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 

3 files changed, 7 insertions, 7 deletions

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index 1dca3b77f38..06a30670e8b 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -1638,7 +1638,7 @@ mod test_map {
 
     use super::HashMap;
     use super::Entry::{Occupied, Vacant};
-    use iter::{range_inclusive, repeat};
+    use iter::range_inclusive;
     use cell::RefCell;
     use rand::{thread_rng, Rng};
 
@@ -1698,7 +1698,7 @@ mod test_map {
     #[test]
     fn test_drops() {
         DROP_VECTOR.with(|slot| {
-            *slot.borrow_mut() = repeat(0).take(200).collect();
+            *slot.borrow_mut() = vec![0; 200];
         });
 
         {
@@ -1757,7 +1757,7 @@ mod test_map {
     #[test]
     fn test_move_iter_drops() {
         DROP_VECTOR.with(|v| {
-            *v.borrow_mut() = repeat(0).take(200).collect();
+            *v.borrow_mut() = vec![0; 200];
         });
 
         let hm = {
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)
diff --git a/src/libstd/sys/windows/stdio.rs b/src/libstd/sys/windows/stdio.rs
index 9961fef714a..356787d5bf0 100644
--- a/src/libstd/sys/windows/stdio.rs
+++ b/src/libstd/sys/windows/stdio.rs
@@ -12,7 +12,6 @@ use prelude::v1::*;
 use io::prelude::*;
 
 use io::{self, Cursor};
-use iter::repeat;
 use libc;
 use ptr;
 use str;
@@ -94,7 +93,7 @@ impl Stdin {
         let mut utf8 = self.utf8.lock().unwrap();
         // Read more if the buffer is empty
         if utf8.position() as usize == utf8.get_ref().len() {
-            let mut utf16: Vec<u16> = repeat(0u16).take(0x1000).collect();
+            let mut utf16 = vec![0u16; 0x1000];
             let mut num = 0;
             try!(cvt(unsafe {
                 c::ReadConsoleW(handle,