error[E0416]: identifier `tail` is bound more than once in the same pattern
  --> $DIR/subslice-only-once-semantic-restriction.rs:4:24
   |
LL |         [1, tail @ .., tail @ ..] => {},
   |                        ^^^^ used in a pattern more than once

error: `..` can only be used once per slice pattern
  --> $DIR/subslice-only-once-semantic-restriction.rs:4:31
   |
LL |         [1, tail @ .., tail @ ..] => {},
   |                    --         ^^ can only be used once per slice pattern
   |                    |
   |                    previously used here

error[E0308]: mismatched types
  --> $DIR/subslice-only-once-semantic-restriction.rs:11:30
   |
LL | const RECOVERY_WITNESS: () = 0;
   |                              ^ expected `()`, found integer

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0308, E0416.
For more information about an error, try `rustc --explain E0308`.