cow_to_ascii_lowercase()

Cargo.lock

@@ -2636,6 +2636,7 @@ dependencies = [
  "cssparser 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "euclid 0.10.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "html5ever-atoms 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "matches 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "owning_ref 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",

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
+}

tests/unit/style/Cargo.toml

@@ -16,6 +16,7 @@ testing = ["style/testing"]
 app_units = "0.3"
 cssparser = {version = "0.7", features = ["heap_size"]}
 euclid = "0.10.1"
+matches = "0.1"
 owning_ref = "0.2.2"
 parking_lot = "0.3"
 rustc-serialize = "0.3"

tests/unit/style/lib.rs

@@ -10,6 +10,7 @@ extern crate app_units;
 extern crate cssparser;
 extern crate euclid;
 #[macro_use] extern crate html5ever_atoms;
+#[macro_use] #[allow(unused_extern_crates)] extern crate matches;
 extern crate owning_ref;
 extern crate parking_lot;
 extern crate rustc_serialize;

tests/unit/style/str.rs

@@ -2,7 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use style::str::{split_html_space_chars, str_join};
+use std::borrow::Cow;
+use style::str::{split_html_space_chars, str_join, cow_into_ascii_lowercase};
 
 #[test]
 pub fn split_html_space_chars_whitespace() {
@@ -33,3 +34,13 @@ pub fn test_str_join_many() {
     let expected = "-alpha--beta-gamma-";
     assert_eq!(actual, expected);
 }
+
+#[test]
+pub fn test_cow_into_ascii_lowercase() {
+    assert!(matches!(cow_into_ascii_lowercase("abc.d"), Cow::Borrowed("abc.d")));
+    let string = String::from("abc.d");
+    assert!(matches!(cow_into_ascii_lowercase(string), Cow::Owned(ref s) if s == "abc.d"));
+    assert!(matches!(cow_into_ascii_lowercase("Abc.d"), Cow::Owned(ref s) if s == "abc.d"));
+    assert!(matches!(cow_into_ascii_lowercase("aBC.D"), Cow::Owned(ref s) if s == "abc.d"));
+    assert!(matches!(cow_into_ascii_lowercase("abc.D"), Cow::Owned(ref s) if s == "abc.d"));
+}