Make TypeCheckedTag a FailureCause

By passing the Starlark representation of a module extension tag to
the new `causes` parameter of `fail`, module extensions can generate
more concise and helpful error messages on user errors.

Before:
```
ERROR: Traceback (most recent call last):
	File "/home/user/.cache/bazel/_bazel_user/466e6feac151a1e22847383f4e503361/external/gazelle~override/internal/bzlmod/go_deps.bzl", line 165, column 21, in _go_deps_impl
		fail("Multiple overrides defined for Go module path \"{}\" in module \"{}\".".format(override_tag.path, module.name))
Error in fail: Multiple overrides defined for Go module path "github.com/stretchr/testify" in module "gazelle_bcr_tests".
```

After:
```
ERROR: Multiple overrides defined for Go module path "github.com/stretchr/testify" in module "gazelle_bcr_tests".
Causes:
	bazel_module_tag in file "<root>/MODULE.bazel", line 39, column 25, with value:
		go_deps.gazelle_override(
		    path = "github.com/stretchr/testify",
		    directives = ["gazelle:go_naming_convention go_default_library"],
		)
```

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

@@ -259,6 +259,7 @@ private ModuleExtensionContext createContext(
             StarlarkBazelModule.create(
                 abridgedModule,
                 extension,
+                extensionId,
                 usagesValue.getRepoMappings().get(moduleKey),
                 usagesValue.getExtensionUsages().get(moduleKey)));
       } catch (ExternalDepsException e) {

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

@@ -101,6 +101,7 @@ private StarlarkBazelModule(String name, String version, Tags tags, boolean isRo
   public static StarlarkBazelModule create(
       AbridgedModule module,
       ModuleExtension extension,
+      ModuleExtensionId extensionId,
       RepositoryMapping repoMapping,
       @Nullable ModuleExtensionUsage usage)
       throws ExternalDepsException {
@@ -130,7 +131,7 @@ public static StarlarkBazelModule create(
       // (for example, String to Label).
       typeCheckedTags
           .get(tag.getTagName())
-          .add(TypeCheckedTag.create(tagClass, tag, labelConverter));
+          .add(TypeCheckedTag.create(tagClass, tag, labelConverter, tag.getTagName(), extensionId));
     }
     return new StarlarkBazelModule(
         module.getName(),

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

@@ -23,25 +23,42 @@
 import javax.annotation.Nullable;
 import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.FailureCause;
+import net.starlark.java.eval.Printer;
+import net.starlark.java.eval.StarlarkSemantics;
 import net.starlark.java.eval.Structure;
 import net.starlark.java.spelling.SpellChecker;
+import net.starlark.java.syntax.Location;
 
 /**
  * A {@link Tag} whose attribute values have been type-checked against the attribute schema define
  * in the {@link TagClass}.
  */
 @StarlarkBuiltin(name = "bazel_module_tag", documented = false)
-public class TypeCheckedTag implements Structure {
+public class TypeCheckedTag implements FailureCause, Structure {
+
   private final TagClass tagClass;
   private final Object[] attrValues;
 
-  private TypeCheckedTag(TagClass tagClass, Object[] attrValues) {
+  // The properties below are only used for error reporting.
+  private final Location location;
+  private final String tagClassName;
+  private final String extensionName;
+
+  private TypeCheckedTag(TagClass tagClass, Object[] attrValues, Location location,
+      String tagClassName, String extensionName) {
     this.tagClass = tagClass;
     this.attrValues = attrValues;
+    this.location = location;
+    this.tagClassName = tagClassName;
+    this.extensionName = extensionName;
   }
 
-  /** Creates a {@link TypeCheckedTag}. */
-  public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter labelConverter)
+  /**
+   * Creates a {@link TypeCheckedTag}.
+   */
+  public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter labelConverter,
+      String tagClassName, ModuleExtensionId extensionId)
       throws ExternalDepsException {
     Object[] attrValues = new Object[tagClass.getAttributes().size()];
     for (Map.Entry<String, Object> attrValue : tag.getAttributeValues().entrySet()) {
@@ -95,7 +112,8 @@ public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter l
         attrValues[i] = Attribute.valueToStarlark(attr.getDefaultValueUnchecked());
       }
     }
-    return new TypeCheckedTag(tagClass, attrValues);
+    return new TypeCheckedTag(tagClass, attrValues, tag.getLocation(), tagClassName,
+        extensionId.getExtensionName());
   }
 
