Specialize many implementations of Read::read_buf_exact

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

@@ -322,6 +322,14 @@ impl<R: ?Sized + Read> Read for BufReader<R> {
         crate::io::default_read_exact(self, buf)
     }
 
+    fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        if self.buf.consume_with(cursor.capacity(), |claimed| cursor.append(claimed)) {
+            return Ok(());
+        }
+
+        crate::io::default_read_buf_exact(self, cursor)
+    }
+
     fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
         if self.buf.pos() == self.buf.filled() && total_len >= self.capacity() {

library/std/src/io/cursor.rs

@@ -355,6 +355,13 @@ where
         self.pos += n as u64;
         Ok(())
     }
+
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        let n = cursor.capacity();
+        Read::read_buf_exact(&mut self.remaining_slice(), cursor)?;
+        self.pos += n as u64;
+        Ok(())
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/std/src/io/impls.rs

@@ -50,6 +50,10 @@ impl<R: Read + ?Sized> Read for &mut R {
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         (**self).read_exact(buf)
     }
+    #[inline]
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        (**self).read_buf_exact(cursor)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<W: Write + ?Sized> Write for &mut W {
@@ -154,6 +158,10 @@ impl<R: Read + ?Sized> Read for Box<R> {
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         (**self).read_exact(buf)
     }
+    #[inline]
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        (**self).read_buf_exact(cursor)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<W: Write + ?Sized> Write for Box<W> {
@@ -301,6 +309,22 @@ impl Read for &[u8] {
         Ok(())
     }
 
+    #[inline]
+    fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        if cursor.capacity() > self.len() {
+            return Err(io::const_io_error!(
+                ErrorKind::UnexpectedEof,
+                "failed to fill whole buffer"
+            ));
+        }
+        let (a, b) = self.split_at(cursor.capacity());
+
+        cursor.append(a);
+
+        *self = b;
+        Ok(())
+    }
+
     #[inline]
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
         let len = self.len();

library/std/src/io/mod.rs

@@ -582,6 +582,29 @@ where
     Ok(())
 }
 
+pub(crate) fn default_read_buf_exact<R: Read + ?Sized>(
+    this: &mut R,
+    mut cursor: BorrowedCursor<'_>,
+) -> Result<()> {
+    while cursor.capacity() > 0 {
+        let prev_written = cursor.written();
+        match this.read_buf(cursor.reborrow()) {
+            Ok(()) => {}
+            Err(e) if e.is_interrupted() => continue,
+            Err(e) => return Err(e),
+        }
+
+        if cursor.written() == prev_written {
+            return Err(error::const_io_error!(
+                ErrorKind::UnexpectedEof,
+                "failed to fill whole buffer"
+            ));
+        }
+    }
+
+    Ok(())
+}
+
 /// The `Read` trait allows for reading bytes from a source.
 ///
 /// Implementors of the `Read` trait are called 'readers'.
@@ -978,24 +1001,8 @@ pub trait Read {
     ///
     /// If this function returns an error, all bytes read will be appended to `cursor`.
     #[unstable(feature = "read_buf", issue = "78485")]
-    fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> Result<()> {
-        while cursor.capacity() > 0 {
-            let prev_written = cursor.written();
-            match self.read_buf(cursor.reborrow()) {
-                Ok(()) => {}
-                Err(e) if e.is_interrupted() => continue,
-                Err(e) => return Err(e),
-            }
-
-            if cursor.written() == prev_written {
-                return Err(error::const_io_error!(
-                    ErrorKind::UnexpectedEof,
-                    "failed to fill whole buffer"
-                ));
-            }
-        }
-
-        Ok(())
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()> {
+        default_read_buf_exact(self, cursor)
     }
 
     /// Creates a "by reference" adaptor for this instance of `Read`.

library/std/src/io/stdio.rs

@@ -451,6 +451,9 @@ impl Read for Stdin {
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         self.lock().read_exact(buf)
     }
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.lock().read_buf_exact(cursor)
+    }
 }
 
 #[stable(feature = "read_shared_stdin", since = "CURRENT_RUSTC_VERSION")]
@@ -477,6 +480,9 @@ impl Read for &Stdin {
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         self.lock().read_exact(buf)
     }
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.lock().read_buf_exact(cursor)
+    }
 }
 
 // only used by platform-dependent io::copy specializations, i.e. unused on some platforms
@@ -517,6 +523,10 @@ impl Read for StdinLock<'_> {
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         self.inner.read_exact(buf)
     }
+
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.inner.read_buf_exact(cursor)
+    }
 }
 
 impl SpecReadByte for StdinLock<'_> {