error: attempt to subtract with overflow
  --> $DIR/const-len-underflow-subspans.rs:18:17
   |
LL |     let a: [i8; ONE - TWO] = unimplemented!();
   |                 ^^^^^^^^^
   |
   = note: #[deny(const_err)] on by default

error[E0080]: could not evaluate constant expression
  --> $DIR/const-len-underflow-subspans.rs:18:12
   |
LL |     let a: [i8; ONE - TWO] = unimplemented!();
   |            ^^^^^---------^
   |                 |
   |                 attempt to subtract with overflow

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0080`.