Add colon constructor for CartesianIndices

See
https://discourse.julialang.org/t/psa-replacement-of-ind2sub-sub2ind-in-julia-0-7/14666/6
and posts below it.

Author: timholy

NEWS.md

@@ -4,6 +4,7 @@ Julia v1.1.0 Release Notes
 New language features
 ---------------------
 
+  * `CartesianIndices` can now be constructed from two `CartesianIndex`es `I` and `J` with `I:J` ([#29440]).
 
 Language changes
 ----------------
@@ -22,3 +23,4 @@ Deprecated or removed
 
 
 <!--- generated by NEWS-update.jl: -->
+[#29440]: https://github.com/JuliaLang/julia/issues/29440

base/multidimensional.jl

@@ -6,7 +6,7 @@ module IteratorsMD
                  setindex!, IndexStyle, min, max, zero, one, isless, eachindex,
                  ndims, IteratorSize, convert, show, iterate, promote_rule
 
-    import .Base: +, -, *
+    import .Base: +, -, *, (:)
     import .Base: simd_outer_range, simd_inner_length, simd_index
     using .Base: IndexLinear, IndexCartesian, AbstractCartesianIndex, fill_to_length, tail
     using .Base.Iterators: Reverse
@@ -228,6 +228,9 @@ module IteratorsMD
 
     CartesianIndices(A::AbstractArray) = CartesianIndices(axes(A))
 
+    (:)(I::CartesianIndex{N}, J::CartesianIndex{N}) where N =
+        CartesianIndices(map((i,j) -> i:j, Tuple(I), Tuple(J)))
+
     promote_rule(::Type{CartesianIndices{N,R1}}, ::Type{CartesianIndices{N,R2}}) where {N,R1,R2} =
         CartesianIndices{N,Base.indices_promote_type(R1,R2)}
 

test/arrayops.jl

@@ -1753,6 +1753,10 @@ end
 
     @test @inferred(convert(NTuple{2,UnitRange}, R)) === (2:5, 3:5)
     @test @inferred(convert(Tuple{Vararg{UnitRange}}, R)) === (2:5, 3:5)
+
+    I = CartesianIndex(2,3)
+    J = CartesianIndex(5,4)
+    @test I:J === CartesianIndices((2:5, 3:4))
 end
 
 # All we really care about is that we have an optimized