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

Author: bors

src/libcollections/bit.rs

@@ -283,7 +283,7 @@ impl BitVec {
     pub fn from_elem(nbits: usize, bit: bool) -> BitVec {
         let nblocks = blocks_for_bits(nbits);
         let mut bit_vec = BitVec {
-            storage: repeat(if bit { !0 } else { 0 }).take(nblocks).collect(),
+            storage: vec![if bit { !0 } else { 0 }; nblocks],
             nbits: nbits
         };
         bit_vec.fix_last_block();

src/libcollectionstest/slice.rs

@@ -1318,7 +1318,7 @@ mod bench {
 
     #[bench]
     fn mut_iterator(b: &mut Bencher) {
-        let mut v: Vec<_> = repeat(0).take(100).collect();
+        let mut v = vec![0; 100];
 
         b.iter(|| {
             let mut i = 0;
@@ -1419,7 +1419,7 @@ mod bench {
     #[bench]
     fn zero_1kb_from_elem(b: &mut Bencher) {
         b.iter(|| {
-            repeat(0u8).take(1024).collect::<Vec<_>>()
+            vec![0u8; 1024]
         });
     }
 
@@ -1467,7 +1467,7 @@ mod bench {
     fn random_inserts(b: &mut Bencher) {
         let mut rng = thread_rng();
         b.iter(|| {
-            let mut v: Vec<_> = repeat((0, 0)).take(30).collect();
+            let mut v = vec![(0, 0); 30];
             for _ in 0..100 {
                 let l = v.len();
                 v.insert(rng.gen::<usize>() % (l + 1),
@@ -1479,7 +1479,7 @@ mod bench {
     fn random_removes(b: &mut Bencher) {
         let mut rng = thread_rng();
         b.iter(|| {
-            let mut v: Vec<_> = repeat((0, 0)).take(130).collect();
+            let mut v = vec![(0, 0); 130];
             for _ in 0..100 {
                 let l = v.len();
                 v.remove(rng.gen::<usize>() % l);

src/libcoretest/ptr.rs

@@ -10,7 +10,6 @@
 
 use core::ptr::*;
 use core::mem;
-use std::iter::repeat;
 
 #[test]
 fn test() {
@@ -110,7 +109,7 @@ fn test_as_mut() {
 #[test]
 fn test_ptr_addition() {
     unsafe {
-        let xs = repeat(5).take(16).collect::<Vec<_>>();
+        let xs = vec![5; 16];
         let mut ptr = xs.as_ptr();
         let end = ptr.offset(16);
 
@@ -128,7 +127,7 @@ fn test_ptr_addition() {
             m_ptr = m_ptr.offset(1);
         }
 
-        assert!(xs_mut == repeat(10).take(16).collect::<Vec<_>>());
+        assert!(xs_mut == vec![10; 16]);
     }
 }
 

src/libgraphviz/lib.rs

@@ -599,7 +599,6 @@ mod tests {
     use std::io;
     use std::io::prelude::*;
     use std::borrow::IntoCow;
-    use std::iter::repeat;
 
     /// each node is an index in a vector in the graph.
     type Node = usize;
@@ -647,7 +646,7 @@ mod tests {
         fn to_opt_strs(self) -> Vec<Option<&'static str>> {
             match self {
                 UnlabelledNodes(len)
-                    => repeat(None).take(len).collect(),
+                    => vec![None; len],
                 AllNodesLabelled(lbls)
                     => lbls.into_iter().map(
                         |l|Some(l)).collect(),

src/librand/reseeding.rs

@@ -186,7 +186,7 @@ mod tests {
     const FILL_BYTES_V_LEN: usize = 13579;
     #[test]
     fn test_rng_fill_bytes() {
-        let mut v = repeat(0).take(FILL_BYTES_V_LEN).collect::<Vec<_>>();
+        let mut v = vec![0; FILL_BYTES_V_LEN];
         ::test::rng().fill_bytes(&mut v);
 
         // Sanity test: if we've gotten here, `fill_bytes` has not infinitely

src/librustc/middle/check_match.rs

@@ -704,7 +704,7 @@ fn is_useful(cx: &MatchCheckCtxt,
                 match is_useful(cx, &matrix, v.tail(), witness) {
                     UsefulWithWitness(pats) => {
                         let arity = constructor_arity(cx, &constructor, left_ty);
-                        let wild_pats: Vec<_> = repeat(DUMMY_WILD_PAT).take(arity).collect();
+                        let wild_pats = vec![DUMMY_WILD_PAT; arity];
                         let enum_pat = construct_witness(cx, &constructor, wild_pats, left_ty);
                         let mut new_pats = vec![enum_pat];
                         new_pats.extend(pats);
@@ -862,7 +862,7 @@ pub fn specialize<'a>(cx: &MatchCheckCtxt, r: &[&'a Pat],
     } = raw_pat(r[col]);
     let head: Option<Vec<&Pat>> = match *node {
         ast::PatWild(_) =>
-            Some(repeat(DUMMY_WILD_PAT).take(arity).collect()),
+            Some(vec![DUMMY_WILD_PAT; arity]),
 
         ast::PatIdent(_, _, _) => {
             let opt_def = cx.tcx.def_map.borrow().get(&pat_id).map(|d| d.full_def());
@@ -875,7 +875,7 @@ pub fn specialize<'a>(cx: &MatchCheckCtxt, r: &[&'a Pat],
                 } else {
                     None
                 },
-                _ => Some(repeat(DUMMY_WILD_PAT).take(arity).collect())
+                _ => Some(vec![DUMMY_WILD_PAT; arity])
             }
         }
 
