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

src/test/bench/core-std.rs

@@ -12,7 +12,6 @@
 
 #![feature(rand, vec_push_all, duration, duration_span)]
 
-use std::iter::repeat;
 use std::mem::swap;
 use std::env;
 use std::__rand::{thread_rng, Rng};
@@ -56,7 +55,7 @@ fn maybe_run_test<F>(argv: &[String], name: String, test: F) where F: FnOnce() {
 }
 
 fn shift_push() {
-    let mut v1 = repeat(1).take(30000).collect::<Vec<_>>();
+    let mut v1 = vec![1; 30000];
     let mut v2 = Vec::new();
 
     while !v1.is_empty() {
@@ -70,7 +69,7 @@ fn vec_plus() {
     let mut v = Vec::new();
     let mut i = 0;
     while i < 1500 {
-        let rv = repeat(i).take(r.gen_range(0, i + 1)).collect::<Vec<_>>();
+        let rv = vec![i; r.gen_range(0, i + 1)];
         if r.gen() {
             v.extend(rv);
         } else {
@@ -88,7 +87,7 @@ fn vec_append() {
     let mut v = Vec::new();
     let mut i = 0;
     while i < 1500 {
-        let rv = repeat(i).take(r.gen_range(0, i + 1)).collect::<Vec<_>>();
+        let rv = vec![i; r.gen_range(0, i + 1)];
         if r.gen() {
             let mut t = v.clone();
             t.push_all(&rv);
@@ -108,7 +107,7 @@ fn vec_push_all() {
 
     let mut v = Vec::new();
     for i in 0..1500 {
-        let mut rv = repeat(i).take(r.gen_range(0, i + 1)).collect::<Vec<_>>();
+        let mut rv = vec![i; r.gen_range(0, i + 1)];
         if r.gen() {
             v.push_all(&rv);
         }

src/test/bench/shootout-fasta-redux.rs

@@ -42,7 +42,6 @@ use std::cmp::min;
 use std::env;
 use std::io;
 use std::io::prelude::*;
-use std::iter::repeat;
 
 const LINE_LEN: usize = 60;
 const LOOKUP_SIZE: usize = 4 * 1024;
@@ -121,7 +120,7 @@ impl<'a, W: Write> RepeatFasta<'a, W> {
 
     fn make(&mut self, n: usize) -> io::Result<()> {
         let alu_len = self.alu.len();
-        let mut buf = repeat(0).take(alu_len + LINE_LEN).collect::<Vec<_>>();
+        let mut buf = vec![0; alu_len + LINE_LEN];
         let alu: &[u8] = self.alu.as_bytes();
 
         for (slot, val) in buf.iter_mut().zip(alu) {

src/test/bench/shootout-meteor.rs

@@ -42,7 +42,6 @@
 
 #![feature(iter_cmp)]
 
-use std::iter::repeat;
 use std::sync::Arc;
 use std::sync::mpsc::channel;
 use std::thread;
@@ -221,7 +220,7 @@ fn get_id(m: u64) -> u8 {
 
 // Converts a list of mask to a Vec<u8>.
 fn to_vec(raw_sol: &List<u64>) -> Vec<u8> {
-    let mut sol = repeat('.' as u8).take(50).collect::<Vec<_>>();
+    let mut sol = vec![b'.'; 50];
     for &m in raw_sol.iter() {
         let id = '0' as u8 + get_id(m);
         for i in 0..50 {

src/test/bench/shootout-spectralnorm.rs

@@ -43,7 +43,6 @@
 #![allow(non_snake_case)]
 #![feature(unboxed_closures, iter_arith, core_simd, scoped)]
 
-use std::iter::repeat;
 use std::thread;
 use std::env;
 use std::simd::f64x2;
@@ -62,7 +61,7 @@ fn main() {
 
 fn spectralnorm(n: usize) -> f64 {
     assert!(n % 2 == 0, "only even lengths are accepted");
-    let mut u = repeat(1.0).take(n).collect::<Vec<_>>();
+    let mut u = vec![1.0; n];
     let mut v = u.clone();
     let mut tmp = v.clone();
     for _ in 0..10 {

src/test/bench/sudoku.rs

@@ -15,7 +15,6 @@
 
 use std::io::prelude::*;
 use std::io;
-use std::iter::repeat;
 use std::env;
 
 // Computes a single solution to a given 9x9 sudoku
@@ -59,8 +58,7 @@ impl Sudoku {
         reader.read_line(&mut s).unwrap();
         assert_eq!(s, "9,9\n");
 
-        let mut g = repeat(vec![0, 0, 0, 0, 0, 0, 0, 0, 0])
-                          .take(10).collect::<Vec<_>>();
+        let mut g = vec![vec![0, 0, 0, 0, 0, 0, 0, 0, 0]; 10];
         for line in reader.lines() {
             let line = line.unwrap();
             let comps: Vec<&str> = line

src/test/run-pass/issue-3563-3.rs

@@ -68,12 +68,9 @@ impl Drop for AsciiArt {
 // If there is a canonical constructor it is typically named the same as the type.
 // Other constructor sort of functions are typically named from_foo, from_bar, etc.
 fn AsciiArt(width: usize, height: usize, fill: char) -> AsciiArt {
-    // Use an anonymous function to build a vector of vectors containing
-    // blank characters for each position in our canvas.
-    let mut lines = Vec::new();
-    for _ in 0..height {
-        lines.push(repeat('.').take(width).collect::<Vec<_>>());
-    }
+    // Build a vector of vectors containing blank characters for each position in
+    // our canvas.
+    let lines = vec![vec!['.'; width]; height];
 
     // Rust code often returns values by omitting the trailing semi-colon
     // instead of using an explicit return statement.

src/test/run-pass/realloc-16687.rs

@@ -19,7 +19,6 @@ extern crate alloc;
 
 use alloc::heap;
 use std::ptr;
-use std::iter::repeat;
 
 fn main() {
     unsafe {
@@ -29,7 +28,7 @@ fn main() {
 
 unsafe fn test_triangle() -> bool {
     static COUNT : usize = 16;
-    let mut ascend = repeat(ptr::null_mut()).take(COUNT).collect::<Vec<_>>();
+    let mut ascend = vec![ptr::null_mut(); COUNT];
     let ascend = &mut *ascend;
     static ALIGN : usize = 1;