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)

Author: N5N3

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