avoid jl_value_ptr

[stevengj]

A cleaner solution to JuliaLang/julia#31473 and #670.

[yuyichao]

Technically this is dangerous since the argument to unsafe_convert is not guaranteed rooted by the caller here. The current implementation my or may not take advantage of this but it is technically allowed to assume that and give wrong result.. pointer_from_objref should be fine.

[stevengj]

@yuyichao, the jlWrapType variable is a const global precisely because we can't allow it to be garbage-collected as long as we are using the tp_base pointer.

So, for the same reason, I don't think rooting is an issue for unsafe_convert?

I tend prefer this to pointer_from_objref because it seems to more clearly indicate the intent: "give me the pointer to the C-like struct for this type" rather than "give me the jl_value_t pointer". Those two happen to be the same thing at the moment, but it is not obvious to me that this will always be the case.

[stevengj]

I guess in theory the RefValue object needs to be rooted too, so I would need a GC.@preserve here?

[yuyichao]

Correct, it's the

argument to unsafe_convert

that's the issue.