cycle errors for unevaluated constants in the param env

[lcnr]

// check-pass
#![feature(generic_const_exprs)]
#![allow(incomplete_features)]

struct Foo<T>(T);

fn test() where [u8; {
    let _: Foo<[u8; 3 + 4]>;
    3
}]: Sized {}

ends up failing with

error[E0391]: cycle detected when simplifying constant for the type system `test::{constant#0}`
  --> src/lib.rs:7:22
   |
7  |   fn test() where [u8; {
   |  ______________________^
8  | |     let _: Foo<[u8; 3 + 4]>;
9  | |     3
10 | | }]: Sized {
   | |_^
   |
note: ...which requires const-evaluating + checking `test::{constant#0}`...
  --> src/lib.rs:7:22
   |
7  |   fn test() where [u8; {
   |  ______________________^
8  | |     let _: Foo<[u8; 3 + 4]>;
9  | |     3
10 | | }]: Sized {
   | |_^
note: ...which requires type-checking `test::{constant#0}`...
  --> src/lib.rs:7:22
   |
7  |   fn test() where [u8; {
   |  ______________________^
8  | |     let _: Foo<[u8; 3 + 4]>;
9  | |     3
10 | | }]: Sized {
   | |_^
   = note: ...which requires evaluating trait selection obligation `[u8; _]: std::marker::Sized`...
   = note: ...which again requires simplifying constant for the type system `test::{constant#0}`, completing the cycle
note: cycle used when checking that `test` is well-formed
  --> src/lib.rs:10:5
   |
10 | }]: Sized {
   |     ^^^^^

The cycle happens while typechecking the outer array length. This happens because Foo<[u8; 3 + 4]> requires [u8; 3 + 4] to be sized. While trying to fulfill this we see the [u8; ...]: Sized bound in the param_env and try to unify these two. This causes us to evaluate both constants which once again relies on typechecking them.

*This test succeeds with min_const_generics and currently fails with const_generics.

[lcnr]

i think the same problem should also be also responsible for the cycle error in this example:

#![feature(generic_const_exprs)]

trait Foo {
    const N: usize;
}

struct Num<const N: usize>
where
    [(); <Self as Foo>::N]: ;

impl<const N: usize> Foo for Num<N> {
    const N: usize = N - 1;
}

[lcnr]

edit: using <X as Trait>::ASSOC not only requires X: Trait but also all predicates of that trait, which is what's causing the cycle here, so this is a slightly different issue

and

#![feature(generic_const_exprs)]

trait Foo where [(); Self::N]: {
  const N: usize;
}

[lcnr]

from zulip: https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/lazy.20norm.20future.20compat/near/217760369

#![feature(const_generics)]
#![allow(incomplete_features)]

struct Foo<T>(T);

fn test() where [u8; {
    let _: Foo<[u8; 3 + 4]>;
    3
}]: Sized,
[u8; {
    let _: Foo<[u8; 3 + 4]>;
    4
}]: Sized  {}

[lcnr]

the array len from the obligation does not have to be unevaluated.

#![feature(generic_const_exprs)]
#![allow(incomplete_features)]

struct Foo<T>(T);

fn test() where [u8; {
    let _: Foo<[u8; 7]>;
    3
}]: Sized {}