cmd/cover: do not report coverage for assembly functions

cover -func mode was reporting a coverage for function
declarations without bodies - assembly functions.
Since we are not annotating their code, we have no data
for those functions and should not report them at all.

Fixes #6880.

Change-Id: I4b8cd90805accf61f54e3ee167f54f4dc10c7c59
Reviewed-on: https://go-review.googlesource.com/77152Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cover/func.go

@@ -113,6 +113,10 @@ type FuncVisitor struct {
 func (v *FuncVisitor) Visit(node ast.Node) ast.Visitor {
 	switch n := node.(type) {
 	case *ast.FuncDecl:
+		if n.Body == nil {
+			// Do not count declarations of assembly functions.
+			break
+		}
 		start := v.fset.Position(n.Pos())
 		end := v.fset.Position(n.End())
 		fe := &FuncExtent{

src/cmd/go/go_test.go

@@ -2457,6 +2457,19 @@ func TestCoverageErrorLine(t *testing.T) {
 	}
 }
 
+func TestCoverageFunc(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+
+	tg.run("test", "-coverprofile="+filepath.Join(tg.tempdir, "cover.out"), "coverasm")
+	tg.run("tool", "cover", "-func="+filepath.Join(tg.tempdir, "cover.out"))
+	tg.grepStdout(`\tg\t*100.0%`, "did not find g 100% covered")
+	tg.grepStdoutNot(`\tf\t*[0-9]`, "reported coverage for assembly function f")
+}
+
 func TestPluginNonMain(t *testing.T) {
 	wd, err := os.Getwd()
 	if err != nil {

src/cmd/go/testdata/src/coverasm/p.go

+package p
+
+func f()
+
+func g() {
+	println("g")
+}

src/cmd/go/testdata/src/coverasm/p.s

+// empty asm file,
+// so go test doesn't complain about declaration of f in p.go.

src/cmd/go/testdata/src/coverasm/p_test.go

+package p
+
+import "testing"
+
+func Test(t *testing.T) {
+	g()
+}