Auto merge of #64525 - nikomatsakis:issue-64512-drop-order-tail-temp, r=davidtwco

adjust desugaring for async fn to correct drop order

Old desugaring, given a user function body `{ $stmts; $expr }`

```
{
    let $param_pattern0 = $raw_param0;
    ...
    let $param_patternN = $raw_paramN;
    $stmts;
    $expr
}
```

New desugaring:

```
{
    let $param_pattern0 = $raw_param0;
    ...
    let $param_patternN = $raw_paramN;
    drop-temps {
        $stmts;
        $expr
    }
}
```

The drop-temps is an internal bit of HIR that drops temporaries from the resulting expression, but it should be equivalent to `return { $stmts; $expr }`.

Fixes #64512
Fixes #64391

2 files changed, 109 insertions, 0 deletions

diff --git a/src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs b/src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs
new file mode 100644
index 00000000000..e40acff6dc1
--- /dev/null
+++ b/src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs
@@ -0,0 +1,95 @@
+// aux-build:arc_wake.rs
+// edition:2018
+// run-pass
+
+#![allow(unused_variables)]
+
+// Test the drop order for parameters relative to local variables and
+// temporaries created in the tail return expression of the function
+// body. In particular, check that this drop order is the same between
+// a `async fn` and an ordinary `fn`. See #64512.
+
+extern crate arc_wake;
+
+use arc_wake::ArcWake;
+use std::cell::RefCell;
+use std::future::Future;
+use std::sync::Arc;
+use std::rc::Rc;
+use std::task::Context;
+
+struct EmptyWaker;
+
+impl ArcWake for EmptyWaker {
+    fn wake(self: Arc<Self>) {}
+}
+
+#[derive(Debug, Eq, PartialEq)]
+enum DropOrder {
+    Function,
+    Val(&'static str),
+}
+
+type DropOrderListPtr = Rc<RefCell<Vec<DropOrder>>>;
+
+struct D(&'static str, DropOrderListPtr);
+
+impl Drop for D {
+    fn drop(&mut self) {
+        self.1.borrow_mut().push(DropOrder::Val(self.0));
+    }
+}
+
+/// Check drop order of temporary "temp" as compared to `x`, `y`, and `z`.
+///
+/// Expected order:
+/// - `z`
+/// - temp
+/// - `y`
+/// - `x`
+async fn foo_async(x: D, _y: D) {
+    let l = x.1.clone();
+    let z = D("z", l.clone());
+    l.borrow_mut().push(DropOrder::Function);
+    helper_async(&D("temp", l)).await
+}
+
+async fn helper_async(v: &D) { }
+
+fn foo_sync(x: D, _y: D) {
+    let l = x.1.clone();
+    let z = D("z", l.clone());
+    l.borrow_mut().push(DropOrder::Function);
+    helper_sync(&D("temp", l))
+}
+
+fn helper_sync(v: &D) { }
+
+fn assert_drop_order_after_poll<Fut: Future<Output = ()>>(
+    f: impl FnOnce(DropOrderListPtr) -> Fut,
+    g: impl FnOnce(DropOrderListPtr),
+) {
+    let empty = Arc::new(EmptyWaker);
+    let waker = ArcWake::into_waker(empty);
+    let mut cx = Context::from_waker(&waker);
+
+    let actual_order = Rc::new(RefCell::new(Vec::new()));
+    let mut fut = Box::pin(f(actual_order.clone()));
+    let r = fut.as_mut().poll(&mut cx);
+
+    assert!(match r {
+        std::task::Poll::Ready(()) => true,
+        _ => false,
+    });
+
+    let expected_order = Rc::new(RefCell::new(Vec::new()));
+    g(expected_order.clone());
+
+    assert_eq!(*actual_order.borrow(), *expected_order.borrow());
+}
+
+fn main() {
+    // Free functions (see doc comment on function for what it tests).
+    assert_drop_order_after_poll(|l| foo_async(D("x", l.clone()), D("_y", l.clone())),
+                                 |l| foo_sync(D("x", l.clone()), D("_y", l.clone())));
+}
diff --git a/src/test/ui/async-await/issue-64391.rs b/src/test/ui/async-await/issue-64391.rs
new file mode 100644
index 00000000000..c6faad3aad0
--- /dev/null
+++ b/src/test/ui/async-await/issue-64391.rs
@@ -0,0 +1,14 @@
+// Regression test for Issue #64391. The goal here is that this
+// function compiles. In the past, due to incorrect drop order for
+// temporaries in the tail expression, we failed to compile this
+// example. The drop order itself is directly tested in
+// `drop-order/drop-order-for-temporary-in-tail-return-expr.rs`.
+//
+// check-pass
+// edition:2018
+
+async fn add(x: u32, y: u32) -> u32 {
+    async { x + y }.await
+}
+
+fn main() { }