error[E0379]: functions in traits cannot be declared const
  --> $DIR/issue-54954.rs:7:5
   |
LL |     const fn const_val<T: Sized>() -> usize {
   |     ^^^^^ functions in traits cannot be const

error[E0283]: type annotations needed
  --> $DIR/issue-54954.rs:3:24
   |
LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
...
LL |     const fn const_val<T: Sized>() -> usize {
   |                        - required by this bound in `Tt::const_val`
   |
   = note: cannot satisfy `_: Tt`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0283, E0379.
For more information about an error, try `rustc --explain E0283`.