Allow overrides in non-root modules

They're simply ignored.

RELNOTES: Overrides specified by non-root modules no longer cause an error, and are silently ignored instead. They were originally treated as an error to allow for the future possibility of overrides in the transitive dependency graph working together; but we've deemed that infeasible (and even if it was, it'd be so complicated and confusing to users that it would not be a good addition).
PiperOrigin-RevId: 529095596
Change-Id: I8b9b7b570b405ee757554accf791d8e4c1ff6528

Author: Wyverald

site/en/external/module.md

@@ -108,12 +108,12 @@ exist in the resolved dependency graph.
 ## Overrides
 
 Specify overrides in the `MODULE.bazel` file to alter the behavior of Bazel
-module resolution. Only the root module can specify overrides — Bazel throws an
-error for dependency modules with overrides.
+module resolution. Only the root module's overrides take effect — if a module is
+used as a dependency, its overrides are ignored.
 
 Each override is specified for a certain module name, affecting all of its
-versions in the dependency graph. Although only the root module can specify
-overrides, they can be for transitive dependencies that the root module does not
+versions in the dependency graph. Although only the root module's overrides take
+effect, they can be for transitive dependencies that the root module does not
 directly depend on.
 
 ### Single-version override

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

@@ -138,9 +138,6 @@ public SkyValue compute(SkyKey skyKey, Environment env)
           moduleKey,
           module.getVersion());
     }
-    if (!moduleFileGlobals.buildOverrides().isEmpty()) {
-      throw errorf(Code.BAD_MODULE, "The MODULE.bazel file of %s declares overrides", moduleKey);
-    }
 
     return NonRootModuleFileValue.create(module, moduleFileHash);
   }

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

@@ -626,8 +626,8 @@ private void addOverride(String moduleName, ModuleOverride override) throws Eval
       doc =
           "Specifies that a dependency should still come from a registry, but its version should"
               + " be pinned, or its registry overridden, or a list of patches applied. This"
-              + " directive can only be used by the root module; in other words, if a module"
-              + " specifies any overrides, it cannot be used as a dependency by others.",
+              + " directive only takes effect in the root module; in other words, if a module"
+              + " is used as a dependency by others, its own overrides are ignored.",
       parameters = {
         @Param(
             name = "module_name",
@@ -708,9 +708,9 @@ public void singleVersionOverride(
           "Specifies that a dependency should still come from a registry, but multiple versions of"
               + " it should be allowed to coexist. See <a"
               + " href=\"/external/module#multiple-version_override\">the documentation</a> for"
-              + " more details. This directive can only be used by the root module; in other words,"
-              + " if a module specifies any overrides, it cannot be used as a dependency by"
-              + " others.",
+              + " more details. This"
+              + " directive only takes effect in the root module; in other words, if a module"
+              + " is used as a dependency by others, its own overrides are ignored.",
       parameters = {
         @Param(
             name = "module_name",
@@ -760,9 +760,9 @@ public void multipleVersionOverride(String moduleName, Iterable<?> versions, Str
       name = "archive_override",
       doc =
           "Specifies that this dependency should come from an archive file (zip, gzip, etc) at a"
-              + " certain location, instead of from a registry. This directive can only be used by"
-              + " the root module; in other words, if a module specifies any overrides, it cannot"
-              + " be used as a dependency by others.",
+              + " certain location, instead of from a registry. This"
+              + " directive only takes effect in the root module; in other words, if a module"
+              + " is used as a dependency by others, its own overrides are ignored.",
       parameters = {
         @Param(
             name = "module_name",
@@ -844,8 +844,8 @@ public void archiveOverride(
       name = "git_override",
       doc =
           "Specifies that a dependency should come from a certain commit of a Git repository. This"
-              + " directive can only be used by the root module; in other words, if a module"
-              + " specifies any overrides, it cannot be used as a dependency by others.",
+              + " directive only takes effect in the root module; in other words, if a module"
+              + " is used as a dependency by others, its own overrides are ignored.",
       parameters = {
         @Param(
             name = "module_name",
@@ -911,8 +911,8 @@ public void gitOverride(
       name = "local_path_override",
       doc =
           "Specifies that a dependency should come from a certain directory on local disk. This"
-              + " directive can only be used by the root module; in other words, if a module"
-              + " specifies any overrides, it cannot be used as a dependency by others.",
+              + " directive only takes effect in the root module; in other words, if a module"
+              + " is used as a dependency by others, its own overrides are ignored.",
       parameters = {
         @Param(
             name = "module_name",

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

@@ -215,7 +215,10 @@ public void testSimpleDiamond() throws Exception {
             .addModule(
                 createModuleKey("ccc", "2.0"),
                 "module(name='ccc', version='2.0');bazel_dep(name='ddd',version='3.0')")
-            .addModule(createModuleKey("ddd", "3.0"), "module(name='ddd', version='3.0')");
+            .addModule(
+                createModuleKey("ddd", "3.0"),
+                // Add a random override here; it should be ignored
+                "module(name='ddd', version='3.0');local_path_override(module_name='ff',path='f')");
     ModuleFileFunction.REGISTRIES.set(differencer, ImmutableList.of(registry.getUrl()));
 
     EvaluationResult<DiscoveryValue> result =