Original `pin!()` macro behavior cannot be expressed in Rust 2024

[m-ou-se]

See #138596

It looks like Rust 2024 regressed in expressivity. The exact original behavior of the pin!() macro cannot be expressed in Rust 2024.

As a temporary workaround for pin!() itself, #138717 adds #[rustc_macro_edition_2021].

Both #138622 (comment) and #138717 mention super let as the proper solution.

One could argue that pin!() cannot be expressed in any Rust edition, because it relies on an unstable public (hidden) field and #[allow_internal_unstable]. This problem is also solved by super let.

[m-ou-se]

Update: Turns out super let is quite simple to implement. Starting an experiment and posting an implementation soon.

[m-ou-se]

@conradludgate In your bug report, you report this code snippet as getting broken:

fn main() {
    match std::pin::pin!(foo(&mut 0)) {
        _f => {}
    }
}

async fn foo(_s: &mut usize) {}

Do you have the real code where this popped up? I'd love to see a more realistic example to understand in which cases supporting this is important.

[conradludgate]

It's not code I'm particularly proud of, but here https://github.com/neondatabase/neon/blob/939354abea3915d9a2a3ccd6b1f7547fe2ab2cd6/proxy/src/serverless/sql_over_http.rs#L874