warning: function cannot return without recursing
  --> $DIR/in-defining-scope.rs:8:1
   |
LL | fn foo() -> impl Sized {
   | ^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
...
LL |         transmute::<_, u8>(foo());
   |                            ----- recursive call site
   |
   = help: a `loop` may express intention better if this is on purpose
   = note: `#[warn(unconditional_recursion)]` on by default

warning: 1 warning emitted