cmd/go: show position in error for wrong signature in test functions

Change-Id: Ie915dc2fc32a31d31f566ac931ccecb506559645
Reviewed-on: https://go-review.googlesource.com/19888Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/test.go

@@ -1352,14 +1352,16 @@ func (t *testFuncs) load(filename, pkg string, doImport, seen *bool) error {
 			t.TestMain = &testFunc{pkg, name, ""}
 			*doImport, *seen = true, true
 		case isTest(name, "Test"):
-			if !isTestFunc(n, "T") {
-				return fmt.Errorf("wrong type for %s", name)
+			err := checkTestFunc(n, "T")
+			if err != nil {
+				return err
 			}
 			t.Tests = append(t.Tests, testFunc{pkg, name, ""})
 			*doImport, *seen = true, true
 		case isTest(name, "Benchmark"):
-			if !isTestFunc(n, "B") {
-				return fmt.Errorf("wrong type for %s", name)
+			err := checkTestFunc(n, "B")
+			if err != nil {
+				return err
 			}
 			t.Benchmarks = append(t.Benchmarks, testFunc{pkg, name, ""})
 			*doImport, *seen = true, true
@@ -1379,6 +1381,15 @@ func (t *testFuncs) load(filename, pkg string, doImport, seen *bool) error {
 	return nil
 }
 
+func checkTestFunc(fn *ast.FuncDecl, arg string) error {
+	if !isTestFunc(fn, arg) {
+		name := fn.Name.String()
+		pos := testFileSet.Position(fn.Pos())
+		return fmt.Errorf("%s: wrong signature for %s, must be: func %s(%s *testing.%s)", pos, name, name, strings.ToLower(arg), arg)
+	}
+	return nil
+}
+
 type byOrder []*doc.Example
 
 func (x byOrder) Len() int           { return len(x) }