@@ -889,7 +889,7 @@ pub fn specialize<'a>(cx: &MatchCheckCtxt, r: &[&'a Pat],
                 DefVariant(..) | DefStruct(..) => {
                     Some(match args {
                         &Some(ref args) => args.iter().map(|p| &**p).collect(),
-                        &None => repeat(DUMMY_WILD_PAT).take(arity).collect(),
+                        &None => vec![DUMMY_WILD_PAT; arity],
                     })
                 }
                 _ => None

src/librustc/middle/dataflow.rs

@@ -21,7 +21,6 @@ use middle::cfg::CFGIndex;
 use middle::ty;
 use std::io;
 use std::usize;
-use std::iter::repeat;
 use syntax::ast;
 use syntax::ast_util::IdRange;
 use syntax::visit;
@@ -239,11 +238,11 @@ impl<'a, 'tcx, O:DataFlowOperator> DataFlowContext<'a, 'tcx, O> {
 
         let entry = if oper.initial_value() { usize::MAX } else {0};
 
-        let zeroes: Vec<_> = repeat(0).take(num_nodes * words_per_id).collect();
-        let gens: Vec<_> = zeroes.clone();
-        let kills1: Vec<_> = zeroes.clone();
-        let kills2: Vec<_> = zeroes;
-        let on_entry: Vec<_> = repeat(entry).take(num_nodes * words_per_id).collect();
+        let zeroes = vec![0; num_nodes * words_per_id];
+        let gens = zeroes.clone();
+        let kills1 = zeroes.clone();
+        let kills2 = zeroes;
+        let on_entry = vec![entry; num_nodes * words_per_id];
 
         let nodeid_to_index = build_nodeid_to_index(decl, cfg);
 
@@ -511,7 +510,7 @@ impl<'a, 'tcx, O:DataFlowOperator+Clone+'static> DataFlowContext<'a, 'tcx, O> {
                 changed: true
             };
 
-            let mut temp: Vec<_> = repeat(0).take(words_per_id).collect();
+            let mut temp = vec![0; words_per_id];
             while propcx.changed {
                 propcx.changed = false;
                 propcx.reset(&mut temp);

src/librustc/middle/infer/region_inference/mod.rs

@@ -34,7 +34,6 @@ use util::nodemap::{FnvHashMap, FnvHashSet};
 use std::cell::{Cell, RefCell};
 use std::cmp::Ordering::{self, Less, Greater, Equal};
 use std::fmt;
-use std::iter::repeat;
 use std::u32;
 use syntax::ast;
 
@@ -1304,7 +1303,7 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
         // idea is to report errors that derive from independent
         // regions of the graph, but not those that derive from
         // overlapping locations.
-        let mut dup_vec: Vec<_> = repeat(u32::MAX).take(self.num_vars() as usize).collect();
+        let mut dup_vec = vec![u32::MAX; self.num_vars() as usize];
 
         for idx in 0..self.num_vars() as usize {
             match var_data[idx].value {

src/librustc/middle/liveness.rs

@@ -119,7 +119,6 @@ use util::nodemap::NodeMap;
 use std::{fmt, usize};
 use std::io::prelude::*;
 use std::io;
-use std::iter::repeat;
 use std::rc::Rc;
 use syntax::ast::{self, NodeId, Expr};
 use syntax::codemap::{BytePos, original_sp, Span};
@@ -566,8 +565,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         Liveness {
             ir: ir,
             s: specials,
-            successors: repeat(invalid_node()).take(num_live_nodes).collect(),
-            users: repeat(invalid_users()).take(num_live_nodes * num_vars).collect(),
+            successors: vec![invalid_node(); num_live_nodes],
+            users: vec![invalid_users(); num_live_nodes * num_vars],
             loop_scope: Vec::new(),
             break_ln: NodeMap(),
             cont_ln: NodeMap(),

src/librustc_back/sha2.rs

@@ -12,7 +12,6 @@
 //! use. This implementation is not intended for external use or for any use where security is
 //! important.
 
-use std::iter::repeat;
 use std::slice::bytes::{MutableByteVector, copy_memory};
 use serialize::hex::ToHex;
 
@@ -255,7 +254,7 @@ pub trait Digest {
     /// Convenience function that retrieves the result of a digest as a
     /// newly allocated vec of bytes.
     fn result_bytes(&mut self) -> Vec<u8> {
-        let mut buf: Vec<u8> = repeat(0).take((self.output_bits()+7)/8).collect();
+        let mut buf = vec![0; (self.output_bits()+7)/8];
         self.result(&mut buf);
         buf
     }
@@ -534,7 +533,6 @@ mod tests {
     use self::rand::Rng;
     use self::rand::isaac::IsaacRng;
     use serialize::hex::FromHex;
-    use std::iter::repeat;
     use std::u64;
     use super::{Digest, Sha256, FixedBuffer};
 
@@ -613,7 +611,7 @@ mod tests {
     /// correct.
     fn test_digest_1million_random<D: Digest>(digest: &mut D, blocksize: usize, expected: &str) {
         let total_size = 1000000;
-        let buffer: Vec<u8> = repeat('a' as u8).take(blocksize * 2).collect();
+        let buffer = vec![b'a'; blocksize * 2];
         let mut rng = IsaacRng::new_unseeded();
         let mut count = 0;