strings: fix Replacer bug with prefix matches

singleStringReplacer had a bug where if a string was replaced
at the beginning and no output had yet been produced into the
temp buffer before matching ended, an invalid nil check (used
as a proxy for having matched anything) meant it always
returned its input.

Fixes #6659

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/16880043

src/pkg/strings/replace.go

@@ -364,17 +364,18 @@ func makeSingleStringReplacer(pattern string, value string) *singleStringReplace
 
 func (r *singleStringReplacer) Replace(s string) string {
 	var buf []byte
-	i := 0
+	i, matched := 0, false
 	for {
 		match := r.finder.next(s[i:])
 		if match == -1 {
 			break
 		}
+		matched = true
 		buf = append(buf, s[i:i+match]...)
 		buf = append(buf, r.value...)
 		i += match + len(r.finder.pattern)
 	}
-	if buf == nil {
+	if !matched {
 		return s
 	}
 	buf = append(buf, s[i:]...)

src/pkg/strings/replace_test.go

@@ -261,10 +261,21 @@ func TestReplacer(t *testing.T) {
 	testCases = append(testCases,
 		testCase{abcMatcher, "", ""},
 		testCase{abcMatcher, "ab", "ab"},
+		testCase{abcMatcher, "abc", "[match]"},
 		testCase{abcMatcher, "abcd", "[match]d"},
 		testCase{abcMatcher, "cabcabcdabca", "c[match][match]d[match]a"},
 	)
 
+	// Issue 6659 cases (more single string replacer)
+
+	noHello := NewReplacer("Hello", "")
+	testCases = append(testCases,
+		testCase{noHello, "Hello", ""},
+		testCase{noHello, "Hellox", "x"},
+		testCase{noHello, "xHello", "x"},
+		testCase{noHello, "xHellox", "xx"},
+	)
+
 	// No-arg test cases.
 
 	nop := NewReplacer()