Rollup merge of rust-lang#102811 - the8472:bufread-memset, r=m-ou-se

Use memset to initialize readbuf

The write loop was found to be slow in rust-lang#102727

The proper fix is in rust-lang#102760 but this might still help debug builds and code running under miri by using the write_bytes intrinsic instead of writing one byte at a time.

library/std/src/io/readbuf.rs

@@ -3,10 +3,10 @@
 #[cfg(test)]
 mod tests;
 
-use crate::cmp;
 use crate::fmt::{self, Debug, Formatter};
 use crate::io::{Result, Write};
 use crate::mem::{self, MaybeUninit};
+use crate::{cmp, ptr};
 
 /// A borrowed byte buffer which is incrementally filled and initialized.
 ///
@@ -250,8 +250,11 @@ impl<'a> BorrowedCursor<'a> {
     /// Initializes all bytes in the cursor.
     #[inline]
     pub fn ensure_init(&mut self) -> &mut Self {
-        for byte in self.uninit_mut() {
-            byte.write(0);
+        let uninit = self.uninit_mut();
+        // SAFETY: 0 is a valid value for MaybeUninit<u8> and the length matches the allocation
+        // since it is comes from a slice reference.
+        unsafe {
+            ptr::write_bytes(uninit.as_mut_ptr(), 0, uninit.len());
         }
         self.buf.init = self.buf.capacity();