Account for an additional reborrow inserted by UniqueImmBorrow and MutBorrow

compiler/rustc_mir_transform/src/coroutine/by_move_body.rs

@@ -289,10 +289,15 @@ impl<'tcx> MutVisitor<'tcx> for MakeByMoveBody<'tcx> {
             // generating, we also are taking that field by value. Peel off a deref,
             // since a layer of reffing has now become redundant.
             let final_deref = if needs_deref {
-                let [mir::ProjectionElem::Deref] = projection else {
-                    bug!("There should only be a single deref for an upvar local initialization");
+                let Some((mir::ProjectionElem::Deref, projection)) = projection.split_first()
+                else {
+                    bug!(
+                        "There should be at least a single deref for an upvar local initialization, found {projection:#?}"
+                    );
                 };
-                &[]
+                // There may be more derefs, since we may also implicitly reborrow
+                // a captured mut pointer.
+                projection
             } else {
                 projection
             };

tests/ui/async-await/async-closures/mut-ref-reborrow.rs

@@ -0,0 +1,27 @@
+//@ aux-build:block-on.rs
+//@ run-pass
+//@ check-run-results
+//@ revisions: e2021 e2018
+//@[e2018] edition:2018
+//@[e2021] edition:2021
+
+#![feature(async_closure)]
+
+extern crate block_on;
+
+async fn call_once(f: impl async FnOnce()) { f().await; }
+
+pub async fn async_closure(x: &mut i32) {
+    let c = async move || {
+        *x += 1;
+    };
+    call_once(c).await;
+}
+
+fn main() {
+    block_on::block_on(async {
+        let mut x = 0;
+        async_closure(&mut x).await;
+        assert_eq!(x, 1);
+    });
+}