> If given explicit support in a language then no it's not. Especially with the keyword approach rust is taking, the compiler will guarantee the semantics.

Then it's TCE not TCO.

> Do you believe the exec system call is similarly unreliable at replacing your process image?

Exec is entirely unlike TCO. If exec was a syscall which sometimes did and sometimes did not replace your process image, then it'd be like TCO, and it'd be similarly unreliable.