Joseph Ferano
e041b2f26c
defer is a compile-time construct: the cleanup is copied into every exit path of the function. That is why a loop body and a branch are refused — a loop body's would fire once at function exit rather than once per iteration, and a branch would have to express "maybe registered", which a form copied into every exit path or into none cannot say. A let is neither. It is not a frame here: its bindings are function slots like any other and nothing is released at scope exit, so a let at the top level of a function body has exactly the function's extent and a defer written in it always registers. It was refused for a reason that does not apply to it. A let nested inside such a let has the same extent and the same permission; a let inside a while or an if has the loop's or the arm's, and inherits the refusal. The permission is granted again before every form of a body, never once around the body: check withdraws it as it starts, so granting it once would let the first defer through and refuse the second — and two resources acquired in one let is the case this exists for. defer-let.flan covers that one specifically, along with nesting, interleaved registration order across the let boundary, and an early return. The two refusals that stay now name what blocks them.
