Use CartesianIndices(Rsrc) as the shared iterator. (#45289)

There's no performance change, if the `indices`s of `Rdest` and `Rsrc` are all `NTuple{N,<:AbstractUnitRange}`.

(cherry picked from commit a91be39)

base/multidimensional.jl

@@ -1076,16 +1076,18 @@ function copyto!(dest::AbstractArray{T1,N}, Rdest::CartesianIndices{N},
  checkbounds(src, first(Rsrc))
  checkbounds(src, last(Rsrc))
  src′ = unalias(dest, src)
- ΔI = first(Rdest) - first(Rsrc)
+ CRdest = CartesianIndices(Rdest)
+ CRsrc = CartesianIndices(Rsrc)
+ ΔI = first(CRdest) - first(CRsrc)
  if @generated
  quote
- @nloops $N i (n->Rsrc.indices[n]) begin
- @inbounds @nref($N,dest,n->i_n+ΔI[n]) = @nref($N,src′,i)
+ @nloops $N i (n->CRsrc.indices[n]) begin
+ @inbounds @nref($N,dest,n->Rdest.indices[n][i_n+ΔI[n]]) = @nref($N,src,n->Rsrc.indices[n][i_n])
  end
  end
  else
- for I in Rsrc
- @inbounds dest[I + ΔI] = src′[I]
+ for I in CRsrc
+ @inbounds dest[Rdest[I + ΔI]] = src′[Rsrc[I]]
  end
  end
  dest

test/copy.jl

@@ -58,11 +58,14 @@ end
  @test B == A
  end
  let A = reshape(1:6, 3, 2), B = zeros(8,8)
- RA = CartesianIndices(axes(A))
- copyto!(B, CartesianIndices((5:7,2:3)), A, RA)
- @test B[5:7,2:3] == A
- B[5:7,2:3] .= 0
- @test all(x->x==0, B)
+ RBs = Any[(5:7,2:3), (3:2:7,1:2:3), (6:-1:4,2:-1:1)]
+ RAs = Any[axes(A), reverse.(axes(A))]
+ for RB in RBs, RA in RAs
+ copyto!(B, CartesianIndices(RB), A, CartesianIndices(RA))
+ @test B[RB...] == A[RA...]
+ B[RB...] .= 0
+ @test all(iszero, B)
+ end
  end
 end