error[E0379]: functions in traits cannot be declared const
  --> $DIR/issue-54954.rs:5:5
   |
LL |     const fn const_val<T: Sized>() -> usize {
   |     ^^^^^-
   |     |
   |     functions in traits cannot be const
   |     help: remove the `const`

error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
  --> $DIR/issue-54954.rs:1:24
   |
LL |   const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
...
LL | /     const fn const_val<T: Sized>() -> usize {
LL | |
LL | |         core::mem::size_of::<T>()
LL | |     }
   | |_____- `Tt::const_val` defined here

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0379, E0790.
For more information about an error, try `rustc --explain E0379`.