collections: Implement String::drain(range) according to RFC 574

`.drain(range)` is unstable and under feature(collections_drain).

This adds a safe way to remove any range of a String as efficiently as
possible.

As noted in the code, this drain iterator has none of the memory safety
issues of the vector version.

RFC tracking issue is #23055

1 files changed, 17 insertions, 0 deletions

diff --git a/src/libcollectionstest/string.rs b/src/libcollectionstest/string.rs
index d842d1e7f27..d4e2ebf4fd1 100644
--- a/src/libcollectionstest/string.rs
+++ b/src/libcollectionstest/string.rs
@@ -348,6 +348,23 @@ fn test_from_iterator() {
     assert_eq!(s, d);
 }
 
+#[test]
+fn test_drain() {
+    let mut s = String::from("αβγ");
+    assert_eq!(s.drain(2..4).collect::<String>(), "β");
+    assert_eq!(s, "αγ");
+
+    let mut t = String::from("abcd");
+    t.drain(..0);
+    assert_eq!(t, "abcd");
+    t.drain(..1);
+    assert_eq!(t, "bcd");
+    t.drain(3..);
+    assert_eq!(t, "bcd");
+    t.drain(..);
+    assert_eq!(t, "");
+}
+
 #[bench]
 fn bench_with_capacity(b: &mut Bencher) {
     b.iter(|| {