warning: unreachable block in `if` expression
  --> $DIR/if-ret.rs:6:24
   |
LL | fn foo() { if (return) { } }
   |               -------- ^^^ unreachable block in `if` expression
   |               |
   |               any code following this expression is unreachable
   |
   = note: `#[warn(unreachable_code)]` on by default