cow_to_ascii_lowercase()

components/style/str.rs

@@ -3,6 +3,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use num_traits::ToPrimitive;
+use std::ascii::AsciiExt;
+use std::borrow::Cow;
 use std::convert::AsRef;
 use std::iter::{Filter, Peekable};
 use std::str::Split;
@@ -125,3 +127,13 @@ pub fn str_join<I, T>(strs: I, join: &str) -> String
         acc
     })
 }
+
+/// Like AsciiExt::to_ascii_lowercase, but avoids allocating when the input is already lower-case.
+pub fn cow_into_ascii_lowercase<'a, S: Into<Cow<'a, str>>>(s: S) -> Cow<'a, str> {
+    let mut cow = s.into();
+    match cow.bytes().position(|byte| byte >= b'A' && byte <= b'Z') {
+        Some(first_uppercase) => cow.to_mut()[first_uppercase..].make_ascii_lowercase(),
+        None => {}
+    }
+    cow
+}