Relax Transformer name rules

Relax the Transformer name to be any Go identifier or qualified identifier.
Clarify in the documentation how this is used (e.g., in Diff) and
what names are valid.

Fixes #102

cmp/options.go

@@ -7,6 +7,7 @@ package cmp
 import (
 	"fmt"
 	"reflect"
+	"regexp"
 	"strings"
 
 	"github.com/google/go-cmp/cmp/internal/function"
@@ -205,6 +206,11 @@ func (invalid) apply(s *state, _, _ reflect.Value) {
 	panic(fmt.Sprintf("cannot handle unexported field: %#v\n%s", s.curPath, help))
 }
 
+// identRx represents a valid identifier according to the Go specification.
+const identRx = `[_\p{L}][_\p{L}\p{N}]*`
+
+var identsRx = regexp.MustCompile(`^` + identRx + `(\.` + identRx + `)*$`)
+
 // Transformer returns an Option that applies a transformation function that
 // converts values of a certain type into that of another.
 //
@@ -222,7 +228,9 @@ func (invalid) apply(s *state, _, _ reflect.Value) {
 // to prevent the transformer from being recursively applied upon itself.
 //
 // The name is a user provided label that is used as the Transform.Name in the
-// transformation PathStep. If empty, an arbitrary name is used.
+// transformation PathStep (and eventually shown in the Diff output).
+// The name must be a valid identifier or qualified identifier in Go syntax.
+// If empty, an arbitrary name is used.
 func Transformer(name string, f interface{}) Option {
 	v := reflect.ValueOf(f)
 	if !function.IsType(v.Type(), function.Transformer) || v.IsNil() {
@@ -231,7 +239,7 @@ func Transformer(name string, f interface{}) Option {
 	if name == "" {
 		name = "λ" // Lambda-symbol as place-holder for anonymous transformer
 	}
-	if !isValid(name) {
+	if !identsRx.MatchString(name) {
 		panic(fmt.Sprintf("invalid name: %q", name))
 	}
 	tr := &transformer{name: name, fnc: reflect.ValueOf(f)}

cmp/options_test.go

@@ -113,10 +113,9 @@ func TestOptionPanic(t *testing.T) {
 		args:      []interface{}{"/*", func(int) bool { return true }},
 		wantPanic: "invalid name",
 	}, {
-		label:     "Transformer",
-		fnc:       Transformer,
-		args:      []interface{}{"_", func(int) bool { return true }},
-		wantPanic: "invalid name",
+		label: "Transformer",
+		fnc:   Transformer,
+		args:  []interface{}{"_", func(int) bool { return true }},
 	}, {
 		label:     "FilterPath",
 		fnc:       FilterPath,

cmp/path.go

@@ -296,14 +296,3 @@ func isExported(id string) bool {
 	r, _ := utf8.DecodeRuneInString(id)
 	return unicode.IsUpper(r)
 }
-
-// isValid reports whether the identifier is valid.
-// Empty and underscore-only strings are not valid.
-func isValid(id string) bool {
-	ok := id != "" && id != "_"
-	for j, c := range id {
-		ok = ok && (j > 0 || !unicode.IsDigit(c))
-		ok = ok && (c == '_' || unicode.IsLetter(c) || unicode.IsDigit(c))
-	}
-	return ok
-}