new lint: find functions that can be "const fn"

[matthiaskrgr]

Maybe we can automatically find simple functions that can be const fn

This was suggested during FOSDEM talk on rusts const evaluation: https://youtu.be/Zz863ksXRhA?t=1102
I didn't see a ticket about this yet so here we go.

[sunjay]

Yes! This would be great! I would be happy with it even if it just started as a simple lint that encouraged people to make code like this const:

impl Game {
    // Could be const
    pub fn new() -> Self {
        Self {
            tiles: Default::default(),
            current_piece: Piece::X,
            winner: None,
        }
    }
}

Of course, it could always grow from there to become more complete. This would just give people a start into using const. Not having const on simple associated functions like this makes it really hard to use these in const declarations in your code. If a library you use doesn't have this, you get completely stuck.

That's why I think it's really important to start getting people in the habit of putting const on functions when they can. I think a clippy lint will go a long way.

[oli-obk]

So... implementing this lint has become significantly easier lately: Just call https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_min_const_fn.rs#L11 and check the result. There are some false positives, but we can't check them right now outside the compiler (because the check emits errors immediately instead of reporting a Result, so if we run it on normal functions we get loads of errors emitted).

[phansch]

Going to give this a try over the next couple of days. First step is getting rust-lang/rust#57342 merged.