Implement `?` in catch expressions

[cramertj]

Builds on #39921. Final part of #39849.

r? @nikomatsakis

[bors]

☔️ The latest upstream changes (presumably #40133) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

at first read, this all looks good, I'll have to revisit it in more detail when it's rebased

[nikomatsakis]

Ah, a thought. It'd be good to have some tests targeting the CFG etc. The idea would be to have initializations and moves that occur during a catch and check that we track it properly. For example:

let x;
let _v = do catch {
    x = 5;
    Ok(());
};
println!("{}", x); // OK

let x;
let _v = do catch {
    x = Ok(5)?;
    Ok(());
};
println!("{}", x); // ERROR

and similar tests for moves that may or may not occur (and perhaps a few for borrows that start in the catch and extend outside, etc).

[bors]

☔️ The latest upstream changes (presumably #40220) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

can you add a comment on this test explaining what the test aims to demonstrate (e.g., that a borrowed valid inside the do catch will propagate out?

I also think it'd be good to possibly break this test into distinct files.

[nikomatsakis]

can you modify the test to demonstrate that cfg_res is usable *within the do catch, if not outside?

Also, add a (run-pass) test like so, where cfg_res is initialized before the ?:

pub fn main() {
    let cfg_res;
    let _: Result<(), ()> = do catch {
        cfg_res = 5;
        Err(())?;
        Ok(())
    };
    use(cfg_res); // OK
}

[nikomatsakis]

I think I would also like some tests around lifetimes like these:

// Test that a borrow which only conditionally occurs still freezes `i` 

fn main() {
    let mut i = 222;
    let j;
    let x = do catch {
        Err(())?;
        Ok(&i)
    };
    i = 0; // ERROR
    println!("{}", i);
}

// Test that a borrow which only conditionally occurs still freezes `i` 

fn main() {
    let mut i = 222;
    let j;
    let x = do catch {
        Err(())?;
        j = &i;
        Ok(())
    };
    i = 0; // ERROR, I ... guess ... maybe not with NLL
    println!("{}", i);
}

[nikomatsakis]

@bors r+

[bors]

📌 Commit bc07fe6 has been approved by nikomatsakis

[bors]

☔️ The latest upstream changes (presumably #40598) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 1f43731 has been approved by nikomatsakis

[bors]

⌛️ Testing commit 1f43731 with merge 58c701f...