RFC: add unsafe_pointer_to_any (fixes #2458)

[stevengj]

This adds a function unsafe_pointer_to_any(p::Ptr) that takes a boxed jl_value_t* and converts it back to a Julia Any. It works by adding a new pointerany intrinsic.

This is useful in callback functions, because raw jl_value_t* pointers to boxed Julia types can be passed as void* to C functions by declaring the argument as Any, but there was previously no way to undo this conversion to get back the native Julia object.

In particular, it should be quite useful in implementing closures, as discussed on the mailing list.

[stevengj]

@JeffBezanson should probably look over the src/intrinsics.cpp patch, to make sure I'm grokking the LLVM stuff.

[vtjnash]

to avoid adding new intrinsics, i was going to give this behavior to unsafe_ref (sorry, I would have responded sooner, but didn't have good internet access earlier). currently unsafe_ref(pointer(Any,uint(0))) throws a compile error only because it was explicitly excluded from things unsafe_ref was allowed to return. i expect i should be able to add that functionality pretty quickly (it's pretty much the same code as you have here, just tacked on a different function)

[stevengj]

@vtjnash, unsafe_ref on Ptr{Any} sounds good to me. Looking forward to your patch.

[JeffBezanson]

I could also just allow Any as an argument type in cfunction. Currently it only accepts concrete types, so for purposes of passing a closure you can use Function as the argument type. I did this in Tk.jl.

[stevengj]

@JeffBezanson, allowing Any in cfunction would be great, although maybe not essential if you can use Function and other Julia leaf types. But it might still be good to have unsafe_ref(p::Ptr{Any}) or whatever, for conversions in other circumstances (e.g. pointers stored in other data structures).

[stevengj]

@JeffBezanson, passing Function to cfunction seems to work some of the time but not all of the time, e.g.

julia> foo(f::Function) = 3
julia> cfunction(foo, Int, (Function,))
ERROR: function is not yet c-callable
 in cfunction at c.jl:30

[vtjnash]

note that if you do know the type, you can add it as typeassert (same as Any return type for ccall)

x = unsafe_ref( pointer(Any, ptr) )::XType