Non-API calls

[lionel-]

Easy:

• SET_ENCLOS(): Used to create new environments, should be easy to fix: Remove SET_ENCLOS() usage rlang#1707
• PREXPR()/R_PromiseExpr()`: Remove PREXPR() usage (as much as we can) rlang#1708

Unknown:

• SET_S4_OBJECT()/UNSET_S4_OBJECT(): Used for proxying / unproxying S4 objects. Can we do better?

Problematic:

• SETLENGTH()/SET_TRUELENGTH(): Used in various algorithms for expansible vectors. Also used in data.table for the same purpose. Would be nice to find a solution with R core.
• STDVEC_DATAPTR(): Part of the ALTREP API. Can we get this part of the experimental API that can be used in packages?

[DavisVaughan]

I think STDVEC_DATAPTR() can be replaced with DATAPTR() but I may be wrong