cmd/go: run vet on packages with only cgo files

CgoFiles is not included in GoFiles, so we need to check both.

Fixes #24193

Change-Id: I6a67bd912e3d9a4be0eae8fa8db6fa8a07fb5df3
Reviewed-on: https://go-review.googlesource.com/99175
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -3233,6 +3233,16 @@ func TestGoVetWithOnlyTestFiles(t *testing.T) {
 	tg.run("vet", "p")
 }
 
+// Issue 24193.
+func TestVetWithOnlyCgoFiles(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/p/p.go", "package p; import \"C\"; func F() {}")
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("vet", "p")
+}
+
 // Issue 9767, 19769.
 func TestGoGetDotSlashDownload(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)

src/cmd/go/internal/vet/vet.go

@@ -62,11 +62,11 @@ func runVet(cmd *base.Command, args []string) {
 			base.Errorf("%v", err)
 			continue
 		}
-		if len(ptest.GoFiles) == 0 && pxtest == nil {
+		if len(ptest.GoFiles) == 0 && len(ptest.CgoFiles) == 0 && pxtest == nil {
 			base.Errorf("go vet %s: no Go files in %s", p.ImportPath, p.Dir)
 			continue
 		}
-		if len(ptest.GoFiles) > 0 {
+		if len(ptest.GoFiles) > 0 || len(ptest.CgoFiles) > 0 {
 			root.Deps = append(root.Deps, b.VetAction(work.ModeBuild, work.ModeBuild, ptest))
 		}
 		if pxtest != nil {