improve type stability with StaticArrays

[mapclyps]

Replace an anonymous function with Base.Fix1 for type stability in some cases, see #639

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.59%. Comparing base (c310fb5) to head (4bc44b6).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #640      +/-   ##
==========================================
+ Coverage   89.57%   89.59%   +0.02%     
==========================================
  Files          11       11              
  Lines         969      971       +2     
==========================================
+ Hits          868      870       +2     
  Misses        101      101              

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[devmotion]

Thank you! Could you add a test for #639?

[mapclyps]

I tried to replicate the type instability in the tests and put a test at the end of JacobianTest.jl like

ff(x) = sin.(x)
ssx = SA[1.0, 2.0]
ssy = ff(ssx)
result_store = DiffResults.JacobianResult(ssx)
@test begin
    result = (@inferred ForwardDiff.jacobian!(result_store, ff, ssx)) #the actual test
    jac = ForwardDiff.jacobian(ff, ssx)
    expected = DiffResults.ImmutableDiffResult(ssy, (jac,)) 
    result == expected
end

But to my surprise, there was no type instability with this test also on master.

However, just spinning up a REPL and executing this throws the error as expected and reported in #639.
Then I just moved this test to the top of the JacobianTest.jl file, and then it also threw the error on master (so it was working correctly as a test for the fix for #639). But it made me wonder what is actually going on there... and where this "hidden state" comes from.

Or is there a better, more stable way to test it?