Auto merge of #52738 - ljedrz:push_to_extend, r=eddyb

Replace push loops with extend() where possible

Or set the vector capacity where I couldn't do it.

According to my [simple benchmark](https://gist.github.com/ljedrz/568e97621b749849684c1da71c27dceb) `extend`ing a vector can be over **10 times** faster than `push`ing to it in a loop:

10 elements (6.1 times faster):
```
test bench_extension ... bench:          75 ns/iter (+/- 23)
test bench_push_loop ... bench:         458 ns/iter (+/- 142)
```

100 elements (11.12 times faster):
```
test bench_extension ... bench:          87 ns/iter (+/- 26)
test bench_push_loop ... bench:         968 ns/iter (+/- 3,528)
```

1000 elements (11.04 times faster):
```
test bench_extension ... bench:         311 ns/iter (+/- 9)
test bench_push_loop ... bench:       3,436 ns/iter (+/- 233)
```

Seems like a good idea to use `extend` as much as possible.

2 files changed, 3 insertions, 11 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 1b6b47f5489..d59c9dd6b53 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -500,10 +500,7 @@ impl Pat {
             PatKind::Slice(pats, None, _) if pats.len() == 1 =>
                 pats[0].to_ty().map(TyKind::Slice)?,
             PatKind::Tuple(pats, None) => {
-                let mut tys = Vec::new();
-                for pat in pats {
-                    tys.push(pat.to_ty()?);
-                }
+                let tys = pats.iter().map(|pat| pat.to_ty()).collect::<Option<Vec<_>>>()?;
                 TyKind::Tup(tys)
             }
             _ => return None,
@@ -949,10 +946,7 @@ impl Expr {
             ExprKind::Array(exprs) if exprs.len() == 1 =>
                 exprs[0].to_ty().map(TyKind::Slice)?,
             ExprKind::Tup(exprs) => {
-                let mut tys = Vec::new();
-                for expr in exprs {
-                    tys.push(expr.to_ty()?);
-                }
+                let tys = exprs.iter().map(|expr| expr.to_ty()).collect::<Option<Vec<_>>>()?;
                 TyKind::Tup(tys)
             }
             ExprKind::Binary(binop, lhs, rhs) if binop.node == BinOpKind::Add =>
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 4b077aa8dd4..f9b9e95ead1 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -265,9 +265,7 @@ impl<'a> StringReader<'a> {
                 m.push(c);
             }
             _ => {
-                for c in c.escape_default() {
-                    m.push(c);
-                }
+                m.extend(c.escape_default());
             }
         }
     }