add better error message for Python in signature

[davidhewitt]

Inspired by #2929, this just adds a better error message when Python arguments are accidentally included in the signature.

[adamreichold]

Was the change from closure to named type required for this to work?

[davidhewitt]

The simplest way I could see to implement this was to use recursion to skip over Pythonarguments, which makes a horrible mess of closures.

Not sure what I've come up with here is the prettiest either. Maybe give me another shot at this tomorrow and I'll try factor it out differently?

[adamreichold]

I see, yes recursion and closure would indeed be a mess. I am not opposed to the change, I just asked to make sure that I did not miss anything. (And indeed I did not see the recursion at a first glance.)

For such cases, I think loop might help, i.e. turn

fn func(iter: I) -> R {
  if let Some(val) = iter.next() {
    if condition(val) {
      return func(iter);
    }

    ...
  } else {
    ...
  }
}

into

fn func(iter: I) -> R {
  loop {
    if let Some(val) = iter.next() {
      if condition(val) {
        continue;
      }

      break ...;
    } else {
      break ...;
    }
  }
}

[mejrs]

I could be missing something, but this looks like an awful way to reinvent retain.

let mut err = None;
arguments.retain(|a| /* delete some things and/or set err */);
if let Some(err){
     return ...
}

[davidhewitt]

Not exactly - we don't want to remove elements from arguments, just enrich them based off the attribute information. This might be able to be refactored better once deprecated #[args] is removed.

I'll have another go at tidying this up now.

[adamreichold]

A for loop is certainly nicer than an open-coded loop loop. LGTM.

(The .by_ref() is required because adding move |... to the closure would move more than just the iterator into it?)

[davidhewitt]

The .by_ref() is needed because I don't want to move the iterator into the closure - I check at the end of the function that we've consumed everything from it.

[davidhewitt]

bors r=adamreichold

[bors]

Build succeeded:

• conclusion