cmd/go: give full import stack for errors in dependencies of test dependencies

Fixes #9558.

Change-Id: I68506af58088155d38d492b49b19c5fc2048b087
Reviewed-on: https://go-review.googlesource.com/12176Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/go_test.go

@@ -2094,3 +2094,17 @@ func TestGoTestRaceInstallCgo(t *testing.T) {
 		t.Fatalf("go test -i runtime/race reinstalled cmd/cgo")
 	}
 }
+
+func TestGoTestImportErrorStack(t *testing.T) {
+	const out = `package testdep/p1 (test)
+	imports testdep/p2
+	imports testdep/p3: no buildable Go source files`
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.runFail("test", "testdep/p1")
+	if !strings.Contains(tg.stderr.String(), out) {
+		t.Fatal("did not give full import stack:\n\n%s", tg.stderr.String())
+	}
+}

src/cmd/go/test.go

@@ -578,6 +578,11 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		if p1.Error != nil {
 			return nil, nil, nil, p1.Error
 		}
+		if len(p1.DepsErrors) > 0 {
+			err := p1.DepsErrors[0]
+			err.Pos = "" // show full import stack
+			return nil, nil, nil, err
+		}
 		if contains(p1.Deps, p.ImportPath) || p1.ImportPath == p.ImportPath {
 			// Same error that loadPackage returns (via reusePackage) in pkg.go.
 			// Can't change that code, because that code is only for loading the
@@ -604,6 +609,11 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		if p1.Error != nil {
 			return nil, nil, nil, p1.Error
 		}
+		if len(p1.DepsErrors) > 0 {
+			err := p1.DepsErrors[0]
+			err.Pos = "" // show full import stack
+			return nil, nil, nil, err
+		}
 		ximports = append(ximports, p1)
 		p.XTestImports[i] = p1.ImportPath
 	}