The $:expr case keeps temporaries alive longer than needed

[dtolnay]

This can lead to non-Sync futures for no good reason, if non-Sync temporaries inside an anyhow! expression are live across an await point.

use anyhow::anyhow;
use std::cell::Cell;
use std::future;

fn assert_sync<T: Sync>(_: T) {}

fn message(cell: Cell<&str>) -> &str {
    cell.get()
}

fn main() {
    assert_sync(async {
        future::ready(anyhow!(message(Cell::new("...")))).await;
    });
}

error: future cannot be shared between threads safely
  --> src/main.rs:12:5
   |
12 |     assert_sync(async {
   |     ^^^^^^^^^^^ future created by async block is not `Sync`
   |
   = help: within `impl Future`, the trait `Sync` is not implemented for `Cell<&str>`
note: future is not `Sync` as this value is used across an await
  --> src/main.rs:13:9
   |
13 |         future::ready(anyhow!(message(Cell::new("...")))).await;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ first, await occurs here, with `Cell::new("...")` maybe used later...
note: `Cell::new("...")` is later dropped here
  --> src/main.rs:13:64
   |
13 |         future::ready(anyhow!(message(Cell::new("...")))).await;
   |                                       ----------------         ^
   |                                       |
   |                                       has type `Cell<&str>` which is not `Sync`
note: required by a bound in `assert_sync`
  --> src/main.rs:5:19
   |
5  | fn assert_sync<T: Sync>(_: T) {}
   |                   ^^^^ required by this bound in `assert_sync`