Deprecate @unpack to reduce specializations (but keep v1.6 compat)

[ChrisRackauckas]

No description provided.

[ranocha]

Did you measure the impact on precompile/load times? This would be quite interesting for a lot of my packages, too.

[ChrisRackauckas]

It's at least a piece of the puzzle

I'll figure out where we're at after #1892 + SciML/SciMLOperators.jl#155

[ChrisRackauckas]

#1892 had a bigger effect so far, but we're almost <100mb now when it's all together.

[ranocha]

Cool - I'm looking forward to it!

[devmotion]

I wonder if this could be made into a custom @unpack macro to avoid the code duplications? Or even just changed in UnPack.jl?

[ChrisRackauckas]

Unpack.jl handles cases other than types, while this does not. And this will be easy to delete once LTS changes.

[devmotion]

I played around a bit and came up with a more lightweight @unpack macro: https://github.com/devmotion/SimpleUnPack.jl On Julia >= 1.7.0-DEV.364 it uses the destructuring syntax and also on older Julia versions it is solely based on getproperty. Thus it's less flexible than UnPack but should also fix compilation times and the large amount of method definitions on older Julia versions.