Auto merge of #50855 - nnethercote:fewer-macro_parser-allocs, r=petrochenkov

Speed up the macro parser

These three commits reduce the number of allocations done by the macro parser, in some cases dramatically. For example, for a clean check builds of html5ever, the number of allocations is reduced by 40%.

Here are the rustc-benchmarks that are sped up by at least 1%.
```
html5ever-check
        avg: -6.6%      min: -10.3%     max: -4.1%
html5ever
        avg: -5.2%      min: -9.5%      max: -2.8%
html5ever-opt
        avg: -4.3%      min: -9.3%      max: -1.6%
crates.io-check
        avg: -1.8%      min: -2.9%      max: -0.6%
crates.io-opt
        avg: -1.0%      min: -2.2%      max: -0.1%
crates.io
        avg: -1.1%      min: -2.2%      max: -0.2%
```

1 files changed, 3 insertions, 2 deletions

diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index f26a6a53074..0abedb99bd0 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -23,6 +23,7 @@ use symbol::Symbol;
 use tokenstream::{TokenStream, TokenTree};
 use diagnostics::plugin::ErrorMap;
 
+use std::borrow::Cow;
 use std::collections::HashSet;
 use std::iter;
 use std::path::{Path, PathBuf};
@@ -89,8 +90,8 @@ impl ParseSess {
 }
 
 #[derive(Clone)]
-pub struct Directory {
-    pub path: PathBuf,
+pub struct Directory<'a> {
+    pub path: Cow<'a, Path>,
     pub ownership: DirectoryOwnership,
 }