Pre-allocate buffers in File::open_buffered and create_buffered

library/std/src/fs.rs

@@ -407,7 +407,10 @@ impl File {
     /// ```
     #[unstable(feature = "file_buffered", issue = "none")]
     pub fn open_buffered<P: AsRef<Path>>(path: P) -> io::Result<io::BufReader<File>> {
-        File::open(path).map(io::BufReader::new)
+        // Allocate the buffer *first* so we don't affect the filesystem otherwise.
+        let buffer = io::BufReader::<Self>::try_new_buffer()?;
+        let file = File::open(path)?;
+        Ok(io::BufReader::with_buffer(file, buffer))
     }
 
     /// Opens a file in write-only mode.
@@ -472,7 +475,10 @@ impl File {
     /// ```
     #[unstable(feature = "file_buffered", issue = "none")]
     pub fn create_buffered<P: AsRef<Path>>(path: P) -> io::Result<io::BufWriter<File>> {
-        File::create(path).map(io::BufWriter::new)
+        // Allocate the buffer *first* so we don't affect the filesystem otherwise.
+        let buffer = io::BufWriter::<Self>::try_new_buffer()?;
+        let file = File::create(path)?;
+        Ok(io::BufWriter::with_buffer(file, buffer))
     }
 
     /// Creates a new file in read-write mode; error if the file exists.

library/std/src/io/buffered/bufreader.rs

@@ -74,6 +74,14 @@ impl<R: Read> BufReader<R> {
         BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
     }
 
+    pub(crate) fn try_new_buffer() -> io::Result<Buffer> {
+        Buffer::try_with_capacity(DEFAULT_BUF_SIZE)
+    }
+
+    pub(crate) fn with_buffer(inner: R, buf: Buffer) -> Self {
+        Self { inner, buf }
+    }
+
     /// Creates a new `BufReader<R>` with the specified buffer capacity.
     ///
     /// # Examples

library/std/src/io/buffered/bufreader/buffer.rs

@@ -10,7 +10,7 @@
 //! without encountering any runtime bounds checks.
 
 use crate::cmp;
-use crate::io::{self, BorrowedBuf, Read};
+use crate::io::{self, BorrowedBuf, ErrorKind, Read};
 use crate::mem::MaybeUninit;
 
 pub struct Buffer {
@@ -36,6 +36,16 @@ impl Buffer {
         Self { buf, pos: 0, filled: 0, initialized: 0 }
     }
 
+    #[inline]
+    pub fn try_with_capacity(capacity: usize) -> io::Result<Self> {
+        match Box::try_new_uninit_slice(capacity) {
+            Ok(buf) => Ok(Self { buf, pos: 0, filled: 0, initialized: 0 }),
+            Err(_) => {
+                Err(io::const_io_error!(ErrorKind::OutOfMemory, "failed to allocate read buffer"))
+            }
+        }
+    }
+
     #[inline]
     pub fn buffer(&self) -> &[u8] {
         // SAFETY: self.pos and self.cap are valid, and self.cap => self.pos, and

library/std/src/io/buffered/bufwriter.rs

@@ -94,6 +94,16 @@ impl<W: Write> BufWriter<W> {
         BufWriter::with_capacity(DEFAULT_BUF_SIZE, inner)
     }
 
+    pub(crate) fn try_new_buffer() -> io::Result<Vec<u8>> {
+        Vec::try_with_capacity(DEFAULT_BUF_SIZE).map_err(|_| {
+            io::const_io_error!(ErrorKind::OutOfMemory, "failed to allocate write buffer")
+        })
+    }
+
+    pub(crate) fn with_buffer(inner: W, buf: Vec<u8>) -> Self {
+        Self { inner, buf, panicked: false }
+    }
+
     /// Creates a new `BufWriter<W>` with at least the specified buffer capacity.
     ///
     /// # Examples

library/std/src/lib.rs

@@ -374,6 +374,7 @@
 #![feature(slice_concat_trait)]
 #![feature(thin_box)]
 #![feature(try_reserve_kind)]
+#![feature(try_with_capacity)]
 #![feature(vec_into_raw_parts)]
 // tidy-alphabetical-end
 //