Add String::write_fmt with #[may_ignore].

Author: m-ou-se

library/alloc/src/lib.rs

@@ -106,6 +106,7 @@
 #![feature(int_bits_const)]
 #![feature(lang_items)]
 #![feature(layout_for_ptr)]
+#![cfg_attr(not(bootstrap), feature(may_ignore))]
 #![feature(maybe_uninit_ref)]
 #![feature(negative_impls)]
 #![feature(never_type)]

library/alloc/src/string.rs

@@ -1587,6 +1587,14 @@ impl String {
         let slice = self.vec.into_boxed_slice();
         unsafe { from_boxed_utf8_unchecked(slice) }
     }
+
+    #[doc(hidden)]
+    #[inline]
+    #[cfg_attr(not(bootstrap), may_ignore)]
+    #[stable(feature = "string_write_fmt", since = "1.49.0")]
+    pub fn write_fmt(mut self: &mut Self, args: fmt::Arguments<'_>) -> fmt::Result {
+        fmt::write(&mut self, args)
+    }
 }
 
 impl FromUtf8Error {