error[E0435]: attempt to use a non-constant value in a constant
  --> $DIR/type-dependent-def-issue-49241.rs:3:22
   |
LL |     const l: usize = v.count();
   |                      ^ non-constant value
   |
help: consider using `let` instead of `const`
   |
LL -     const l: usize = v.count();
LL +     let l: usize = v.count();
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0435`.