Add Write::write_fmt

src/io/write/mod.rs

@@ -1,24 +1,26 @@
 mod flush;
 mod write;
 mod write_all;
+mod write_fmt;
 mod write_vectored;
 
 use flush::FlushFuture;
 use write::WriteFuture;
 use write_all::WriteAllFuture;
+use write_fmt::WriteFmtFuture;
 use write_vectored::WriteVectoredFuture;
 
 use cfg_if::cfg_if;
 
 use crate::io::IoSlice;
 use crate::utils::extension_trait;
 
+use crate::io;
+
 cfg_if! {
     if #[cfg(feature = "docs")] {
         use std::pin::Pin;
         use std::ops::{Deref, DerefMut};
-
-        use crate::io;
         use crate::task::{Context, Poll};
     }
 }
@@ -197,6 +199,50 @@ extension_trait! {
         {
             WriteAllFuture { writer: self, buf }
         }
+
+        #[doc = r#"
+            Writes a formatted string into this writer, returning any error encountered.
+
+            This method will continuously call [`write`] until there is no more data to be
+            written or an error is returned. This future will not resolve until the entire
+            buffer has been successfully written or such an error occurs.
+
+            [`write`]: #tymethod.write
+
+            # Examples
+
+            ```no_run
+            # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
+            #
+            use async_std::io::prelude::*;
+            use async_std::fs::File;
+
+            let mut buffer = File::create("foo.txt").await?;
+
+            // this call
+            write!(buffer, "{:.*}", 2, 1.234567).await?;
+            // turns into this:
+            buffer.write_fmt(format_args!("{:.*}", 2, 1.234567)).await?;
+            #
+            # Ok(()) }) }
+            ```
+        "#]
+        fn write_fmt<'a>(
+            &'a mut self,
+            fmt: std::fmt::Arguments<'_>,
+        ) -> impl Future<Output = io::Result<()>> + 'a [WriteFmtFuture<'a, Self>]
+        where
+            Self: Unpin,
+        {
+            // In order to not have to implement an async version of `fmt` including private types
+            // and all, we convert `Arguments` to a `Result<Vec<u8>>` and pass that to the Future.
+            // Doing an owned conversion saves us from juggling references.
+            let mut string = String::new();
+            let res = std::fmt::write(&mut string, fmt)
+                .map(|_| string.into_bytes())
+                .map_err(|_| io::Error::new(io::ErrorKind::Other, "formatter error"));
+            WriteFmtFuture { writer: self, res: Some(res), buffer: None, amt: 0 }
+        }
     }
 
     impl<T: Write + Unpin + ?Sized> Write for Box<T> {

src/io/write/write_fmt.rs

@@ -0,0 +1,50 @@
+use std::pin::Pin;
+
+use crate::future::Future;
+use crate::io::{self, Write};
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct WriteFmtFuture<'a, T: Unpin + ?Sized> {
+    pub(crate) writer: &'a mut T,
+    pub(crate) res: Option<io::Result<Vec<u8>>>,
+    pub(crate) buffer: Option<Vec<u8>>,
+    pub(crate) amt: u64,
+}
+
+impl<T: Write + Unpin + ?Sized> Future for WriteFmtFuture<'_, T> {
+    type Output = io::Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // Process the interal Result the first time we run.
+        if self.buffer.is_none() {
+            match self.res.take().unwrap() {
+                Err(err) => return Poll::Ready(Err(err)),
+                Ok(buffer) => self.buffer = Some(buffer),
+            };
+        }
+
+        // Get the types from the future.
+        let Self {
+            writer,
+            amt,
+            buffer,
+            ..
+        } = &mut *self;
+        let mut buffer = buffer.as_mut().unwrap();
+
+        // Copy the data from the buffer into the writer until it's done.
+        loop {
+            if buffer.is_empty() {
+                futures_core::ready!(Pin::new(&mut **writer).poll_flush(cx))?;
+                return Poll::Ready(Ok(()));
+            }
+            let i = futures_core::ready!(Pin::new(&mut **writer).poll_write(cx, &mut buffer))?;
+            if i == 0 {
+                return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
+            }
+            *amt += i as u64;
+        }
+    }
+}

src/lib.rs

@@ -77,3 +77,6 @@ cfg_if! {
 }
 
 pub(crate) mod utils;
+
+#[doc(inline)]
+pub use std::{write, writeln};