mapslices for sparse matrices is wrong

[andreasnoack]

E.g.

julia> mapslices(nnz, speye(3), 1)
1×3 Array{Int64,2}:
 1  2  3

The problem seems to be

julia/base/abstractarray.jl

Line 1830 in 90cfb82

_unsafe_getindex!(Aslice, A, idx...)

which doesn't set structural zeros in the slice variable, e.g.

julia> Base._unsafe_getindex!(sparse(ones(3)), speye(3), 1:3, 1)
3-element SparseVector{Float64,Int64} with 3 stored entries:
  [1]  =  1.0
  [2]  =  0.0
  [3]  =  0.0

I don't know the behavior of _unsafe_getindex! should change or if mapslices should use a different function. I'm on

julia> versioninfo()
Julia Version 0.6.0-pre.alpha.180
Commit a365da4c6c* (2017-03-19 13:41 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin16.5.0)
  CPU: Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT NO_AFFINITY HASWELL)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, haswell)

cc: @mbauman, @timholy

[mbauman]

It's not so much _unsafe_getindex! as it is sparse setindex. s[1]=0 doesn't set a structural zero. Ref: #15568

We just need to be more strict in the determination of slices that are deemed safe_for_reuse. Not sure what the exact criterion should be here.