unevaluated consts in param_env query cycle

[lcnr]

const CONST: &str = "hi";

trait ToUnit {
    type Assoc;
}
impl<T> ToUnit for T {
    type Assoc = ();
}

fn foo()
where
    <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
{}

results in

error[E0391]: cycle detected when evaluating type-level constant
  --> src/main.rs:12:11
   |
12 |     <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
   |           ^^^^^^^^^^^
   |
note: ...which requires const-evaluating + checking `foo::{constant#0}`...
  --> src/main.rs:12:11
   |
12 |     <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
   |           ^^^^^
note: ...which requires simplifying constant for the type system `CONST`...
  --> src/main.rs:1:1
   |
 1 | const CONST: &str = "hi";
   | ^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating + checking `CONST`...
  --> src/main.rs:1:1
   |
 1 | const CONST: &str = "hi";
   | ^^^^^^^^^^^^^^^^^
   = note: ...which requires computing layout of `&str`...
note: ...which requires computing whether `str` is `Sized`...
  --> src/main.rs:12:11
   |
12 |     <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
   |           ^^^^^^^^^^^
   = note: ...which again requires evaluating type-level constant, completing the cycle
note: cycle used when checking that `foo` is well-formed
  --> src/main.rs:12:11
   |
12 |     <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
   |           ^^^^^^^^^^^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

with the new solver as we evaluate CONST.len() in the param_env of the impl, we then try to use the where-bound containing that const when checking whether str: Sized holds while computing the layout of the return type of CONST.

The reason this doesn't have a cycle error in the old solver is that we don't try to normalize where-bounds while trait solving.