Place tail expression behind terminating scope

[dingxiangfei2009]

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.

match into_async_iter($iter_expr) {
  ref mut iter => match unsafe { Pin::unchecked_new(iter) } {
    ...
  }
}

[rustbot]

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[dingxiangfei2009]

cc @traviscross for edition 2024 update

cc @eholk for context who has worked on async iterator

[petrochenkov]

r? compiler

[SparrowLii]

Many tests should be added. Such as

1. code will still panic, but leave the mutex unpoisoned

fn f(m: &Mutex<i32>) -> i32 {
    let _x = PanicOnDrop;
    *m.lock().unwrap()
}

2. mutable reference

fn main() {
    let c = std::cell::RefCell::new(123);

    if let Ok(mut b) = c.try_borrow_mut() {
        *b = 321;
    }
}

3. Explicit drop order
   https://github.com/rust-lang/rfcs/pull/3606/files#r1548272644

4. injects a borrow to a temporary into an already existing local variable that borrows it on drop

fn why_would_you_do_this() -> bool {
    let mut x = None;
    // Make a temporary `RefCell` and put a `Ref` that borrows it in `x`.
    x.replace(RefCell::new(123).borrow()).is_some()
}

5. block could be a subexpression of a larger expression

    let zero = { String::new().as_str() }.len();

Considering this is a breaking feature, more tests would be better.