internal/gcimporter: set correct Package.Path for main packages

The compiler emits Path="main" for main packages because that's
the linker symbol prefix that it uses; but go/packages and x/tools
in general wants types.Package.Path to represent the path
as reported by go list.

This CL works around the bug by decoding path="main" as the
correct path for the package being decoded.

+ a test in go/packages

Fixes golang/go#70742

Change-Id: I90f9234976cbfbe2a39c4da4852bc4a134fffd88
Reviewed-on: https://go-review.googlesource.com/c/tools/+/634922
Reviewed-by: Tim King <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

go/packages/packages_test.go

@@ -3339,6 +3339,31 @@ func Foo() int { return a.Foo() }
 	t.Logf("Packages: %+v", pkgs)
 }
 
+// TestMainPackagePathInModeTypes tests (*types.Package).Path() for
+// main packages in mode NeedTypes, a regression test for #70742, a
+// bug in cmd/compile's export data that caused them to appear as
+// "main". (The PkgPath field was always correct.)
+func TestMainPackagePathInModeTypes(t *testing.T) {
+	testenv.NeedsGoPackages(t)
+
+	cfg := &packages.Config{Mode: packages.NeedName | packages.NeedTypes}
+	pkgs, err := packages.Load(cfg, "cmd/go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	p := pkgs[0]
+	if p.PkgPath != "cmd/go" ||
+		p.Name != "main" ||
+		p.Types.Path() != "cmd/go" ||
+		p.Types.Name() != "main" {
+		t.Errorf("PkgPath=%q Name=%q Types.Path=%q Types.Name=%q; want (cmd/go, main) both times)",
+			p.PkgPath,
+			p.Name,
+			p.Types.Name(),
+			p.Types.Path())
+	}
+}
+
 func writeTree(t *testing.T, archive string) string {
 	root := t.TempDir()
 

internal/gcimporter/ureader_yes.go

@@ -264,7 +264,12 @@ func (pr *pkgReader) pkgIdx(idx pkgbits.Index) *types.Package {
 func (r *reader) doPkg() *types.Package {
 	path := r.String()
 	switch path {
-	case "":
+	// cmd/compile emits path="main" for main packages because
+	// that's the linker symbol prefix it used; but we need
+	// the package's path as it would be reported by go list,
+	// hence "main" below.
+	// See test at go/packages.TestMainPackagePathInModeTypes.
+	case "", "main":
 		path = r.p.PkgPath()
 	case "builtin":
 		return nil // universe