Make view of range with range be a range

Typical `AbstractRange` types are compact and immutable, and we can
remove the overhead of `SubArray` in this case while the cost of `view`
remains O(1).

Here we specialize only on valid combinations of integer ranges,
otherwise still relying on `SubArray`.

Author: Andy Ferris

base/subarray.jl

@@ -134,6 +134,24 @@ function view(A::AbstractArray, I::Vararg{Any,N}) where {N}
     unsafe_view(_maybe_reshape_parent(A, index_ndims(J...)), J...)
 end
 
+# Ranges tend to be compact & immutable
+function view(r1::OneTo, r2::OneTo)
+    @_propagate_inbounds_meta
+    getindex(r1, r2)
+end
+function view(r1::AbstractUnitRange, r2::AbstractUnitRange{<:Integer})
+    @_propagate_inbounds_meta
+    getindex(r1, r2)
+end
+function view(r1::AbstractUnitRange, r2::StepRange{<:Integer})
+    @_propagate_inbounds_meta
+    getindex(r1, r2)
+end
+function view(r1::StepRange, r2::AbstractRange{<:Integer})
+    @_propagate_inbounds_meta
+    getindex(r1, r2)
+end
+
 function unsafe_view(A::AbstractArray, I::Vararg{ViewIndex,N}) where {N}
     @_inline_meta
     SubArray(A, I)

test/ranges.jl

@@ -1263,3 +1263,10 @@ end
     @test step(x) == 0.0
     @test x isa StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}
 end
+
+@testset "Views of ranges" begin
+    @test view(Base.OneTo(10), Base.OneTo(5)) === Base.OneTo(5)
+    @test view(1:10, 1:5) === 1:5
+    @test view(1:10, 1:2:5) === 1:2:5
+    @test view(1:2:9, 1:5) === 1:2:9
+end