What is the cost of adding TCO to rust / why does not rust have TCO?

6

In particular, it is not obvious to me what the "cost" of TCO in Rust would be. Suppose:

fn A() {
  return B();
}

fn C() {
  A();
}

then what we have here is:

| stack frame C | stack frame A |

without TCO, we do:

| stack frame C | stack frame A | stack frame B |

then when B returns, we pop B's frame, then pop A's frame. With TCO, we just pop A's frame before hand, and do:

| stack frame C | stack frame B |

instead.

====

Atleast with sysv c abi calling convention, I do not see any additional overhead that TCO creates -- so I am trying to understand: what is the overhead / cost of adding TCO to rust ?