Auto merge of #24689 - SimonSapin:formatter-write-char, r=alexcrichton

This is the logical next step after #24661, but I’m less sure about this one.

r? @alexcrichton

src/libcore/fmt/mod.rs

@@ -892,6 +892,21 @@ impl<'a> Formatter<'a> {
     }
 }
 
+#[stable(since = "1.2.0", feature = "formatter_write")]
+impl<'a> Write for Formatter<'a> {
+    fn write_str(&mut self, s: &str) -> Result {
+        self.buf.write_str(s)
+    }
+
+    fn write_char(&mut self, c: char) -> Result {
+        self.buf.write_char(c)
+    }
+
+    fn write_fmt(&mut self, args: Arguments) -> Result {
+        write(self.buf, args)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Display for Error {
     fn fmt(&self, f: &mut Formatter) -> Result {
@@ -965,10 +980,7 @@ impl Debug for char {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Display for char {
     fn fmt(&self, f: &mut Formatter) -> Result {
-        let mut utf8 = [0; 4];
-        let amt = self.encode_utf8(&mut utf8).unwrap_or(0);
-        let s: &str = unsafe { mem::transmute(&utf8[..amt]) };
-        Display::fmt(s, f)
+        f.write_char(*self)
     }
 }
 

src/test/run-pass/ifmt.rs

@@ -15,12 +15,13 @@
 #![allow(unknown_features)]
 #![feature(box_syntax)]
 
-use std::fmt;
+use std::fmt::{self, Write};
 use std::usize;
 
 struct A;
 struct B;
 struct C;
+struct D;
 
 impl fmt::LowerHex for A {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -37,6 +38,13 @@ impl fmt::Display for C {
         f.pad_integral(true, "☃", "123")
     }
 }
+impl fmt::Binary for D {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        try!(f.write_str("aa"));
+        try!(f.write_char('☃'));
+        f.write_str("bb")
+    }
+}
 
 macro_rules! t {
     ($a:expr, $b:expr) => { assert_eq!($a, $b) }
@@ -90,6 +98,7 @@ pub fn main() {
     t!(format!("{foo_bar}", foo_bar=1), "1");
     t!(format!("{}", 5 + 5), "10");
     t!(format!("{:#4}", C), "☃123");
+    t!(format!("{:b}", D), "aa☃bb");
 
     let a: &fmt::Debug = &1;
     t!(format!("{:?}", a), "1");