cmd/go: extend the ignore directive for indexed modules

For modules that have already been indexed, we can skip ignored paths.
We already skip 'testdata' and '_' for this case so we can extend the
ignore directive for this case as well.

Updates: #42965
Change-Id: I076a242ba65c7b905b9dc65dcfb0a0247cbd68d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/674076
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/modload/search.go

@@ -208,7 +208,7 @@ func matchPackages(ctx context.Context, m *search.Match, tags map[string]bool, f
 			modPrefix = mod.Path
 		}
 		if mi, err := modindex.GetModule(root); err == nil {
-			walkFromIndex(mi, modPrefix, isMatch, treeCanMatch, tags, have, addPkg)
+			walkFromIndex(mi, modPrefix, isMatch, treeCanMatch, tags, have, addPkg, ignorePatternsMap[root], root)
 			continue
 		} else if !errors.Is(err, modindex.ErrNotIndexed) {
 			m.AddError(err)
@@ -225,7 +225,7 @@ func matchPackages(ctx context.Context, m *search.Match, tags map[string]bool, f
 // walkFromIndex matches packages in a module using the module index. modroot
 // is the module's root directory on disk, index is the modindex.Module for the
 // module, and importPathRoot is the module's path prefix.
-func walkFromIndex(index *modindex.Module, importPathRoot string, isMatch, treeCanMatch func(string) bool, tags, have map[string]bool, addPkg func(string)) {
+func walkFromIndex(index *modindex.Module, importPathRoot string, isMatch, treeCanMatch func(string) bool, tags, have map[string]bool, addPkg func(string), ignorePatterns *search.IgnorePatterns, modRoot string) {
 	index.Walk(func(reldir string) {
 		// Avoid .foo, _foo, and testdata subdirectory trees.
 		p := reldir
@@ -248,6 +248,14 @@ func walkFromIndex(index *modindex.Module, importPathRoot string, isMatch, treeC
 			p = rest
 		}
 
+		if ignorePatterns != nil && ignorePatterns.ShouldIgnore(reldir) {
+			if cfg.BuildX {
+				absPath := filepath.Join(modRoot, reldir)
+				fmt.Fprintf(os.Stderr, "# ignoring directory %s\n", absPath)
+			}
+			return
+		}
+
 		// Don't use GOROOT/src.
 		if reldir == "" && importPathRoot == "" {
 			return

src/cmd/go/testdata/mod/example.com_ignore_v1.0.0.txt

@@ -0,0 +1,11 @@
+-- .mod --
+module example.com/ignore
+
+ignore ./foo
+-- .info --
+{"Version":"v1.0.0"}
+
+-- foo/foo.go --
+package foo
+
+const Bar = "Hello from foo!"

src/cmd/go/testdata/script/list_ignore_modcache.txt

@@ -0,0 +1,17 @@
+# go list should skip 'ignore' directives for indexed modules in the module cache
+# See golang.org/issue/42965
+
+env GOMODCACHE=$WORK${/}modcache
+go get example.com/ignore/...@v1.0.0
+go list -x example.com/ignore/...
+stderr 'ignoring directory '$GOMODCACHE''${/}'example.com'${/}'ignore@v1.0.0'${/}'foo'
+
+-- go.mod --
+module example
+
+go 1.24
+
+-- main.go --
+package main
+
+func main() {}