Rollup merge of #125293 - dingxiangfei2009:tail-expr-temp-lifetime, r=estebank,davidtwco

Place tail expression behind terminating scope

This PR implements #123739 so that we can do further experiments in nightly.

A little rewrite has been applied to `for await` lowering. It was previously `unsafe { Pin::unchecked_new(into_async_iter(..)) }`. Under the edition 2024 rule, however, `into_async_iter` gets dropped at the end of the `unsafe` block. This presumably the first Edition 2024 migration rule goes by hoisting `into_async_iter(..)` into `match` one level above, so it now looks like the following.
```rust
match into_async_iter($iter_expr) {
  ref mut iter => match unsafe { Pin::unchecked_new(iter) } {
    ...
  }
}
```

0 files changed, 0 insertions, 0 deletions