Macro parameters are not substituted in docs for const generics

[Flying-Toast]

The generated documentation for const generic expressions generated by macros doesn't substitute macro parameters.

The docs for this code:

pub trait ConstGeneric<const N: usize> {}

macro_rules! trait_impl {
    ($typ:ty) => {
        impl ConstGeneric<{core::mem::size_of::<$typ>()}> for $typ {}
    }
}

trait_impl!(i64);

Look like this:

Note that the $typ macro parameter shows up literally in the const expression of the generic. This bug only happens if the macro parameter is in a const expression within the const generic; other occurrences of the macro parameter are properly substituted.

Produced on rustdoc 1.52.0-nightly (3a5d45f68 2021-03-09) and rustdoc 1.51.0-beta.4 (4d25f4607 2021-03-05)

[jyn514]

This looks like a duplicate of #82852.

[Flying-Toast]

@jyn514 I think #82852 is a bit different - that’s about substituting const values, this is relating to substitution of macro parameters.

Unless the intended behavior is for rustdoc to show impl ConstGeneric<8> for i64?

[jyn514]

Unless the intended behavior is for rustdoc to show impl ConstGeneric<8> for i64?

I think that's what it should show, yes. I'm ok with keeping this issue open for the intermediate step of impl ConstGeneric<{core::mem::size_of::<i64>()}> for i64 {}, but I think in practice it will be easier to fix both at once.

[Flying-Toast]

Ok, makes sense. I’ll close this since it’s really just a subset of #82852 and I don’t want to clutter the issue tracker.