Disallow path label syntax for restricted register_toolchains

This change only applies to --experimental_single_package_toolchain_binding mode, which restricts register_toolchains to be clear about binding toolchains in only a single package.

path label syntax should be disabled given the semantic intention of --experimental_single_package_toolchain_binding because: If one were to specify register_toolchains("foo/bar/baz"), it may result in bindings to either foo/bar/baz:baz, foo/bar:baz, or foo:bar/baz (depending on which BUILD files are present).

Also: add a test that this restriction already exists for register_toolchains in module files (regardless of the restriction flag)
PiperOrigin-RevId: 632505958
Change-Id: I38a0b1e12fc4b975ae02f9e5d4e141a3c45d06da

src/main/java/com/google/devtools/build/lib/packages/WorkspaceGlobals.java

@@ -131,6 +131,12 @@ public void registerToolchains(Sequence<?> toolchainLabels, StarlarkThread threa
               "invalid target pattern \"%s\": register_toolchain target patterns may only refer to "
                   + "targets within a single package",
               tp.getOriginalPattern());
+        } else if (tp.getType() == TargetPattern.Type.PATH_AS_TARGET) {
+          throw Starlark.errorf(
+              "invalid target pattern \"%s\": register_toolchain target patterns may only refer to "
+                  + "targets with a declared package (relative path syntax omitting ':' is "
+                  + "ambiguous)",
+              tp.getOriginalPattern());
         }
       }
     }

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

@@ -1535,7 +1535,7 @@ public void isolatedUsage_notEnabled() throws Exception {
   }
 
   @Test
-  public void testRegisterToolchains_singlePackageRestriction_error() throws Exception {
+  public void testRegisterToolchains_singlePackageRestriction_underDir() throws Exception {
     // Test intentionally introduces errors.
     reporter.removeHandler(failFastHandler);
     PrecomputedValue.STARLARK_SEMANTICS.set(
@@ -1562,6 +1562,29 @@ public void testRegisterToolchains_singlePackageRestriction_error() throws Excep
             + "may only refer to targets within a single package");
   }
 
+  @Test
+  public void testRegisterToolchains_pathSyntax() throws Exception {
+    // Test intentionally introduces errors.
+    reporter.removeHandler(failFastHandler);
+
+    scratch.overwriteFile(
+        rootDirectory.getRelative("MODULE.bazel").getPathString(),
+        "module(name='aaa')",
+        "register_toolchains('bar/baz')");
+
+    FakeRegistry registry = registryFactory.newFakeRegistry("/foo");
+    ModuleFileFunction.REGISTRIES.set(differencer, ImmutableSet.of(registry.getUrl()));
+
+    EvaluationResult<RootModuleFileValue> result =
+        evaluator.evaluate(
+            ImmutableList.of(ModuleFileValue.KEY_FOR_ROOT_MODULE), evaluationContext);
+    assertThat(result.hasError()).isTrue();
+
+    assertContainsEvent(
+        "Expected absolute target patterns (must begin with '//' or '@') for 'register_toolchains'"
+            + " argument, but got 'bar/baz' as an argument");
+  }
+
   @Test
   public void testRegisterToolchains_singlePackageRestriction() throws Exception {
     PrecomputedValue.STARLARK_SEMANTICS.set(

src/test/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunctionTest.java

@@ -271,7 +271,7 @@ public void testRegisterToolchains_singlePackageRestriction() throws Exception {
   }
 
   @Test
-  public void testRegisterToolchains_singlePackageRestriction_error() throws Exception {
+  public void testRegisterToolchains_singlePackageRestriction_underDir() throws Exception {
     // Test intentionally introduces errors.
     reporter.removeHandler(failFastHandler);
 
@@ -289,6 +289,26 @@ public void testRegisterToolchains_singlePackageRestriction_error() throws Excep
             + "may only refer to targets within a single package");
   }
 
+  @Test
+  public void testRegisterToolchains_singlePackageRestriction_pathSyntax() throws Exception {
+    // Test intentionally introduces errors.
+    reporter.removeHandler(failFastHandler);
+
+    setBuildLanguageOptions("--experimental_single_package_toolchain_binding");
+
+    String[] lines = {"register_toolchains('foo/bar')"};
+    createWorkspaceFile(lines);
+
+    SkyKey key = ExternalPackageFunction.key();
+    EvaluationResult<PackageValue> evaluationResult = eval(key);
+    Package pkg = evaluationResult.get(key).getPackage();
+    assertThat(pkg.containsErrors()).isTrue();
+    assertContainsEvent(
+        "invalid target pattern \"foo/bar\": register_toolchain target patterns may only refer "
+            + "to targets with a declared package (relative path syntax omitting ':' is "
+            + "ambiguous)");
+  }
+
   @Test
   public void testNoWorkspaceFile() throws Exception {
     // Create and immediately delete to make sure we got the right file.