Allow custom string types as map keys (#414)

Because the map keys were converted to []string,
rv.MapIndex(reflect.ValueOf(mapKey)) would panic as it's the "wrong"
type (string, instead of the custom string type).

Just keep track of the reflect.Value instead; don't really need to
convert it to strings.

Fixes #413

encode.go

@@ -402,31 +402,30 @@ func (enc *Encoder) eMap(key Key, rv reflect.Value, inline bool) {
 
 	// Sort keys so that we have deterministic output. And write keys directly
 	// underneath this key first, before writing sub-structs or sub-maps.
-	var mapKeysDirect, mapKeysSub []string
+	var mapKeysDirect, mapKeysSub []reflect.Value
 	for _, mapKey := range rv.MapKeys() {
-		k := mapKey.String()
 		if typeIsTable(tomlTypeOfGo(eindirect(rv.MapIndex(mapKey)))) {
-			mapKeysSub = append(mapKeysSub, k)
+			mapKeysSub = append(mapKeysSub, mapKey)
 		} else {
-			mapKeysDirect = append(mapKeysDirect, k)
+			mapKeysDirect = append(mapKeysDirect, mapKey)
 		}
 	}
 
-	var writeMapKeys = func(mapKeys []string, trailC bool) {
-		sort.Strings(mapKeys)
+	writeMapKeys := func(mapKeys []reflect.Value, trailC bool) {
+		sort.Slice(mapKeys, func(i, j int) bool { return mapKeys[i].String() < mapKeys[j].String() })
 		for i, mapKey := range mapKeys {
-			val := eindirect(rv.MapIndex(reflect.ValueOf(mapKey)))
+			val := eindirect(rv.MapIndex(mapKey))
 			if isNil(val) {
 				continue
 			}
 
 			if inline {
-				enc.writeKeyValue(Key{mapKey}, val, true)
+				enc.writeKeyValue(Key{mapKey.String()}, val, true)
 				if trailC || i != len(mapKeys)-1 {
 					enc.wf(", ")
 				}
 			} else {
-				enc.encode(key.add(mapKey), val)
+				enc.encode(key.add(mapKey.String()), val)
 			}
 		}
 	}

encode_test.go

@@ -837,10 +837,13 @@ func TestEncodeJSONNumber(t *testing.T) {
 }
 
 func TestEncode(t *testing.T) {
-	type Embedded struct {
-		Int int `toml:"_int"`
-	}
-	type NonStruct int
+	type (
+		Embedded struct {
+			Int int `toml:"_int"`
+		}
+		NonStruct int
+		MyInt     int
+	)
 
 	date := time.Date(2014, 5, 11, 19, 30, 40, 0, time.UTC)
 	dateStr := "2014-05-11T19:30:40Z"
@@ -1165,6 +1168,10 @@ ArrayOfMixedSlices = [[1, 2], ["a", "b"]]
 			input:     map[int]string{1: ""},
 			wantError: errNonString,
 		},
+		"(error) map no string key indirect": {
+			input:     map[MyInt]string{1: ""},
+			wantError: errNonString,
+		},
 
 		"tbl-in-arr-struct": {
 			input: struct {
@@ -1279,7 +1286,7 @@ func encodeExpected(t *testing.T, label string, val any, want string, wantErr er
 				if wantErr == errAnything && err != nil {
 					return
 				}
-				t.Errorf("want Encode error %v, got %v", wantErr, err)
+				t.Errorf("wrong error:\nwant: %v\nhave: %v", wantErr, err)
 			} else {
 				t.Errorf("Encode failed: %s", err)
 			}
@@ -1297,3 +1304,40 @@ func encodeExpected(t *testing.T, label string, val any, want string, wantErr er
 		}
 	})
 }
+
+func TestMapCustomKeytype(t *testing.T) {
+	type (
+		MyString string
+		MyMap    map[MyString]any
+	)
+
+	m := MyMap{
+		"k1":     "a",
+		"nested": MyMap{"k2": "b"},
+	}
+	have := new(bytes.Buffer)
+	err := NewEncoder(have).Encode(m)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := `
+k1 = "a"
+
+[nested]
+  k2 = "b"
+`[1:]
+
+	if have.String() != want {
+		t.Fatalf("\nhave: %q\nwant: %q", have, want)
+	}
+
+	var m2 MyMap
+	_, err = Decode(have.String(), &m2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if h, w := fmt.Sprintf("%s", m2), fmt.Sprintf("%s", m); h != w {
+		t.Fatalf("\nhave: %s\nwant: %s", h, w)
+	}
+}