expand: fix SplitBraces v3 TODO

Leave an internal TODO for skipping the allocations if there are no
brace expansions. The important bit is to simplify the API, as we can't
do that once v3.0.0 is out.

expand/braces_test.go

@@ -156,7 +156,7 @@ func TestBraces(t *testing.T) {
 			wantStr := printWords(tc.want...)
 			wantBraceExpParts(t, tc.in, false)
 
-			inBraces, _ := syntax.SplitBraces(tc.in)
+			inBraces := syntax.SplitBraces(tc.in)
 			wantBraceExpParts(t, inBraces, inStr != wantStr)
 
 			got := Braces(inBraces)

expand/expand.go

@@ -365,11 +365,11 @@ func Fields(cfg *Config, words ...*syntax.Word) ([]string, error) {
 	fields := make([]string, 0, len(words))
 	dir := cfg.envGet("PWD")
 	for _, word := range words {
-		expBraces := []*syntax.Word{word}
-		if expWord, any := syntax.SplitBraces(word); any {
-			expBraces = Braces(expWord)
+		afterBraces := []*syntax.Word{word}
+		if w2 := syntax.SplitBraces(word); w2 != word {
+			afterBraces = Braces(w2)
 		}
-		for _, word2 := range expBraces {
+		for _, word2 := range afterBraces {
 			wfields, err := cfg.wordFields(word2.Parts)
 			if err != nil {
 				return nil, err

syntax/braces.go

@@ -12,16 +12,16 @@ var (
 	litRightBrace = &Lit{Value: "}"}
 )
 
-// TODO: remove bool return parameter, make it equivalent to input != output.
-
-// SplitBraces may introduce a number of BraceExp parts to a word, which can be
-// used to perform brace expansion with expand.Braces. For example, passing it a
-// literal word "foo{bar,baz}" will result in a word containing the literal
-// "foo", and a brace expansion with the elements "bar" and "baz".
+// SplitBraces will return a new Word with a number of BraceExp parts, if the
+// input word contained brace expansions within its literal parts. Otherwise,
+// the input word is returned intact.
+//
+// For example, a literal word "foo{bar,baz}" will result in a word containing
+// the literal "foo", and a brace expansion with the elements "bar" and "baz".
 //
 // It does not return an error; malformed brace expansions are simply skipped.
 // For example, the literal word "a{b" is returned unchanged.
-func SplitBraces(word *Word) (*Word, bool) {
+func SplitBraces(word *Word) *Word {
 	any := false
 	top := &Word{}
 	acc := top
@@ -169,5 +169,9 @@ func SplitBraces(word *Word) (*Word, bool) {
 			acc.Parts = append(acc.Parts, elem.Parts...)
 		}
 	}
-	return top, any
+	if !any {
+		// TODO: avoid allocations in this case
+		return word
+	}
+	return top
 }