Code action to apply DeMorgan's law

[DougGregor]

Description

Sometimes in code we end up with something like:

if !x && y != z { ... }

and it would be cleaner to apply DeMorgan's law to make this:

if !(x || y == z) { ... }

We should have a code action to do this correctly, which requires dealing with inverting conditions and dealing with operator precedence. The SwiftOperators module of swift-syntax can help here.

[ahoppen]

Synced to Apple’s issue tracker as rdar://128016621

[AppAppWorks]

Could this code action extend to cover bitwise operators as well?

[ahoppen]

Yes, that would make sense.

[AppAppWorks]

@ahoppen as you mentioned in #1539, being overly restrictive is more of a problem than being too permissive. However for DeMorgan's law, the situation is a bit different. We may identify multiple candidate expressions as we walk up the syntax tree, e.g. !(a != b || !(c && d)), which I believe isn't a terribly uncommon form of expression.

I can think of two possible ways to deal with the ambiguity,

1. pick the nearest or the farthermost candidate expression;
2. provide a code action for each candidate expression encountered as we ascend to the root expression.

[ahoppen]

It’s probably one of these situations where we need some reference implementation and then use it to decide what feels best. But I think starting with the outermost expression makes sense as a start.

[AppAppWorks]

Shall we support generalised DeMorgan's in addition to dual DeMorgan's?
e.g. !a && !b && !c -> !(a || b || c)

[ahoppen]

I would start with one direction. The dual would be a nice follow-up after that.