go/packages: internally expose ForTests in go/packages

This change eliminates our need to guess what the package under test is
in gopls, since `go list` always knows the answer.

Change-Id: I16e482ed3b452bd57cd478b1f8280fcea56474d3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/215020
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

go/packages/golist.go

@@ -729,6 +729,7 @@ func golistDriver(cfg *Config, rootsDirs func() *goInfo, words ...string) (*driv
 			GoFiles:         absJoin(p.Dir, p.GoFiles, p.CgoFiles),
 			CompiledGoFiles: absJoin(p.Dir, p.CompiledGoFiles),
 			OtherFiles:      absJoin(p.Dir, otherFiles(p)...),
+			forTest:         p.ForTest,
 		}
 
 		// Work around https://golang.org/issue/28749:

go/packages/packages.go

@@ -23,6 +23,7 @@ import (
 	"sync"
 
 	"golang.org/x/tools/go/gcexportdata"
+	"golang.org/x/tools/internal/packagesinternal"
 )
 
 // A LoadMode controls the amount of detail to return when loading.
@@ -292,6 +293,15 @@ type Package struct {
 
 	// TypesSizes provides the effective size function for types in TypesInfo.
 	TypesSizes types.Sizes
+
+	// forTest is the package under test, if any.
+	forTest string
+}
+
+func init() {
+	packagesinternal.GetForTest = func(p interface{}) string {
+		return p.(*Package).forTest
+	}
 }
 
 // An Error describes a problem with a package's metadata, syntax, or types.

go/packages/packages_test.go

@@ -27,6 +27,7 @@ import (
 
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/tools/go/packages/packagestest"
+	"golang.org/x/tools/internal/packagesinternal"
 	"golang.org/x/tools/internal/testenv"
 )
 
@@ -2588,6 +2589,52 @@ func testCycleImportStack(t *testing.T, exporter packagestest.Exporter) {
 	}
 }
 
+func TestForTestField(t *testing.T) {
+	packagestest.TestAll(t, testForTestField)
+}
+func testForTestField(t *testing.T, exporter packagestest.Exporter) {
+	exported := packagestest.Export(t, exporter, []packagestest.Module{{
+		Name: "golang.org/fake",
+		Files: map[string]interface{}{
+			"a/a.go":      `package a; func hello() {};`,
+			"a/a_test.go": `package a; import "testing"; func TestA1(t *testing.T) {};`,
+			"a/x_test.go": `package a_test; import "testing"; func TestA2(t *testing.T) {};`,
+		}}})
+	defer exported.Cleanup()
+
+	// Add overlays to make sure they don't affect anything.
+	exported.Config.Overlay = map[string][]byte{
+		"a/a_test.go": []byte(`package a; import "testing"; func TestA1(t *testing.T) { hello(); };`),
+		"a/x_test.go": []byte(`package a_test; import "testing"; func TestA2(t *testing.T) { hello(); };`),
+	}
+	exported.Config.Tests = true
+	exported.Config.Mode = packages.NeedName | packages.NeedImports
+	forTest := "golang.org/fake/a"
+	pkgs, err := packages.Load(exported.Config, forTest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(pkgs) != 4 {
+		t.Errorf("expected 4 packages, got %v", len(pkgs))
+	}
+	for _, pkg := range pkgs {
+		var hasTestFile bool
+		for _, f := range pkg.CompiledGoFiles {
+			if strings.Contains(f, "a_test.go") || strings.Contains(f, "x_test.go") {
+				hasTestFile = true
+				break
+			}
+		}
+		if !hasTestFile {
+			continue
+		}
+		got := packagesinternal.GetForTest(pkg)
+		if got != forTest {
+			t.Errorf("expected %q, got %q", forTest, got)
+		}
+	}
+}
+
 func errorMessages(errors []packages.Error) []string {
 	var msgs []string
 	for _, err := range errors {

internal/lsp/cache/load.go

@@ -13,6 +13,7 @@ import (
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/lsp/telemetry"
+	"golang.org/x/tools/internal/packagesinternal"
 	"golang.org/x/tools/internal/span"
 	"golang.org/x/tools/internal/telemetry/log"
 	"golang.org/x/tools/internal/telemetry/tag"
@@ -26,6 +27,7 @@ type metadata struct {
 	name            string
 	goFiles         []span.URI
 	compiledGoFiles []span.URI
+	forTest         packagePath
 	typesSizes      types.Sizes
 	errors          []packages.Error
 	deps            []packageID
@@ -179,6 +181,7 @@ func (s *snapshot) updateImports(ctx context.Context, pkgPath packagePath, pkg *
 		id:         id,
 		pkgPath:    pkgPath,
 		name:       pkg.Name,
+		forTest:    packagePath(packagesinternal.GetForTest(pkg)),
 		typesSizes: pkg.TypesSizes,
 		errors:     pkg.Errors,
 		config:     cfg,

internal/lsp/cache/view.go

@@ -576,7 +576,11 @@ func (v *view) initialize(ctx context.Context, s *snapshot) {
 				}
 				// A test variant of a package can only be loaded directly by loading
 				// the non-test variant with -test. Track the import path of the non-test variant.
-				s.setWorkspacePackage(m.id, m.pkgPath)
+				pkgPath := m.pkgPath
+				if m.forTest != "" {
+					pkgPath = m.forTest
+				}
+				s.setWorkspacePackage(m.id, pkgPath)
 				if _, err := s.packageHandle(ctx, m.id); err != nil {
 					return err
 				}

internal/packagesinternal/packages.go

@@ -0,0 +1,4 @@
+// Package packagesinternal exposes internal-only fields from go/packages.
+package packagesinternal
+
+var GetForTest = func(p interface{}) string { return "" }