       "-experimental_enable_android_migration_apis";
+  public static final String EXPERIMENTAL_ENABLE_SCL_DIALECT = "-experimental_enable_scl_dialect";
   public static final String ENABLE_BZLMOD = "-enable_bzlmod";
   public static final String EXPERIMENTAL_JAVA_PROTO_LIBRARY_DEFAULT_HAS_SERVICES =
       "+experimental_java_proto_library_default_has_services";

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

@@ -86,7 +86,11 @@
 import net.starlark.java.syntax.StringLiteral;
 
 /**
- * A Skyframe function to look up and load a single .bzl module.
+ * A Skyframe function to look up and load a single .bzl (or .scl) module.
+ *
+ * <p>Note: Historically, all modules had the .bzl suffix, but this is no longer true now that Bazel
+ * supports the .scl dialect. In identifiers, code comments, and documentation, you should generally
+ * assume any "bzl" term could mean a .scl file as well.
  *
  * <p>Given a {@link Label} referencing a .bzl file, attempts to locate the file and load it. The
  * Label must be absolute, and must not reference the special {@code external} package. If loading
@@ -736,6 +740,14 @@ private BzlLoadValue computeInternalWithCompiledBzl(
     Label label = key.getLabel();
     PackageIdentifier pkg = label.getPackageIdentifier();
 
+    boolean isSclFlagEnabled =
+        builtins.starlarkSemantics.getBool(BuildLanguageOptions.EXPERIMENTAL_ENABLE_SCL_DIALECT);
+    if (key.isSclDialect() && !isSclFlagEnabled) {
+      throw new BzlLoadFailedException(
+          "loading .scl files requires setting --experimental_enable_scl_dialect",
+          Code.PARSE_ERROR);
+    }
+
     // Determine dependency BzlLoadValue keys for the load statements in this bzl.
     // Labels are resolved relative to the current repo mapping.
     RepositoryMapping repoMapping = getRepositoryMapping(key, builtins.starlarkSemantics, env);
@@ -744,7 +756,13 @@ private BzlLoadValue computeInternalWithCompiledBzl(
     }
     ImmutableList<Pair<String, Location>> programLoads = getLoadsFromProgram(prog);
     ImmutableList<Label> loadLabels =
-        getLoadLabels(env.getListener(), programLoads, pkg, repoMapping);
+        getLoadLabels(
+            env.getListener(),
+            programLoads,
+            pkg,
+            repoMapping,
+            key.isSclDialect(),
+            isSclFlagEnabled);
     if (loadLabels == null) {
       throw new BzlLoadFailedException(
           String.format(
@@ -933,11 +951,59 @@ private static RepositoryMapping getRepositoryMapping(
     return repositoryMappingValue.getRepositoryMapping();
   }
 
-  public static void checkValidLoadLabel(Label label, boolean fromBuiltinsRepo)
+  /**
+   * Validates a label appearing in a {@code load()} statement, throwing {@link
+   * LabelSyntaxException} on failure.
+   *
+   * <p>Different restrictions apply depending on what type of source file the load appears in. For
+   * all kinds of files, {@code label}:
+   *
+   * <ul>
+   *   <li>may not be within {@code @//external}.
+   *   <li>must end with either {@code .bzl} or {@code .scl}.
+   * </ul>
+   *
+   * <p>For source files appearing within {@code @_builtins}, {@code label} must also be within
+   * {@code @_builtins}. (The reverse, that those files may not be loaded by user-defined files, is
+   * enforced by the fact that the {@code @_builtins} pseudorepo cannot be resolved as an ordinary
+   * repo.)
+   *
+   * <p>For .scl files only, {@code label} must end with {@code .scl} (not {@code .bzl}). (Loads in
+   * .scl also should always begin with {@code //}, but that's syntactic and can't be enforced in
+   * this method.)
+   *
+   * @param label the label to validate
+   * @param fromBuiltinsRepo true if the file containing the load is within {@code @_builtins}
+   * @param withinSclDialect true if the file containing the load is a .scl file
+   * @param mentionSclInErrorMessage true if ".scl" should be advertised as a possible extension in
+   *     error messaging
+   */
+  private static void checkValidLoadLabel(
+      Label label,
+      boolean fromBuiltinsRepo,
+      boolean withinSclDialect,
+      boolean mentionSclInErrorMessage)
       throws LabelSyntaxException {
-    if (!label.getName().endsWith(".bzl")) {
-      throw new LabelSyntaxException("The label must reference a file with extension '.bzl'");
+    // Check file extension.
+    String baseName = label.getName();
+    if (withinSclDialect) {
+      if (!baseName.endsWith(".scl")) {
+        String msg = "The label must reference a file with extension \".scl\"";
+        if (baseName.endsWith(".bzl")) {
+          msg += " (.scl files cannot load .bzl files)";
+        }
+        throw new LabelSyntaxException(msg);
+      }
+    } else {
+      if (!(baseName.endsWith(".scl") || baseName.endsWith(".bzl"))) {
+        String msg = "The label must reference a file with extension \".bzl\"";
+        if (mentionSclInErrorMessage) {
+          msg += " or \".scl\"";
+        }
+        throw new LabelSyntaxException(msg);
+      }
     }
+
     if (label.getPackageIdentifier().equals(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER)) {
       throw new LabelSyntaxException(
           "Starlark files may not be loaded from the //external package");
@@ -948,35 +1014,57 @@ public static void checkValidLoadLabel(Label label, boolean fromBuiltinsRepo)
     }
   }
 
