%q in fmt: if the object is a Stringer, use String() to get the value to quote.

R=rsc
CC=golang-dev
https://golang.org/cl/224051

src/pkg/fmt/fmt_test.go

@@ -217,6 +217,9 @@ var fmttests = []fmtTest{
 	fmtTest{"%+v", B{1, 2}, `{i:<1> j:2}`},
 	fmtTest{"%+v", C{1, B{2, 3}}, `{i:1 B:{i:<2> j:3}}`},
 
+	// q on Stringable items
+	fmtTest{"%q", I(23), `"<23>"`},
+
 	// %p on non-pointers
 	fmtTest{"%p", make(chan int), "PTR"},
 	fmtTest{"%p", make(map[int]int), "PTR"},

src/pkg/fmt/print.go

@@ -912,6 +912,13 @@ func (p *pp) doprintf(format string, a []interface{}) {
 				goto badtype
 			}
 		case 'q':
+			if field != nil {
+				// if object implements String, use the result.
+				if stringer, ok := field.(Stringer); ok {
+					p.fmt.fmt_q(stringer.String())
+					break
+				}
+			}
 			if v, ok := getString(field); ok {
 				p.fmt.fmt_q(v)
 			} else {

src/pkg/fmt/stringer_test.go

@@ -41,7 +41,7 @@ func (v TF) String() string   { return Sprintf("F: %f", v) }
 func (v TF32) String() string { return Sprintf("F32: %f", v) }
 func (v TF64) String() string { return Sprintf("F64: %f", v) }
 func (v TB) String() string   { return Sprintf("B: %t", v) }
-func (v TS) String() string   { return Sprintf("S: %q", v) }
+func (v TS) String() string   { return Sprintf("S: %q", string(v)) }
 
 func check(t *testing.T, got, want string) {
 	if got != want {