error[E0038]: the trait `Bar` cannot be made into an object
  --> $DIR/issue-48027.rs:6:6
   |
LL |     const X: usize;
   |           - the trait cannot contain associated consts like `X`
...
LL | impl dyn Bar {}
   |      ^^^^^^^ the trait `Bar` cannot be made into an object

error[E0283]: type annotations needed
  --> $DIR/issue-48027.rs:3:32
   |
LL |     const X: usize;
   |     --------------- required by `Bar::X`
LL |     fn return_n(&self) -> [u8; Bar::X];
   |                                ^^^^^^ cannot infer type
   |
   = note: cannot resolve `_: Bar`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0038, E0283.
For more information about an error, try `rustc --explain E0038`.