error: function cannot return without recursing
  --> $DIR/lint-unconditional-recursion-tail-calls.rs:5:1
   |
LL | fn f(x: bool) {
   | ^^^^^^^^^^^^^ cannot return without recursing
...
LL |         f(!x)
   |         ----- recursive call site
   |
   = help: a `loop` may express intention better if this is on purpose
note: the lint level is defined here
  --> $DIR/lint-unconditional-recursion-tail-calls.rs:2:9
   |
LL | #![deny(unconditional_recursion)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error