review + add tests

Author: lcnr

compiler/rustc_ty_utils/src/needs_drop.rs

@@ -67,7 +67,7 @@ struct NeedsDropTypes<'tcx, F> {
  tcx: TyCtxt<'tcx>,
  param_env: ty::ParamEnv<'tcx>,
  // Whether to reveal coroutine witnesses, this is set
- // to `false` unless we compute `needs_drop` for a generator witness.
+ // to `false` unless we compute `needs_drop` for a coroutine witness.
  reveal_coroutine_witnesses: bool,
  query_ty: Ty<'tcx>,
  seen_tys: FxHashSet<Ty<'tcx>>,
@@ -138,11 +138,11 @@ where
  // computed on MIR, while this very method is used to build MIR.
  // To avoid cycles, we consider that coroutines always require drop.
  //
- // HACK: Because we erase regions contained in the generator witness, we
+ // HACK: Because we erase regions contained in the coroutine witness, we
  // have to conservatively assume that every region captured by the
- // generator has to be live when dropped. This results in a lot of
+ // coroutine has to be live when dropped. This results in a lot of
  // undesirable borrowck errors. During borrowck, we call `needs_drop`
- // for the generator witness and check whether any of the contained types
+ // for the coroutine witness and check whether any of the contained types
  // need to be dropped, and only require the captured types to be live
  // if they do.
  ty::Coroutine(_, args, _) => {

tests/ui/dropck/coroutine-liveness-1.rs

@@ -0,0 +1,18 @@
+// check-pass
+// edition: 2021
+
+// regression test for #116242.
+use std::future;
+
+fn main() {
+ let mut recv = future::ready(());
+ let _combined_fut = async {
+ let _ = || read(&mut recv);
+ };
+
+ drop(recv);
+}
+
+fn read<F: future::Future>(_: &mut F) -> F::Output {
+ todo!()
+}

tests/ui/dropck/coroutine-liveness-2.rs

@@ -0,0 +1,23 @@
+// check-pass
+// edition: 2021
+
+// regression test found while working on #117134.
+use std::future;
+
+fn main() {
+ let mut recv = future::ready(());
+ let _combined_fut = async {
+ let _ = || read(&mut recv);
+ };
+
+ let _uwu = (String::new(), _combined_fut);
+ // Dropping a coroutine as part of a more complex
+ // types should not add unnecessary liveness
+ // constraints.
+
+ drop(recv);
+}
+
+fn read<F: future::Future>(_: &mut F) -> F::Output {
+ todo!()
+}