+  /**
+   * Validates a label appearing in a {@code load()} statement, throwing {@link
+   * LabelSyntaxException} on failure.
+   */
+  public static void checkValidLoadLabel(Label label, StarlarkSemantics starlarkSemantics)
+      throws LabelSyntaxException {
+    checkValidLoadLabel(
+        label,
+        /* fromBuiltinsRepo= */ false,
+        /* withinSclDialect= */ false,
+        /* mentionSclInErrorMessage= */ starlarkSemantics.getBool(
+            BuildLanguageOptions.EXPERIMENTAL_ENABLE_SCL_DIALECT));
+  }
+
   /**
    * Given a list of {@code load("module")} strings and their locations, in source order, returns a
    * corresponding list of Labels they each resolve to. Labels are resolved relative to {@code
    * base}, the file's package. If any label is malformed, the function reports one or more errors
    * to the handler and returns null.
+   *
+   * <p>If {@code withinSclDialect} is true, the labels are validated according to the rules of the
+   * .scl dialect: Only strings beginning with {@code //} are allowed (no repo syntax, no relative
+   * labels), and only .scl files may be loaded (not .bzl). If {@code isSclFlagEnabled} is true,
+   * then ".scl" is mentioned as a possible file extension in error messages.
    */
   @Nullable
-  static ImmutableList<Label> getLoadLabels(
+  private static ImmutableList<Label> getLoadLabels(
       EventHandler handler,
       ImmutableList<Pair<String, Location>> loads,
       PackageIdentifier base,
-      RepositoryMapping repoMapping) {
-    // It's redundant that getRelativeWithRemapping needs a Label;
-    // a PackageIdentifier should suffice. Make one here.
-    Label buildLabel = getBUILDLabel(base);
-
+      RepositoryMapping repoMapping,
+      boolean withinSclDialect,
+      boolean isSclFlagEnabled) {
     boolean ok = true;
 
     ImmutableList.Builder<Label> loadLabels = ImmutableList.builderWithExpectedSize(loads.size());
     for (Pair<String, Location> load : loads) {
-      // Parse the load statement's module string as a label.
-      // It must end in .bzl and not be in package "//external".
+      // Parse the load statement's module string as a label. Validate the unparsed string for
+      // syntax and the parsed label for structure.
+      String unparsedLabel = load.first;
       try {
+        if (withinSclDialect && !unparsedLabel.startsWith("//")) {
+          throw new LabelSyntaxException("in .scl files, load labels must begin with \"//\"");
+        }
         Label label =
-            Label.parseWithPackageContext(
-                load.first, PackageContext.of(buildLabel.getPackageIdentifier(), repoMapping));
+            Label.parseWithPackageContext(unparsedLabel, PackageContext.of(base, repoMapping));
         checkValidLoadLabel(
             label,
-            /* fromBuiltinsRepo= */ StarlarkBuiltinsValue.isBuiltinsRepo(base.getRepository()));
+            /* fromBuiltinsRepo= */ StarlarkBuiltinsValue.isBuiltinsRepo(base.getRepository()),
+            /* withinSclDialect= */ withinSclDialect,
+            /* mentionSclInErrorMessage= */ isSclFlagEnabled);
         loadLabels.add(label);
       } catch (LabelSyntaxException ex) {
         handler.handle(Event.error(load.second, "in load statement: " + ex.getMessage()));
@@ -986,6 +1074,29 @@ static ImmutableList<Label> getLoadLabels(
     return ok ? loadLabels.build() : null;
   }
 
