Rollup merge of rust-lang#73466 - matthiaskrgr:char_into_string, r=dtolnay

impl From<char> for String

This allows us to write

````rust
fn char_to_string() -> String {
    'a'.into()
}
````

which was not possible before.

Author: Manishearth

src/liballoc/string.rs

@@ -2518,3 +2518,11 @@ impl DoubleEndedIterator for Drain<'_> {
 
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for Drain<'_> {}
+
+#[stable(feature = "from_char_for_string", since = "1.46.0")]
+impl From<char> for String {
+    #[inline]
+    fn from(c: char) -> Self {
+        c.to_string()
+    }
+}

src/liballoc/tests/string.rs

@@ -714,3 +714,10 @@ fn test_try_reserve_exact() {
         }
     }
 }
+
+#[test]
+fn test_from_char() {
+    assert_eq!(String::from('a'), 'a'.to_string());
+    let s: String = 'x'.into();
+    assert_eq!(s, 'x'.to_string());
+}