go/importer: disable TestForCompiler/LookupCustom for GOEXPERIMENT=un…

…ified

The TestForCompiler/LookupCustom test tries to read in the export data
for "math/big", but with a package path of "math/bigger" instead. This
has historically worked because the export data formats were designed
to not assume the package's own path, but I expect we can safely
remove support for this now.

However, since that would be a user-visible change, for now just
disable the test for GOEXPERIMENT=unified so we can land CL 393715. We
can revisit whether it's actually safe to break that go/importer use
case later.

Updates #51734.

Change-Id: I5e89314511bd1352a9f5e14a2e218a5ab00cab3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/406319
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/importer/importer_test.go

@@ -7,6 +7,7 @@ package importer
 import (
 	"go/build"
 	"go/token"
+	"internal/buildcfg"
 	"internal/testenv"
 	"io"
 	"os"
@@ -67,6 +68,14 @@ func TestForCompiler(t *testing.T) {
 	})
 
 	t.Run("LookupCustom", func(t *testing.T) {
+		// TODO(mdempsky): Decide whether to remove this test, or to fix
+		// support for it in unified IR. It's not clear that we actually
+		// need to support importing "math/big" as "math/bigger", for
+		// example. cmd/link no longer supports that.
+		if buildcfg.Experiment.Unified {
+			t.Skip("not supported by GOEXPERIMENT=unified; see go.dev/cl/406319")
+		}
+
 		lookup := func(path string) (io.ReadCloser, error) {
 			if path != "math/bigger" {
 				t.Fatalf("lookup called with unexpected path %q", path)