Make Option::unwrap unstably const

[ecstatic-morse]

This is lumped into the const_option feature gate (#67441), which enables a potpourri of Option methods.

cc @rust-lang/wg-const-eval

r? @oli-obk

[ecstatic-morse]

The error annotations in the test are a bit weird, but it's more of a problem for compiletest than for users.

[leonardo-m]

This is useful for:

const N: NonZeroU32 = NonZeroU32::new(10).unwrap();

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit 96c84ac has been approved by oli-obk

[RalfJung]

What is this? I'm in the const-eval GH group and I don't think I ever saw this or #73255 ever before.^^

[RalfJung]

If anything it is because const-eval panic reporting ignores track_caller.

[bors]

⌛ Testing commit 96c84ac with merge 3a92b99...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing 3a92b99 to master...

[leonardo-m]

This is still not allowed:

    const N: usize = 20;
    const NI: i32 = i32::try_from(N).unwrap();

[oli-obk]

The unwrap is not the issue here, it's try_from, which requires #67792

[oli-obk]

Once this PR lands in nightly you should be able to do nope, see below for details

#![feature(const_trait_impl, const_option)]
use std::convert::TryFrom;

struct A(usize);
struct B(i32);

impl const TryFrom<A> for B {
    type Error = &'static str;
    fn try_from(x: A) -> Result<Self, &'static str> {
        let converted = x.0 as i32;
        if x.0 == converted as usize {
            Ok(B(converted))
        } else {
            Err("no")
        }
    }
}

const N: usize = 20;
const NI: i32 = B::try_from(A(N)).unwrap().0;

EDIT: ah nevermind, this unwraps a Result, which requires a Debug impl, so this is way more complex. This is tracked under panic! with formatting in https://static.turbo.fish/const-eval-skill-tree/skill-tree.html

We can't use .ok() either, because that drops the error, so we need to have the Drop node in the skill tree