context: improve valueCtx.String

Check for stringer on the key itself.
This is useful for locally defined context key types,
where there may be multiple instances of that type.
For example, see http.contextKey,
which can now be called after this change.

For the value itself, print the type at least
instead of just resorting to "<not stringer>".

Change-Id: I588ef1df34e90fb9ebd83cb180fea495e1fedaa8
Reviewed-on: https://go-review.googlesource.com/c/go/+/555697
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Joseph Tsai <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: dsnet

src/context/context.go

@@ -740,13 +740,13 @@ func stringify(v any) string {
 	case string:
 		return s
 	}
-	return "<not Stringer>"
+	return reflectlite.TypeOf(v).String()
 }
 
 func (c *valueCtx) String() string {
-	return contextName(c.Context) + ".WithValue(type " +
-		reflectlite.TypeOf(c.key).String() +
-		", val " + stringify(c.val) + ")"
+	return contextName(c.Context) + ".WithValue(" +
+		stringify(c.key) + ", " +
+		stringify(c.val) + ")"
 }
 
 func (c *valueCtx) Value(key any) any {

src/context/x_test.go

@@ -201,6 +201,8 @@ func TestCanceledTimeout(t *testing.T) {
 type key1 int
 type key2 int
 
+func (k key2) String() string { return fmt.Sprintf("%[1]T(%[1]d)", k) }
+
 var k1 = key1(1)
 var k2 = key2(1) // same int as k1, different type
 var k3 = key2(3) // same type as k2, different int
@@ -224,13 +226,17 @@ func TestValues(t *testing.T) {
 	c1 := WithValue(Background(), k1, "c1k1")
 	check(c1, "c1", "c1k1", "", "")
 
-	if got, want := fmt.Sprint(c1), `context.Background.WithValue(type context_test.key1, val c1k1)`; got != want {
+	if got, want := fmt.Sprint(c1), `context.Background.WithValue(context_test.key1, c1k1)`; got != want {
 		t.Errorf("c.String() = %q want %q", got, want)
 	}
 
 	c2 := WithValue(c1, k2, "c2k2")
 	check(c2, "c2", "c1k1", "c2k2", "")
 
+	if got, want := fmt.Sprint(c2), `context.Background.WithValue(context_test.key1, c1k1).WithValue(context_test.key2(1), c2k2)`; got != want {
+		t.Errorf("c.String() = %q want %q", got, want)
+	}
+
 	c3 := WithValue(c2, k3, "c3k3")
 	check(c3, "c2", "c1k1", "c2k2", "c3k3")