fix #36753: fix write(::SubArray) by restoring pointer support (#36757)

Followup to #36739 (and currently built atop it), this restores the `pointer(::SubArray{<:Any,<:Any,<:Array,<:Tuple{Vararg{RangeIndex}}}, ::Tuple)` method that was removed in #36405. It does so, however, as a deprecated method, with the actual method being implemented on `(::SubArray{...}, ::CartesianIndex)`.

The rationale here is because the `::Tuple` method was undocumented and only supported on that one _highly_ specific `SubArray` type. I am keeping this patch as minimal as possible for backporting; in the future I aim to support `Vararg{CartesianIndex, Integer}` locations to make this more `getindex`-y and move farther away from the conflation with memory offsets.

base/deprecated.jl

@@ -207,6 +207,8 @@ macro get!(h, key0, default)
     end
 end
 
+pointer(V::SubArray{<:Any,<:Any,<:Array,<:Tuple{Vararg{RangeIndex}}}, is::Tuple) = pointer(V, CartesianIndex(is))
+
 # END 1.5 deprecations
 
 # BEGIN 1.6 deprecations

base/io.jl

@@ -680,14 +680,14 @@ function write(s::IO, a::SubArray{T,N,<:Array}) where {T,N}
     colsz = size(a,1) * elsz
     GC.@preserve a if stride(a,1) != 1
         for idxs in CartesianIndices(size(a))
-            unsafe_write(s, pointer(a, idxs.I), elsz)
+            unsafe_write(s, pointer(a, idxs), elsz)
         end
         return elsz * length(a)
     elseif N <= 1
         return unsafe_write(s, pointer(a, 1), colsz)
     else
-        for idxs in CartesianIndices((1, size(a)[2:end]...))
-            unsafe_write(s, pointer(a, idxs.I), colsz)
+        for colstart in CartesianIndices((1, size(a)[2:end]...))
+            unsafe_write(s, pointer(a, colstart), colsz)
         end
         return colsz * trailingsize(a,2)
     end

base/subarray.jl

@@ -408,6 +408,15 @@ end
 pointer(V::FastSubArray, i::Int) = pointer(V.parent, V.offset1 + V.stride1*i)
 pointer(V::FastContiguousSubArray, i::Int) = pointer(V.parent, V.offset1 + i)
 
+function pointer(V::SubArray{<:Any,<:Any,<:Array,<:Tuple{Vararg{RangeIndex}}}, is::AbstractCartesianIndex{N}) where {N}
+    index = first_index(V)
+    strds = strides(V)
+    for d = 1:N
+        index += (is[d]-1)*strds[d]
+    end
+    return pointer(V.parent, index)
+end
+
 # indices are taken from the range/vector
 # Since bounds-checking is performance-critical and uses
 # indices, it's worth optimizing these implementations thoroughly

test/subarray.jl

@@ -615,6 +615,27 @@ end
     v = Vector{UInt48}(undef, 5)
     read!(io, v)
     @test v == view(a, :, 2)
+
+    seekstart(io)
+    @test write(io, view(a, 2:5, 1:4)) == 4*4*8
+    seekstart(io)
+    v = Matrix{UInt48}(undef, 4, 4)
+    read!(io, v)
+    @test v == view(a, 2:5, 1:4)
+
+    seekstart(io)
+    @test write(io, view(a, 5:-1:1, 3)) == 5*8
+    seekstart(io)
+    v = Vector{UInt48}(undef, 5)
+    read!(io, v)
+    @test v == view(a, 5:-1:1, 3)
+
+    seekstart(io)
+    @test write(io, view(a, 1:2:5, :)) == 3*5*8
+    seekstart(io)
+    v = Matrix{UInt48}(undef, 3, 5)
+    read!(io, v)
+    @test v == view(a, 1:2:5, :)
 end
 
 @testset "unaliascopy trimming; Issue #26263" begin