Add specialized unique(::AbstractRange) method

Author: sostock

base/set.jl

@@ -166,6 +166,8 @@ _unique_from(itr, out, seen, i) = unique_from(itr, out, seen, i)
     return out
 end
 
+unique(r::AbstractRange) = allunique(r) ? r : oftype(r, r[begin:begin])
+
 """
     unique(f, itr)
 

test/sets.jl

@@ -392,6 +392,13 @@ end
     @test @inferred(unique(x->x^2, Integer[3, -4, 5, 4])) == Integer[3, -4, 5]
     @test @inferred(unique(iseven, Integer[3, -4, 5, 4]; seen=Set{Bool}())) == Integer[3, -4]
     @test @inferred(unique(n -> n % 3, [5, 1, 8, 9, 3, 4, 10, 7, 2, 6])) == [5, 1, 9]
+    for r = (Base.OneTo(-1), Base.OneTo(0), Base.OneTo(1), Base.OneTo(5),
+             1:0, 1:1, 1:2, 1:10, 1:.5:.5, 1:.5:1, 1:.5:10, 3:-2:5, 3:-2:3, 3:-2:1,
+             StepRangeLen(1.0, 2.0, 0), StepRangeLen(1.0, 2.0, 2), StepRangeLen(1.0, 2.0, 3),
+             StepRangeLen(1.0, 0.0, 0), StepRangeLen(1.0, -0.0, 1), StepRangeLen(1.0, 0.0, 2),
+             LinRange(1, 2, 3), LinRange(1, 1, 0), LinRange(1, 1, 1), LinRange(1, 1, 10))
+        @test @inferred(unique(r)) == invoke(unique, Tuple{Any}, r)
+    end
 end
 
 @testset "issue 20105" begin