rollup merge of #24661: SimonSapin/fmt-write-char

as accepted in [RFC 526](https://github.com/rust-lang/rfcs/blob/master/text/0526-fmt-text-writer.md).

Note that this brand new method is marked as **stable**. I judged this safe enough: it’s simple enough that it’s very unlikely to change. Still, I can mark it unstable instead if you prefer.

r? @alexcrichton

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libcollections/string.rs b/src/libcollections/string.rs
index 85f8a9caaaa..25f450e53a4 100644
--- a/src/libcollections/string.rs
+++ b/src/libcollections/string.rs
@@ -1082,4 +1082,10 @@ impl fmt::Write for String {
         self.push_str(s);
         Ok(())
     }
+
+    #[inline]
+    fn write_char(&mut self, c: char) -> fmt::Result {
+        self.push(c);
+        Ok(())
+    }
 }