Auto merge of #26904 - bluss:no-repeat, r=alexcrichton

In a followup to PR #26849, improve one more location for I/O where
we can use `Vec::resize` to ensure better performance when zeroing
buffers.

Use the `vec![elt; n]` macro everywhere we can in the tree. It replaces
`repeat(elt).take(n).collect()` which is more verbose, requires type
hints, and right now produces worse code. `vec![]` is preferable for vector
initialization.

The `vec![]` replacement touches upon one I/O path too, Stdin::read
for windows, and that should be a small improvement.

r? @alexcrichton 

1 files changed, 3 insertions, 2 deletions

diff --git a/src/libstd/io/cursor.rs b/src/libstd/io/cursor.rs
index 72743106abf..f059b24baf6 100644
--- a/src/libstd/io/cursor.rs
+++ b/src/libstd/io/cursor.rs
@@ -13,7 +13,6 @@ use io::prelude::*;
 
 use cmp;
 use io::{self, SeekFrom, Error, ErrorKind};
-use iter::repeat;
 use slice;
 
 /// A `Cursor` is a type which wraps a non-I/O object to provide a `Seek`
@@ -143,7 +142,9 @@ impl Write for Cursor<Vec<u8>> {
         // currently are
         let pos = self.position();
         let amt = pos.saturating_sub(self.inner.len() as u64);
-        self.inner.extend(repeat(0).take(amt as usize));
+        // use `resize` so that the zero filling is as efficient as possible
+        let len = self.inner.len();
+        self.inner.resize(len + amt as usize, 0);
 
         // Figure out what bytes will be used to overwrite what's currently
         // there (left), and what will be appended on the end (right)