Rollup merge of #138092 - lqd:revert-136731, r=SparrowLii

Re-add `DynSend` and `DynSync` impls for `TyCtxt`

They were somewhat unexpectedly removed in #136731. This PR adds them back, as requested in #136731 (comment). I've also tried to expand the comments a bit to make it less likely that they're removed again in the future.

r? `@SparrowLii`

Author: compiler-errors

compiler/rustc_middle/src/ty/context.rs

@@ -1327,6 +1327,11 @@ pub struct TyCtxt<'tcx> {
     gcx: &'tcx GlobalCtxt<'tcx>,
 }
 
+// Explicitly implement `DynSync` and `DynSend` for `TyCtxt` to short circuit trait resolution. Its
+// field are asserted to implement these traits below, so this is trivially safe, and it greatly
+// speeds-up compilation of this crate and its dependents.
+unsafe impl DynSend for TyCtxt<'_> {}
+unsafe impl DynSync for TyCtxt<'_> {}
 fn _assert_tcx_fields() {
     sync::assert_dyn_sync::<&'_ GlobalCtxt<'_>>();
     sync::assert_dyn_send::<&'_ GlobalCtxt<'_>>();