A defer discussion

language design
47

Sorry for the bike-shed, but I think using a closure syntax when we don't use closure semantics (because we don't want to move/borrow the values the defer is supposed to clean up) can be confusing. If you want the ability to conditionally pass the result to the defer, there are two syntax styles for that that are already used in other Rust constructs (and are therefore more teachable):

  1. Always pass the result, and just ignore it with _ when you don't actually need it:
    // Without result:
defer _ { some_queue.push(item); }
// With result:
defer result { result.and_then(|| take_action()); }
  2. dever let
    // Without result:
defer { some_queue.push(item); }
// With result:
defer let result { result.and_then(|| take_action()); }

Also, with both styles (though it may be more natural for defer let) we can support refutable patterns:

// First style
defer Ok(result) { take_action(); }
// Second style
defer let Ok(result) { take_action(); }