Macro call 

call!() { /* proc-macro */ }

Marks an explicit trampoline-backed tail-call site inside a #[tailcall] function.

The macro expects either a direct function call or a method call on self, such as:

tailcall::call! { factorial_inner(acc * input, input - 1) }
tailcall::call! { self.is_odd(x - 1) }

It expands to a call to the hidden trampoline builder generated by the #[tailcall] attribute. The call site itself must remain in tail position.