Auto merge of #43500 - murarth:string-retain, r=alexcrichton

Add method `String::retain`

Behaves like `Vec::retain`, accepting a predicate `FnMut(char) -> bool`
and reducing the string to only characters for which the predicate
returns `true`.

2 files changed, 21 insertions, 0 deletions

diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index 27b23d14059..34b0a137b10 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -25,6 +25,7 @@
 #![feature(slice_rotate)]
 #![feature(splice)]
 #![feature(str_escape)]
+#![feature(string_retain)]
 #![feature(test)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
diff --git a/src/liballoc/tests/string.rs b/src/liballoc/tests/string.rs
index b1731b2a5dc..f5c124c6b44 100644
--- a/src/liballoc/tests/string.rs
+++ b/src/liballoc/tests/string.rs
@@ -333,6 +333,26 @@ fn remove_bad() {
 }
 
 #[test]
+fn test_retain() {
+    let mut s = String::from("α_β_γ");
+
+    s.retain(|_| true);
+    assert_eq!(s, "α_β_γ");
+
+    s.retain(|c| c != '_');
+    assert_eq!(s, "αβγ");
+
+    s.retain(|c| c != 'β');
+    assert_eq!(s, "αγ");
+
+    s.retain(|c| c == 'α');
+    assert_eq!(s, "α");
+
+    s.retain(|_| false);
+    assert_eq!(s, "");
+}
+
+#[test]
 fn insert() {
     let mut s = "foobar".to_string();
     s.insert(0, 'ệ');