expand comments of TyCtxt impl of DynSync/DynSend

so that it doesn't get removed again

Author: lqd

compiler/rustc_middle/src/ty/context.rs

@@ -1327,7 +1327,9 @@ pub struct TyCtxt<'tcx> {
     gcx: &'tcx GlobalCtxt<'tcx>,
 }
 
-// Explicitly implement `DynSync` and `DynSend` for `TyCtxt` to short circuit trait resolution.
+// 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.
 unsafe impl DynSend for TyCtxt<'_> {}
 unsafe impl DynSync for TyCtxt<'_> {}
 fn _assert_tcx_fields() {