Auto merge of #46779 - Zoxc:par-merge-without-sync, r=arielb1

Work towards thread safety in rustc

This PR is split out from https://github.com/rust-lang/rust/pull/45912. It contains changes which do not require the `sync` module.

2 files changed, 8 insertions, 9 deletions

diff --git a/src/libsyntax/attr.rs b/src/libsyntax/attr.rs
index e5e95002e10..cc72ff7d15d 100644
--- a/src/libsyntax/attr.rs
+++ b/src/libsyntax/attr.rs
@@ -31,7 +31,7 @@ use symbol::Symbol;
 use tokenstream::{TokenStream, TokenTree, Delimited};
 use util::ThinVec;
 
-use std::cell::{RefCell, Cell};
+use std::cell::RefCell;
 use std::iter;
 
 thread_local! {
@@ -419,16 +419,14 @@ pub fn mk_spanned_word_item(sp: Span, name: Name) -> MetaItem {
     MetaItem { span: sp, name: name, node: MetaItemKind::Word }
 }
 
+pub fn mk_attr_id() -> AttrId {
+    use std::sync::atomic::AtomicUsize;
+    use std::sync::atomic::Ordering;
 
+    static NEXT_ATTR_ID: AtomicUsize = AtomicUsize::new(0);
 
-thread_local! { static NEXT_ATTR_ID: Cell<usize> = Cell::new(0) }
-
-pub fn mk_attr_id() -> AttrId {
-    let id = NEXT_ATTR_ID.with(|slot| {
-        let r = slot.get();
-        slot.set(r + 1);
-        r
-    });
+    let id = NEXT_ATTR_ID.fetch_add(1, Ordering::SeqCst);
+    assert!(id != ::std::usize::MAX);
     AttrId(id)
 }
 
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 44383233a8a..0b51f2e9814 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -24,6 +24,7 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(match_default_bindings)]
 #![feature(i128_type)]
+#![feature(const_atomic_usize_new)]
 
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
 #[allow(unused_extern_crates)]