async fn drop order for temporaries in return expression does not match sync

[nikomatsakis]

This asynchronous function:

async fn foo_async(x: D, y: D) {
    helper_async(&D()).await
    //                ^^^ temporary D
}

async fn helper_async(v: &D) { }

will drop first y, then x, then the temporary D. However an equivalent synchronous function (playground) would drop the temporary, then y, then x. The problem is our desugaring for async fn, which looks something like this:

// async fn foo_async($parameter_patterns) { $body }
fn foo_async(raw_parameters) {
  async move {
    let $parameter_patterns = raw-parameters;
    $body // body of the async fn
  }
}

here, the temporaries in $body wind up being dropped after the let-bound variables of the block. Proposed fix in a comment below.

[nikomatsakis]

Proposed fix is to alter the desugaring to:

// async fn foo_async($parameter_patterns) { $body }
fn foo_async(raw_parameters) {
  async move {
    let $parameter_patterns = raw-parameters;
    return $body; // body of the async fn
  }
}

Changing to return $body; (note the semicolon!) will cause any temporaries in the tail expression of the body to be scoped to that final statement, and hence to be dropped before the parameters.

[nikomatsakis]

I've got a branch in my repository that adds a testcase for this:

https://github.com/nikomatsakis/rust/tree/issue-64512-drop-order-tail-temp

[nikomatsakis]

I'm working on a fix, so self-assigning.