Tracking Issue for `assert_eq` in constants

[oli-obk]

In order to use assert_eq! in constants, we first need to have panic! with formatting (#51999) and const fn PartialEq::eq implementations (#67792) implemented and stabilized. If you look at the desugaring of assert_eq!, this quickly becomes obvious:

assert_eq!(a, b);

desugars to

        match (&a, &b) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    // The reborrows below are intentional. Without them, the stack slot for the
                    // borrow is initialized even before the values are compared, leading to a
                    // noticeable slow down.
                    panic!(r#"assertion failed: `(left == right)`
  left: `{:?}`,
 right: `{:?}`"#, &*left_val, &*right_val)
                }
            }
        }

where the == operation will desugar to PartialEq::eq(left_val, right_val) for all non-primites. If we want to, we can support assert_eq! for primitives (integers, chars, bools) with only panicking with formatting.

[mental32]

@oli-obk does this only apply to assert_eq or does it cover all of assert/assert_eq/assert_ne/debug_* variants?

[oli-obk]

It covers all of these unless there's no formatting in assert or debug_assert, then those will work even without the formatting part of #51999

[lcnr]

discussed in a compiler meeting: https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bsteering.20meeting.5D.202022-09-16.20JIT.20backlog.20b'za/near/299173821

closing as the information provided here is also part of the const-eval skill tree where it's probably more discoverable: https://rust-lang.github.io/const-eval/

[ilyvion]

1. For some reason, this "const-eval skill tree" is presented so small as to be basically unreadable:
   
2. There's no instructions on how to read this thing anyway, so even if I could read the text in the dark orange/brown boxes, I'm not sure what it's supposed to be telling me about why I can't use assert_eq! in a const context.
3. As far as I can tell, based on the error you get, the problem is that there's a call to a non-const function core::panicking::assert_failed, and although this issue was closed over a year ago, it's still happening in Rust today.

[ChrisDenton]

As the OP says, this is requires const traits (#67792).