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

src/libcollections/string.rs

@@ -1082,4 +1082,10 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
         self.push_str(s);
         Ok(())
     }
+
+    #[inline]
+    fn write_char(&mut self, c: char) -> fmt::Result {
+        self.push(c);
+        Ok(())
+    }
 }

src/libcore/fmt/mod.rs

@@ -83,6 +83,23 @@ pub trait Write {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn write_str(&mut self, s: &str) -> Result;
 
+    /// Writes a `char` into this writer, returning whether the write succeeded.
+    ///
+    /// A single `char` may be encoded as more than one byte.
+    /// This method can only succeed if the entire byte sequence was successfully
+    /// written, and this method will not return until all data has been
+    /// written or an error occurs.
+    ///
+    /// # Errors
+    ///
+    /// This function will return an instance of `FormatError` on error.
+    #[stable(feature = "fmt_write_char", since = "1.1.0")]
+    fn write_char(&mut self, c: char) -> Result {
+        let mut utf_8 = [0u8; 4];
+        let bytes_written = c.encode_utf8(&mut utf_8).unwrap_or(0);
+        self.write_str(unsafe { mem::transmute(&utf_8[..bytes_written]) })
+    }
+
     /// Glue for usage of the `write!` macro with implementers of this trait.
     ///
     /// This method should generally not be invoked manually, but rather through