Allow using interface{} as map keys when decoding

Ref: #181

decode.go

@@ -305,10 +305,10 @@ func (md *MetaData) unifyStruct(mapping interface{}, rv reflect.Value) error {
 }
 
 func (md *MetaData) unifyMap(mapping interface{}, rv reflect.Value) error {
-	if k := rv.Type().Key().Kind(); k != reflect.String {
-		return fmt.Errorf(
-			"toml: cannot decode to a map with non-string key type (%s in %q)",
-			k, rv.Type())
+	keyType := rv.Type().Key().Kind()
+	if keyType != reflect.String && keyType != reflect.Interface {
+		return fmt.Errorf("toml: cannot decode to a map with non-string key type (%s in %q)",
+			keyType, rv.Type())
 	}
 
 	tmap, ok := mapping.(map[string]interface{})
@@ -334,7 +334,14 @@ func (md *MetaData) unifyMap(mapping interface{}, rv reflect.Value) error {
 		md.context = md.context[0 : len(md.context)-1]
 
 		rvkey := indirect(reflect.New(rv.Type().Key()))
-		rvkey.SetString(k)
+
+		switch keyType {
+		case reflect.Interface:
+			rvkey.Set(reflect.ValueOf(k))
+		case reflect.String:
+			rvkey.SetString(k)
+		}
+
 		rv.SetMapIndex(rvkey, rvval)
 	}
 	return nil

decode_test.go

@@ -430,38 +430,56 @@ func TestDecodeSizedInts(t *testing.T) {
 
 type NopUnmarshalTOML int
 
-func (NopUnmarshalTOML) UnmarshalTOML(p interface{}) error { return nil }
+func (n *NopUnmarshalTOML) UnmarshalTOML(p interface{}) error {
+	*n = 42
+	return nil
+}
 
 func TestDecodeTypes(t *testing.T) {
-	type mystr string
+	type (
+		mystr   string
+		myiface interface{}
+	)
 
 	for _, tt := range []struct {
-		v    interface{}
-		want string
+		v       interface{}
+		want    string
+		wantErr string
 	}{
-		{new(map[string]bool), ""},
-		{new(map[mystr]bool), ""},
-		{new(NopUnmarshalTOML), ""},
+		{new(map[string]bool), "&map[F:true]", ""},
+		{new(map[mystr]bool), "&map[F:true]", ""},
+		{new(NopUnmarshalTOML), "42", ""},
+		{new(map[interface{}]bool), "&map[F:true]", ""},
+		{new(map[myiface]bool), "&map[F:true]", ""},
 
-		{3, `toml: cannot decode to non-pointer "int"`},
-		{map[string]interface{}{}, `toml: cannot decode to non-pointer "map[string]interface {}"`},
+		{3, "", `toml: cannot decode to non-pointer "int"`},
+		{map[string]interface{}{}, "", `toml: cannot decode to non-pointer "map[string]interface {}"`},
 
-		{(*int)(nil), `toml: cannot decode to nil value of "*int"`},
-		{(*Unmarshaler)(nil), `toml: cannot decode to nil value of "*toml.Unmarshaler"`},
-		{nil, `toml: cannot decode to non-pointer <nil>`},
+		{(*int)(nil), "", `toml: cannot decode to nil value of "*int"`},
+		{(*Unmarshaler)(nil), "", `toml: cannot decode to nil value of "*toml.Unmarshaler"`},
+		{nil, "", `toml: cannot decode to non-pointer <nil>`},
 
-		{new(map[int]string), "toml: cannot decode to a map with non-string key type"},
-		{new(map[interface{}]string), "toml: cannot decode to a map with non-string key type"},
+		{new(map[int]string), "", "toml: cannot decode to a map with non-string key type"},
 
-		{new(struct{ F int }), `toml: line 1 (last key "F"): incompatible types: TOML value has type bool; destination has type integer`},
-		{new(map[string]int), `toml: line 1 (last key "F"): incompatible types: TOML value has type bool; destination has type integer`},
-		{new(int), `toml: cannot decode to type int`},
-		{new([]int), "toml: cannot decode to type []int"},
+		{new(struct{ F int }), "", `toml: line 1 (last key "F"): incompatible types: TOML value has type bool; destination has type integer`},
+		{new(map[string]int), "", `toml: line 1 (last key "F"): incompatible types: TOML value has type bool; destination has type integer`},
+		{new(int), "", `toml: cannot decode to type int`},
+		{new([]int), "", "toml: cannot decode to type []int"},
 	} {
 		t.Run(fmt.Sprintf("%T", tt.v), func(t *testing.T) {
 			_, err := Decode(`F = true`, tt.v)
-			if !errorContains(err, tt.want) {
-				t.Errorf("wrong error\nhave: %q\nwant: %q", err, tt.want)
+			if !errorContains(err, tt.wantErr) {
+				t.Fatalf("wrong error\nhave: %q\nwant: %q", err, tt.wantErr)
+			}
+
+			if err == nil {
+				have := fmt.Sprintf("%v", tt.v)
+				if n, ok := tt.v.(*NopUnmarshalTOML); ok {
+					have = fmt.Sprintf("%v", *n)
+				}
+				if have != tt.want {
+					t.Errorf("\nhave: %s\nwant: %s", have, tt.want)
+				}
 			}
 		})
 	}