Simplify with_tlv.

Author: nnethercote

compiler/rustc_middle/src/lib.rs

@@ -34,6 +34,7 @@
 #![feature(get_mut_unchecked)]
 #![feature(if_let_guard)]
 #![feature(iter_from_generator)]
+#![feature(local_key_cell_methods)]
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(extern_types)]

compiler/rustc_middle/src/ty/context/tls.rs

@@ -89,9 +89,8 @@ mod tlv {
     /// This is used to set the pointer to the new `ImplicitCtxt`.
     #[inline]
     pub(super) fn with_tlv<F: FnOnce() -> R, R>(value: *const (), f: F) -> R {
-        let old = get_tlv();
-        let _reset = rustc_data_structures::OnDrop(move || TLV.with(|tlv| tlv.set(old)));
-        TLV.with(|tlv| tlv.set(value));
+        let old = TLV.replace(value);
+        let _reset = rustc_data_structures::OnDrop(move || TLV.set(old));
         f()
     }
 }