CrossStageSafety

Checks that staging level consistency holds and heals staged types.

Local term references are level consistent if and only if they are used at the same level as their definition.

Local type references can be used at the level of their definition or lower. If used used at a higher level, it will be healed if possible, otherwise it is inconsistent.

Healing a type consists in replacing locally defined types defined at staging level 0 and used in higher levels. For each type local T that is defined at level 0 and used in a quote, we summon a tag t: Type[T]. This t tag must be defined at level 0. The tags will be listed in the tags of the level 0 quote ('<t>{ ... }) and each reference to T will be replaced by t.Underlying in the body of the quote.

We delay the healing of types in quotes at level 1 or higher until those quotes reach level 0. At this point more types will be statically known and fewer types will need to be healed. This also keeps the nested quotes in their original form, we do not want macro users to see any artifacts of this phase in quoted expressions they might inspect.

Type heal example:

'{ val x: List[T] = ListT '{ .. T .. } () }

is transformed to

' { // where t is a given term of type Type[T] val x: List[t.Underlying] = List t.Underlying; '{ .. t.Underlying .. } () }