Explicitly implement DynSync and DynSend for TyCtxt

compiler/rustc_middle/src/ty/context.rs

@@ -40,6 +40,8 @@ use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{FreezeReadGuard, Lock, WorkerLocal};
+#[cfg(parallel_compiler)]
+use rustc_data_structures::sync::{self, DynSend, DynSync};
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::{
     DecorateLint, DiagnosticBuilder, DiagnosticMessage, ErrorGuaranteed, MultiSpan,
@@ -517,6 +519,16 @@ pub struct TyCtxt<'tcx> {
     gcx: &'tcx GlobalCtxt<'tcx>,
 }
 
+// Explicitly implement `DynSync` and `DynSend` for `TyCtxt` to short circuit trait resolution.
+#[cfg(parallel_compiler)]
+unsafe impl DynSend for TyCtxt<'_> {}
+#[cfg(parallel_compiler)]
+unsafe impl DynSync for TyCtxt<'_> {}
+fn _assert_tcx_fields() {
+    sync::assert_dyn_sync::<&'_ GlobalCtxt<'_>>();
+    sync::assert_dyn_send::<&'_ GlobalCtxt<'_>>();
+}
+
 impl<'tcx> Deref for TyCtxt<'tcx> {
     type Target = &'tcx GlobalCtxt<'tcx>;
     #[inline(always)]