   @Override
@@ -123,4 +141,28 @@ public ImmutableCollection<String> getFieldNames() {
   public String getErrorMessageForUnknownField(String field) {
     return "unknown attribute " + field;
   }
+
+  @Override
+  public void debugPrint(Printer printer, StarlarkSemantics semantics) {
+    printer.append(extensionName);
+    printer.append('.');
+    printer.append(tagClassName);
+    printer.append("(\n");
+
+    for (int i = 0; i < attrValues.length; i++) {
+      Attribute attr = tagClass.getAttributes().get(i);
+      printer.append("    ");
+      printer.append(attr.getPublicName());
+      printer.append(" = ");
+      printer.repr(attrValues[i]);
+      printer.append(",\n");
+    }
+
+    printer.append(")");
+  }
+
+  @Override
+  public Location getLocation() {
+    return location;
+  }
 }

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

@@ -89,7 +89,9 @@ public void basic() throws Exception {
 
     StarlarkBazelModule moduleProxy =
         StarlarkBazelModule.create(
-            abridgedModule, extension, module.getRepoMappingWithBazelDepsOnly(), usage);
+            abridgedModule, extension,
+            ModuleExtensionId.create(Label.parseCanonicalUnchecked("@foo//:extensions.bzl"), "ext"),
+            module.getRepoMappingWithBazelDepsOnly(), usage);
 
     assertThat(moduleProxy.getName()).isEqualTo("foo");
     assertThat(moduleProxy.getVersion()).isEqualTo("1.0");
@@ -136,7 +138,11 @@ public void unknownTagClass() throws Exception {
             ExternalDepsException.class,
             () ->
                 StarlarkBazelModule.create(
-                    abridgedModule, extension, module.getRepoMappingWithBazelDepsOnly(), usage));
+                    abridgedModule, extension,
+                    ModuleExtensionId.create(Label.parseCanonicalUnchecked("@foo//:extensions.bzl"),
+                        "ext"),
+                    module.getRepoMappingWithBazelDepsOnly(),
+                    usage));
     assertThat(e).hasMessageThat().contains("does not have a tag class named blep");
   }
 }

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

@@ -43,10 +43,15 @@
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
-/** Tests for {@link TypeCheckedTag}. */
+/**
+ * Tests for {@link TypeCheckedTag}.
+ */
 @RunWith(JUnit4.class)
 public class TypeCheckedTagTest {
 