+  /**
+   * Given a list of {@code load("module")} strings and their locations, in source order, returns a
+   * corresponding list of Labels they each resolve to. Labels are resolved relative to {@code
+   * base}, the file's package. If any label is malformed, the function reports one or more errors
+   * to the handler and returns null.
+   */
+  @Nullable
+  static ImmutableList<Label> getLoadLabels(
+      EventHandler handler,
+      ImmutableList<Pair<String, Location>> loads,
+      PackageIdentifier base,
+      RepositoryMapping repoMapping,
+      StarlarkSemantics starlarkSemantics) {
+    return getLoadLabels(
+        handler,
+        loads,
+        base,
+        repoMapping,
+        /* withinSclDialect= */ false,
+        /* isSclFlagEnabled= */ starlarkSemantics.getBool(
+            BuildLanguageOptions.EXPERIMENTAL_ENABLE_SCL_DIALECT));
+  }
+
   /** Extracts load statements from compiled program (see {@link #getLoadLabels}). */
   static ImmutableList<Pair<String, Location>> getLoadsFromProgram(Program prog) {
     int n = prog.getLoads().size();
@@ -1010,15 +1121,6 @@ static ImmutableList<Pair<String, Location>> getLoadsFromStarlarkFiles(List<Star
     return loads.build();
   }
 
-  private static Label getBUILDLabel(PackageIdentifier pkgid) {
-    try {
-      return Label.create(pkgid, "BUILD");
-    } catch (LabelSyntaxException e) {
-      // Shouldn't happen; the Label is well-formed by construction.
-      throw new IllegalStateException(e);
-    }
-  }
-
   /**
    * Computes the BzlLoadValue for all given .bzl load keys using ordinary Skyframe evaluation,
    * returning {@code null} if Skyframe deps were missing and have been requested. {@code

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

@@ -31,7 +31,11 @@
 import net.starlark.java.eval.Module;
 
 /**
- * A value that represents the .bzl module loaded by a Starlark {@code load()} statement.
+ * A value that represents the .bzl (or .scl) module loaded by a Starlark {@code load()} statement.
+ *
+ * <p>Note: Historically, all modules had the .bzl suffix, but this is no longer true now that Bazel
+ * supports the .scl dialect. In identifiers, code comments, and documentation, you should generally
+ * assume any "bzl" term could mean a .scl file as well.
  *
  * <p>The key consists of an absolute {@link Label} and the context in which the load occurs. The
  * Label should not reference the special {@code external} package.
@@ -95,6 +99,22 @@ boolean isBuiltins() {
       return false;
     }
 
+    /** Returns true if the requested file follows the .scl dialect. */
+    // Note: Just as with .bzl, the same .scl file can be referred to from multiple key types, for
+    // instance if a BUILD file and a module rule both load foo.scl. Conceptually, .scl files
+    // shouldn't depend on what kind of top-level file caused them to load, but in practice, this
+    // implementation quirk means that the .scl file will be loaded twice as separate copies.
+    //
+    // This shouldn't matter except in rare edge cases, such as if a Starlark function is loaded
+    // from both copies and compared for equality. Performance wise, it also means that all
+    // transitive .scl files will be double-loaded, but we don't expect that to be significant.
+    //
+    // The alternative is to use a separate key type just for .scl, but that complicates repo logic;
+    // see BzlLoadFunction#getRepositoryMapping.
+    final boolean isSclDialect() {
+      return getLabel().getName().endsWith(".scl");
+    }
+
     /**
      * Constructs a new key suitable for evaluating a {@code load()} dependency of this key's .bzl
      * file.

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

@@ -220,7 +220,8 @@ private ImmutableMap<String, Module> loadBzlModules(
             env.getListener(),
             programLoads,
             PackageIdentifier.EMPTY_PACKAGE_ID,
-            EMPTY_MAIN_REPO_MAPPING);
+            EMPTY_MAIN_REPO_MAPPING,
+            starlarkSemantics);
     if (loadLabels == null) {
       NoSuchPackageException e =
           PackageFunction.PackageFunctionException.builder()

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

@@ -1288,7 +1288,11 @@ private LoadedPackage loadPackage(
             BzlLoadFunction.getLoadsFromProgram(compiled.prog);
         ImmutableList<Label> loadLabels =
             BzlLoadFunction.getLoadLabels(
-                env.getListener(), programLoads, packageId, repositoryMapping);
+                env.getListener(),
+                programLoads,
+                packageId,
+                repositoryMapping,
+                starlarkBuiltinsValue.starlarkSemantics);
         if (loadLabels == null) {
           throw PackageFunctionException.builder()
               .setType(PackageFunctionException.Type.BUILD_FILE_CONTAINS_ERRORS)

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

@@ -297,7 +297,8 @@ public SkyValue compute(SkyKey skyKey, Environment env)
     ImmutableList<Pair<String, Location>> programLoads =
         BzlLoadFunction.getLoadsFromStarlarkFiles(chunk);
     ImmutableList<Label> loadLabels =
-        BzlLoadFunction.getLoadLabels(env.getListener(), programLoads, rootPackage, repoMapping);
+        BzlLoadFunction.getLoadLabels(
+            env.getListener(), programLoads, rootPackage, repoMapping, starlarkSemantics);
     if (loadLabels == null) {
       NoSuchPackageException e =
           PackageFunction.PackageFunctionException.builder()