Auto merge of #130803 - cuviper:file-buffered, r=joshtriplett

Add `File` constructors that return files wrapped with a buffer

In addition to the light convenience, these are intended to raise visibility that buffering is something you should consider when opening a file, since unbuffered I/O is a common performance footgun to Rust newcomers.

ACP: https://github.com/rust-lang/libs-team/issues/446
Tracking Issue: #130804

3 files changed, 29 insertions, 1 deletions

diff --git a/library/std/src/io/buffered/bufreader.rs b/library/std/src/io/buffered/bufreader.rs
index e51dde994de..fcb3e36027b 100644
--- a/library/std/src/io/buffered/bufreader.rs
+++ b/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
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 1bf84d8bef3..3df7e3971da 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/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 {
@@ -37,6 +37,16 @@ impl Buffer {
     }
 
     #[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
         // that region is initialized because those are all invariants of this type.
diff --git a/library/std/src/io/buffered/bufwriter.rs b/library/std/src/io/buffered/bufwriter.rs
index 13516d3b961..c41bae2aa4e 100644
--- a/library/std/src/io/buffered/bufwriter.rs
+++ b/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