cmd/go: skip vet when package cannot be build under "go test/vet"

If the the package cannot be built,
"go test" and "go vet" should not run the "vet" tool.
In that case only errors from the compilers will be displayed.

Fixes #26125

Change-Id: I5da6ba64bae5f44feaf5bd4e765eea85533cddd4
Reviewed-on: https://go-review.googlesource.com/123938
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -5526,6 +5526,21 @@ func TestTestVet(t *testing.T) {
 	tg.grepStderrNot(`invalid.*constraint`, "did diagnose bad build constraint in vetxonly mode")
 }
 
+func TestTestSkipVetAfterFailedBuild(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("x_test.go", `package x
+		func f() {
+			return 1
+		}
+	`)
+
+	tg.runFail("test", tg.path("x_test.go"))
+	tg.grepStderrNot(`vet`, "vet should be skipped after the failed build")
+}
+
 func TestTestVetRebuild(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()

src/cmd/go/internal/work/exec.go

@@ -915,14 +915,18 @@ func (b *Builder) vet(a *Action) error {
 
 	a.Failed = false // vet of dependency may have failed but we can still succeed
 
+	if a.Deps[0].Failed {
+		// The build of the package has failed. Skip vet check.
+		// Vet could return export data for non-typecheck errors,
+		// but we ignore it because the package cannot be compiled.
+		return nil
+	}
+
 	vcfg := a.Deps[0].vetCfg
 	if vcfg == nil {
 		// Vet config should only be missing if the build failed.
-		if !a.Deps[0].Failed {
 		return fmt.Errorf("vet config not found")
 	}
-		return nil
-	}
 
 	vcfg.VetxOnly = a.VetxOnly
 	vcfg.VetxOutput = a.Objdir + "vet.out"