Fix for async drop inside async gen fn

Author: azhogin

compiler/rustc_mir_transform/src/coroutine.rs

@@ -317,6 +317,23 @@ impl<'tcx> TransformVisitor<'tcx> {
                         ),
                         user_ty: None,
                     })))
+                } else if matches!(
+                    val,
+                    Operand::Constant(box ConstOperand {
+                        const_: Const::Val(ConstValue::Scalar(_), _),
+                        ..
+                    })
+                ) {
+                    // yield (const false) is used for async drop yields inside AsyncGen
+                    // We need to convert value to Poll<OptRet>::Pending
+                    let poll_def_id = self.tcx.require_lang_item(LangItem::Poll, None);
+                    let ty::Adt(_poll_adt, args) = *self.old_yield_ty.kind() else { bug!() };
+                    make_aggregate_adt(
+                        poll_def_id,
+                        VariantIdx::from_usize(1),
+                        args,
+                        IndexVec::new(),
+                    )
                 } else {
                     Rvalue::Use(val)
                 }

tests/ui/async-await/async-drop/assign-incompatible-types.rs

@@ -0,0 +1,9 @@
+// ex-ice: #140530
+//@ edition: 2024
+//@ build-pass
+#![feature(async_drop, gen_blocks)]
+#![allow(incomplete_features)]
+async gen fn a() {
+  _ = async {}
+}
+fn main() {}