+  private static final ModuleExtensionId EXT_ID = ModuleExtensionId.create(
+      Label.parseCanonicalUnchecked("@foo//:extensions.bzl"), "ext");
+
   private static Object getattr(Structure structure, String fieldName) throws Exception {
     return Starlark.getattr(
         Mutability.IMMUTABLE,
@@ -61,8 +66,8 @@ public void basic() throws Exception {
     TypeCheckedTag typeCheckedTag =
         TypeCheckedTag.create(
             createTagClass(attr("foo", Type.INTEGER).build()),
-            buildTag("tag_name").addAttr("foo", StarlarkInt.of(3)).build(),
-            /*labelConverter=*/ null);
+            buildTag("tag_name").addAttr("foo", StarlarkInt.of(3)).build(), /*labelConverter=*/
+            null, "tag_name", EXT_ID);
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
     assertThat(getattr(typeCheckedTag, "foo")).isEqualTo(StarlarkInt.of(3));
   }
@@ -76,10 +81,11 @@ public void label() throws Exception {
             buildTag("tag_name")
                 .addAttr(
                     "foo", StarlarkList.immutableOf(":thing1", "//pkg:thing2", "@repo//pkg:thing3"))
-                .build(),
-            new LabelConverter(
+                .build(), new LabelConverter(
                 PackageIdentifier.parse("@myrepo//mypkg"),
-                createRepositoryMapping(createModuleKey("test", "1.0"), "repo", "other_repo")));
+                createRepositoryMapping(createModuleKey("test", "1.0"), "repo", "other_repo")),
+            "tag_name", EXT_ID
+        );
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
     assertThat(getattr(typeCheckedTag, "foo"))
         .isEqualTo(
@@ -95,10 +101,11 @@ public void label_withoutDefaultValue() throws Exception {
         TypeCheckedTag.create(
             createTagClass(
                 attr("foo", BuildType.LABEL).allowedFileTypes(FileTypeSet.ANY_FILE).build()),
-            buildTag("tag_name").build(),
-            new LabelConverter(
+            buildTag("tag_name").build(), new LabelConverter(
                 PackageIdentifier.parse("@myrepo//mypkg"),
-                createRepositoryMapping(createModuleKey("test", "1.0"), "repo", "other_repo")));
+                createRepositoryMapping(createModuleKey("test", "1.0"), "repo", "other_repo")),
+            "tag_name", EXT_ID
+        );
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
     assertThat(getattr(typeCheckedTag, "foo")).isEqualTo(Starlark.NONE);
   }
@@ -110,9 +117,8 @@ public void stringListDict_default() throws Exception {
             createTagClass(
                 attr("foo", Type.STRING_LIST_DICT)
                     .value(ImmutableMap.of("key", ImmutableList.of("value1", "value2")))
-                    .build()),
-            buildTag("tag_name").build(),
-            null);
+                    .build()), buildTag("tag_name").build(), null, "tag_name", EXT_ID
+        );
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
     assertThat(getattr(typeCheckedTag, "foo"))
         .isEqualTo(
@@ -128,12 +134,10 @@ public void multipleAttributesAndDefaults() throws Exception {
             createTagClass(
                 attr("foo", Type.STRING).mandatory().build(),
                 attr("bar", Type.INTEGER).value(StarlarkInt.of(3)).build(),
-                attr("quux", Type.STRING_LIST).build()),
-            buildTag("tag_name")
+                attr("quux", Type.STRING_LIST).build()), buildTag("tag_name")
                 .addAttr("foo", "fooValue")
                 .addAttr("quux", StarlarkList.immutableOf("quuxValue1", "quuxValue2"))
-                .build(),
-            /*labelConverter=*/ null);
+                .build(), /*labelConverter=*/null, "tag_name", EXT_ID);
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo", "bar", "quux");
     assertThat(getattr(typeCheckedTag, "foo")).isEqualTo("fooValue");
     assertThat(getattr(typeCheckedTag, "bar")).isEqualTo(StarlarkInt.of(3));
@@ -149,8 +153,7 @@ public void mandatory() throws Exception {
             () ->
                 TypeCheckedTag.create(
                     createTagClass(attr("foo", Type.STRING).mandatory().build()),
-                    buildTag("tag_name").build(),
-                    /*labelConverter=*/ null));
+                    buildTag("tag_name").build(), /*labelConverter=*/null, "tag_name", EXT_ID));
     assertThat(e).hasMessageThat().contains("mandatory attribute foo isn't being specified");
   }
 
@@ -165,8 +168,9 @@ public void allowedValues() throws Exception {
                         attr("foo", Type.STRING)
                             .allowedValues(new AllowedValueSet("yes", "no"))
                             .build()),
-                    buildTag("tag_name").addAttr("foo", "maybe").build(),
-                    /*labelConverter=*/ null));
+                    buildTag("tag_name").addAttr("foo", "maybe").build(), /*labelConverter=*/null,
+                    "tag_name", EXT_ID
+                ));
     assertThat(e)
         .hasMessageThat()
         .contains("the value for attribute foo has to be one of 'yes' or 'no' instead of 'maybe'");
@@ -180,8 +184,9 @@ public void unknownAttr() throws Exception {
             () ->
                 TypeCheckedTag.create(
                     createTagClass(attr("foo", Type.STRING).build()),
-                    buildTag("tag_name").addAttr("bar", "maybe").build(),
-                    /*labelConverter=*/ null));
+                    buildTag("tag_name").addAttr("bar", "maybe").build(), /*labelConverter=*/null,
+                    "tag_name", EXT_ID
+                ));
     assertThat(e).hasMessageThat().contains("unknown attribute bar provided");
   }
 }