error: unused variable: `x`
  --> $DIR/issue-17999.rs:15:13
   |
LL |         let x = (); //~ ERROR: unused variable: `x`
   |             ^ help: consider using `_x` instead
   |
note: lint level defined here
  --> $DIR/issue-17999.rs:11:9
   |
LL | #![deny(unused_variables)]
   |         ^^^^^^^^^^^^^^^^

error: unused variable: `a`
  --> $DIR/issue-17999.rs:17:13
   |
LL |             a => {} //~ ERROR: unused variable: `a`
   |             ^ help: consider using `_a` instead

error: aborting due to 2 previous errors