2 files changed, 42 insertions, 1 deletions
diff --git a/src/strings/strings.go b/src/strings/strings.go
index 73bba9278c..8520f8a732 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -571,7 +571,33 @@ func ToUpper(s string) string {
 }
 
 // ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
-func ToLower(s string) string { return Map(unicode.ToLower, s) }
+func ToLower(s string) string {
+	isASCII, hasUpper := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasUpper = hasUpper || (c >= 'A' && c <= 'Z')
+	}
+
+	if isASCII { // optimize for ASCII-only strings.
+		if !hasUpper {
+			return s
+		}
+		b := make([]byte, len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if c >= 'A' && c <= 'Z' {
+				c += 'a' - 'A'
+			}
+			b[i] = c
+		}
+		return string(b)
+	}
+	return Map(unicode.ToLower, s)
+}
 
 // ToTitle returns a copy of the string s with all Unicode letters mapped to their title case.
 func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index b185e7eec8..289dd92d51 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -532,6 +532,8 @@ var lowerTests = []StringTest{
 	{"abc", "abc"},
 	{"AbC123", "abc123"},
 	{"azAZ09_", "azaz09_"},
+	{"longStrinGwitHmixofsmaLLandcAps", "longstringwithmixofsmallandcaps"},
+	{"LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS", "long\u0250string\u0250with\u0250nonascii\u0250chars"},
 	{"\u2C6D\u2C6D\u2C6D\u2C6D\u2C6D", "\u0251\u0251\u0251\u0251\u0251"}, // shrinks one byte per char
 }
 
@@ -664,6 +666,19 @@ func BenchmarkToUpper(b *testing.B) {
 	}
 }
 
+func BenchmarkToLower(b *testing.B) {
+	for _, tc := range lowerTests {
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToLower(tc.in)
+				if actual != tc.out {
+					b.Errorf("ToLower(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
 func BenchmarkMapNoChanges(b *testing.B) {
 	identity := func(r rune) rune {
 		return r