error[E0435]: attempt to use a non-constant value in a constant
  --> $DIR/E0435.rs:5:17
   |
LL |     let _: [u8; foo];
   |                 ^^^ non-constant value
   |
help: consider using `const` instead of `let`
   |
LL |     const foo: usize = 42;
   |     ~~~~~

error: aborting due to 1 previous error

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