Move SparseArrays support to an extension

[devmotion]

This PR moves support for SparseArrays to an extension.

In Julia >= 1.10, SparseArrays won't be included in the system image anymore. Therefore, to reduce loading times SparseArrays was or is about to be demoted from a regular to a weak dependency for many packages in the ecosystem such as FillArrays, Distances and PDMats. However, if a package in the dependency stack loads SparseArrays all these extensions will be compiled and loaded.

Ideally, if possible SparseArrays support in ChainRules should also be moved to an extension. I haven't checked yet if there are any obstacles (e.g., whether SparseArrays is also used for rules of functions not defined in SparseArrays itself). Since, however, mainly AD backends depend on ChainRules it might be a bit less important though.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (efc2f86) 93.90% compared to head (518b07a) 93.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #638   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files          14       15    +1     
  Lines         903      903           
=======================================
  Hits          848      848           
  Misses         55       55           

Files	Coverage Δ
src/ChainRulesCore.jl	100.00% <ø> (ø)
src/accumulation.jl	97.05% <ø> (-0.17%)	⬇️
src/projection.jl	97.26% <ø> (+0.21%)	⬆️
ext/ChainRulesCoreSparseArraysExt.jl	96.29% <96.29%> (ø)

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[devmotion]

The PDMatsExtras test errors are unrelated (and fixed by invenia/PDMatsExtras.jl#35).

[mcabbott]

This seems fine. Are you able to time any improvements in loading time? Not sure whether SparseArrays is already out of system image, on master I see this:

julia> @time_imports using ChainRulesCore
      0.5 ms  Compat
      0.2 ms  Compat → CompatLinearAlgebraExt
     16.0 ms  ChainRulesCore

[devmotion]

Not sure whether SparseArrays is already out of system image

This is an upcoming change in Julia 1.10: https://github.com/JuliaLang/julia/blob/master/HISTORY.md#build-system-changes

I assume @time_imports doesn't show loading time of stdlibs and/or packages in the system image since LinearAlgebra is another dependency that does not show up in your output.

[devmotion]

With Julia 1.10.0-beta3 I see (no startup file, after precompilation):

main

julia> @time_imports using ChainRulesCore
               ┌ 6.0 ms SuiteSparse_jll.__init__()
     23.4 ms  SuiteSparse_jll 69.28% compilation time
               ┌ 4.0 ms SparseArrays.CHOLMOD.__init__() 85.89% compilation time
     90.1 ms  SparseArrays 3.82% compilation time
      0.3 ms  Compat
      0.2 ms  Compat → CompatLinearAlgebraExt
      8.0 ms  ChainRulesCore

This PR

julia> @time_imports using ChainRulesCore
      0.9 ms  Compat
      0.5 ms  Compat → CompatLinearAlgebraExt
     14.8 ms  ChainRulesCore

That confirms my general impression from other packages: SparseArrays significantly increases loading time.

[oxinabox]

This seems good to me.
Bump the minor version and tag a release once you merge

[devmotion]

Bump the minor version

Already done 🙂 I'll merge and release.