Add a few more comments

compiler/rustc_mir_transform/src/shim.rs

@@ -1023,7 +1023,16 @@ fn build_construct_coroutine_by_move_shim<'tcx>(
         bug!();
     };
 
+    // We use `*mut Self` here because we only need to emit an ABI-compatible shim body,
+    // rather than match the signature exactly.
+    //
+    // The self type here is a coroutine-closure, not a coroutine, and we never read from
+    // it because it never has any captures, because this is only true in the Fn/FnMut
+    // implementation, not the AsyncFn/AsyncFnMut implementation, which is implemented only
+    // if the coroutine-closure has no captures.
     if receiver_by_ref {
+        // Triple-check that there's no captures here.
+        assert_eq!(args.as_coroutine_closure().tupled_upvars_ty(), tcx.types.unit);
         self_ty = Ty::new_mut_ptr(tcx, self_ty);
     }