testing/cover: improve message when a package has no statements

Fixes golang#25492

Change-Id: Ic1496857524dad0c0a77f3bb80fa084c9bf00aa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/155777
Run-TryBot: Agniva De Sarker <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

src/cmd/go/go_test.go

@@ -2612,6 +2612,14 @@ func TestCoverageDepLoop(t *testing.T) {
 	tg.grepStdout("coverage: 100.0% of statements", "expected 100.0% coverage")
 }
 
+func TestCoverageNoStatements(t *testing.T) {
+	tooSlow(t)
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.run("test", "-cover", "./testdata/testcover/pkg4")
+	tg.grepStdout("[no statements]", "expected [no statements] for pkg4")
+}
+
 func TestCoverageImportMainLoop(t *testing.T) {
 	skipIfGccgo(t, "gccgo has no cover tool")
 	tg := testgo(t)

src/cmd/go/testdata/testcover/pkg4/a.go

@@ -0,0 +1,5 @@
+package pkg4
+
+type T struct {
+	X bool
+}

src/cmd/go/testdata/testcover/pkg4/a_test.go

@@ -0,0 +1,9 @@
+package pkg4
+
+import (
+	"testing"
+)
+
+func TestT(t *testing.T) {
+	_ = T{}
+}

src/testing/cover.go

@@ -109,7 +109,8 @@ func coverReport() {
 		}
 	}
 	if total == 0 {
-		total = 1
+		fmt.Println("coverage: [no statements]")
+		return
 	}
 	fmt.Printf("coverage: %.1f%% of statements%s\n", 100*float64(active)/float64(total), cover.CoveredPackages)
 }