min_const_generics allows defining new types in const argument

[Aaron1011]

I recently discovered that the following code compiles:

#![feature(min_const_generics)]
struct Bar<const C: bool>;

struct Foo {
    outer: Bar<{
        struct Inner {
            field: bool
        }
        true
    }>,
}

It looks like an AnonConst is allowed to contain arbitrary expressions (as long as no const or type parameters are used), which means that new types can be defined.

I'm not certain whether this should be allowed or not. However, I haven't seen this discussed before, so I thought it was worth bringing up.

[scottmcm]

Feels to me like this, at the very least, shouldn't be min_const_generics.

(I'd personally be happy to block it from const_generics too, but there are consistency arguments that's it's not unreasonable to allow types in there, since they're allowed in other blocks.)

[petrochenkov]

Items were allowed in constants expressions since forever, not sure why const generic arguments should be any different.

#![allow(unused)]

type A = [u8; { struct S; 1 }]; // OK since 1.0

fn main() {}

https://rust.godbolt.org/z/Ts6YT1

[Aaron1011]

@petrochenkov: Wow, I didn't realize that we already supported that.

I'm going to close this issue - since we already supprt this in the array repeat position, there's nothing to gain from banning it in other positions.