cmd/go: avoid indexing GOROOT packages when the compiler is 'gccgo'

The gccgo compiler does not load standard-library packages from
GOROOT/src, so we cannot load those packages from the GOROOT/src
index when using that compiler.

This fixes TestScript/gccgo_link_c (and perhaps other gccgo tests)
when a 'gccgo' executable is present. Unfortunately, only a few
builders caught the broken test because 'gccgo' is not installed
on most Go project builders (see #35786).

For #53577.
Fixes #53815.

Change-Id: I11a5cf6dbf4ac9893c4d02bd6ab7ef60f67b1e87
Reviewed-on: https://go-review.googlesource.com/c/go/+/417094
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

src/cmd/go/internal/modindex/read.go

@@ -139,6 +139,9 @@ func GetPackage(modroot, pkgdir string) (*IndexPackage, error) {
 	if !errors.Is(err, errNotFromModuleCache) {
 		return nil, err
 	}
+	if cfg.BuildContext.Compiler == "gccgo" && str.HasPathPrefix(modroot, cfg.GOROOTsrc) {
+		return nil, err // gccgo has no sources for GOROOT packages.
+	}
 	return openIndexPackage(modroot, pkgdir)
 }
 

src/go/build/build.go

@@ -715,6 +715,9 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				tried.goroot = dir
 			}
 			if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
+				// TODO(bcmills): Setting p.Dir here is misleading, because gccgo
+				// doesn't actually load its standard-library packages from this
+				// directory. See if we can leave it unset.
 				p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
 				p.Goroot = true
 				p.Root = ctxt.GOROOT