overhead of getfield projection approach

[jrevels]

For the sake of metadata propagation, Cassette will intercept getfield(x, :f) (before type inference) and convert it into something like Cassette._getfield(x, Val(:f)). Bringing :f into the type domain is necessary to enable the type-stable construction of "shadow structs" that can store metadata alongside normal structs.

According to a comment by @vtjnash in the base getfield overloading issue, there is some concern that such an approach may thwart necessary optimizations (such as optimizing namespace references).

[jrevels]

ref JuliaLang/julia#24362

[jrevels]

This is basically resolved upstream at this point.