strings: additional tests

This CL re-applies the tests added in CL 101330053 and subsequently rolled back in CL 102610043.

The original author of this change was Rui Ueyama <ruiu@google.com>

LGTM=r, ruiu
R=ruiu, r
CC=golang-codereviews
https://golang.org/cl/109170043

src/pkg/strings/replace_test.go

@@ -308,20 +308,21 @@ func TestReplacer(t *testing.T) {
 	}
 }
 
-// TestPickAlgorithm tests that NewReplacer picks the correct algorithm.
-func TestPickAlgorithm(t *testing.T) {
-	testCases := []struct {
+var algorithmTestCases = []struct {
 	r    *Replacer
 	want string
-	}{
+}{
 	{capitalLetters, "*strings.byteReplacer"},
 	{htmlEscaper, "*strings.byteStringReplacer"},
 	{NewReplacer("12", "123"), "*strings.singleStringReplacer"},
 	{NewReplacer("1", "12"), "*strings.byteStringReplacer"},
 	{NewReplacer("", "X"), "*strings.genericReplacer"},
 	{NewReplacer("a", "1", "b", "12", "cde", "123"), "*strings.genericReplacer"},
-	}
-	for i, tc := range testCases {
+}
+
+// TestPickAlgorithm tests that NewReplacer picks the correct algorithm.
+func TestPickAlgorithm(t *testing.T) {
+	for i, tc := range algorithmTestCases {
 		got := fmt.Sprintf("%T", tc.r.Replacer())
 		if got != tc.want {
 			t.Errorf("%d. algorithm = %s, want %s", i, got, tc.want)
@@ -329,6 +330,23 @@ func TestPickAlgorithm(t *testing.T) {
 	}
 }
 
+type errWriter struct{}
+
+func (errWriter) Write(p []byte) (n int, err error) {
+	return 0, fmt.Errorf("unwritable")
+}
+
+// TestWriteStringError tests that WriteString returns an error
+// received from the underlying io.Writer.
+func TestWriteStringError(t *testing.T) {
+	for i, tc := range algorithmTestCases {
+		n, err := tc.r.WriteString(errWriter{}, "abc")
+		if n != 0 || err == nil || err.Error() != "unwritable" {
+			t.Errorf("%d. WriteStringError = %d, %v, want 0, unwritable", i, n, err)
+		}
+	}
+}
+
 // TestGenericTrieBuilding verifies the structure of the generated trie. There
 // is one node per line, and the key ending with the current line is in the
 // trie if it ends with a "+".