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/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(),