encoding/json: allow non-string type keys for (un-)marshal

This CL allows JSON-encoding & -decoding maps whose keys are types that
implement encoding.TextMarshaler / TextUnmarshaler.

During encode, the map keys are marshaled upfront so that they can be
sorted.

Fixes #12146

Change-Id: I43809750a7ad82a3603662f095c7baf75fd172da
Reviewed-on: https://go-review.googlesource.com/20356
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index c00491e00c..eed40a4272 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -536,3 +536,19 @@ func TestEncodeString(t *testing.T) {
 		}
 	}
 }
+
+func TestTextMarshalerMapKeysAreSorted(t *testing.T) {
+	b, err := Marshal(map[unmarshalerText]int{
+		{"x", "y"}: 1,
+		{"y", "x"}: 2,
+		{"a", "z"}: 3,
+		{"z", "a"}: 4,
+	})
+	if err != nil {
+		t.Fatalf("Failed to Marshal text.Marshaler: %v", err)
+	}
+	const want = `{"a:z":3,"x:y":1,"y:x":2,"z:a":4}`
+	if string(b) != want {
+		t.Errorf("Marshal map with text.Marshaler keys: got %#q, want %#q", b, want)
+	}
+}