Add benchmark and fast path for BufReader::read_exact

library/std/src/io/mod.rs

@@ -416,6 +416,25 @@ where
     write(buf)
 }
 
+pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {
+    while !buf.is_empty() {
+        match this.read(buf) {
+            Ok(0) => break,
+            Ok(n) => {
+                let tmp = buf;
+                buf = &mut tmp[n..];
+            }
+            Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+            Err(e) => return Err(e),
+        }
+    }
+    if !buf.is_empty() {
+        Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
+    } else {
+        Ok(())
+    }
+}
+
 /// The `Read` trait allows for reading bytes from a source.
 ///
 /// Implementors of the `Read` trait are called 'readers'.
@@ -766,23 +785,8 @@ pub trait Read {
     /// }
     /// ```
     #[stable(feature = "read_exact", since = "1.6.0")]
-    fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
-        while !buf.is_empty() {
-            match self.read(buf) {
-                Ok(0) => break,
-                Ok(n) => {
-                    let tmp = buf;
-                    buf = &mut tmp[n..];
-                }
-                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
-                Err(e) => return Err(e),
-            }
-        }
-        if !buf.is_empty() {
-            Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
-        } else {
-            Ok(())
-        }
+    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
+        default_read_exact(self, buf)
     }
 
     /// Creates a "by reference" adaptor for this instance of `Read`.