Provide itoa::fmt to write to fmt::Write

Fixes #5.

README.md

@@ -5,7 +5,8 @@ itoa
 [![Latest Version](https://img.shields.io/crates/v/itoa.svg)](https://crates.io/crates/itoa)
 
 This crate provides fast functions for printing integer primitives to an
-[`io::Write`](https://doc.rust-lang.org/std/io/trait.Write.html). The
+[`io::Write`](https://doc.rust-lang.org/std/io/trait.Write.html) or a
+[`fmt::Write`](https://doc.rust-lang.org/core/fmt/trait.Write.html). The
 implementation comes straight from
 [libcore](https://github.com/rust-lang/rust/blob/b8214dc6c6fc20d0a660fb5700dca9ebf51ebe89/src/libcore/fmt/num.rs#L201-L254)
 but avoids the performance penalty of going through
@@ -32,12 +33,19 @@ println!("{:?}", buf);
 let mut bytes = [b'\0'; 20];
 let n = itoa::write(&mut bytes[..], 128u64)?;
 println!("{:?}", &bytes[..n]);
+
+// write to a String
+let mut s = String::new();
+itoa::fmt(&mut s, 128u64)?;
+println!("{}", s);
 ```
 
-The function signature is:
+The function signatures are:
 
 ```rust
 fn write<W: io::Write, V: itoa::Integer>(writer: W, value: V) -> io::Result<usize>
+
+fn fmt<W: fmt::Write, V: itoa::Integer>(writer: W, value: V) -> fmt::Result
 ```
 
 where `itoa::Integer` is implemented for `i8`, `u8`, `i16`, `u16`, `i32`, `u32`,

src/lib.rs

@@ -15,13 +15,18 @@
 #[cfg(feature = "i128")]
 mod udiv128;
 
-use std::{io, mem, ptr, slice};
+use std::{fmt, io, mem, ptr, slice, str};
 
 #[inline]
 pub fn write<W: io::Write, V: Integer>(wr: W, value: V) -> io::Result<usize> {
     value.write(wr)
 }
 
+#[inline]
+pub fn fmt<W: fmt::Write, V: Integer>(wr: W, value: V) -> fmt::Result {
+    value.fmt(wr)
+}
+
 // Seal to prevent downstream implementations of the Integer trait.
 mod private {
     pub trait Sealed {}
@@ -29,6 +34,8 @@ mod private {
 
 pub trait Integer: private::Sealed {
     fn write<W: io::Write>(self, W) -> io::Result<usize>;
+
+    fn fmt<W: fmt::Write>(self, W) -> fmt::Result;
 }
 
 trait IntegerPrivate {
@@ -55,6 +62,12 @@ macro_rules! impl_IntegerCommon {
                 try!(wr.write_all(bytes));
                 Ok(bytes.len())
             }
+
+            fn fmt<W: fmt::Write>(self, mut wr: W) -> fmt::Result {
+                let mut buf = unsafe { mem::uninitialized() };
+                let bytes = self.write_to(&mut buf);
+                wr.write_str(unsafe { str::from_utf8_unchecked(bytes) })
+            }
         }
 
         impl private::Sealed for $t {}

tests/test.rs

@@ -20,6 +20,10 @@ macro_rules! test {
                 let mut buf = [b'\0'; 40];
                 let len = itoa::write(&mut buf[..], $value).unwrap();
                 assert_eq!(&buf[0..len], $expected.as_bytes());
+
+                let mut s = String::new();
+                itoa::fmt(&mut s, $value).unwrap();
+                assert_eq!(s, $expected);
             }
         )*
     }