Double check that the deadlock handler doesn't get overwritten

compiler/rustc_interface/src/interface.rs

@@ -43,6 +43,9 @@ pub struct Compiler {
     pub compiler_for_deadlock_handler: Arc<AtomicPtr<()>>,
 }
 
+impl !Sync for Compiler {}
+impl !rustc_data_structures::sync::DynSync for Compiler {}
+
 /// Converts strings provided as `--cfg [cfgspec]` into a `Cfg`.
 pub(crate) fn parse_cfg(dcx: &DiagCtxt, cfgs: Vec<String>) -> Cfg {
     cfgs.into_iter()

compiler/rustc_interface/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(let_chains)]
 #![feature(thread_spawn_unchecked)]
 #![feature(try_blocks)]
+#![feature(negative_impls)]
 
 #[macro_use]
 extern crate tracing;

compiler/rustc_interface/src/queries.rs

@@ -161,9 +161,11 @@ impl<'tcx> Queries<'tcx> {
                 &self.hir_arena,
             );
 
-            self.compiler
+            let old = self
+                .compiler
                 .compiler_for_deadlock_handler
-                .store(qcx as *const _ as *mut _, Ordering::Relaxed);
+                .swap(qcx as *const _ as *mut _, Ordering::Relaxed);
+            assert!(old.is_null());
 
             qcx.enter(|tcx| {
                 let feed = tcx.feed_local_crate();
@@ -321,6 +323,7 @@ impl Compiler {
             // is accessing the `GlobalCtxt`.
             self.compiler_for_deadlock_handler.store(std::ptr::null_mut(), Ordering::Relaxed);
         });
+        assert!(self.compiler_for_deadlock_handler.load(Ordering::Relaxed).is_null());
         let queries = Queries::new(self);
         let ret = f(&queries);