overload Base.rest (#844)

* overload Base.rest

* Update test/abstractarray.jl

Co-authored-by: Mateusz Baran <mateuszbaran89@gmail.com>

* address review comments

* Update src/abstractarray.jl

Co-authored-by: Mateusz Baran <mateuszbaran89@gmail.com>

Co-authored-by: Mateusz Baran <mateuszbaran89@gmail.com>

Author: simeonschaub

src/abstractarray.jl

@@ -286,3 +286,12 @@ end
         @inbounds return similar_type(a, promote_type(eltype(a), eltype(b)), Size($Snew))(tuple($(exprs...)))
     end
 end
+
+if VERSION >= v"1.6.0-DEV.1334"
+    # FIXME: This always assumes one-based linear indexing and that subtypes of StaticArray
+    # don't overload iterate
+    @inline function Base.rest(a::StaticArray{S}, (_, i) = (nothing, 0)) where {S}
+        newlen = tuple_prod(S) - i
+        return similar_type(typeof(a), Size(newlen))(Base.rest(Tuple(a), i + 1))
+    end
+end

test/abstractarray.jl

@@ -273,3 +273,39 @@ end
     @test @inferred(hcat(SA[1,2,3])) === SMatrix{3,1}(1,2,3)
     @test @inferred(hcat(SA[1 2 3])) === SA[1 2 3]
 end
+
+@static if VERSION >= v"1.6.0-DEV.1334"
+    @testset "Base.rest" begin
+        x = SA[1, 2, 3]
+        @test Base.rest(x) == x
+        a, b... = x
+        @test b == SA[2, 3]
+    
+        x = SA[1 2; 3 4]
+        @test Base.rest(x) == vec(x)
+        a, b... = x
+        @test b == SA[3, 2, 4]
+
+        a, b... = SA[1]
+        @test b == []
+        @test b isa SVector{0}
+    
+        for (Vec, Mat) in [(MVector, MMatrix), (SizedVector, SizedMatrix)]
+            x = Vec(1, 2, 3)
+            @test Base.rest(x) == x
+            @test pointer(Base.rest(x)) != pointer(x)
+            a, b... = x
+            @test b == Vec(2, 3)
+        
+            x = Mat{2,2}(1, 2, 3, 4)
+            @test Base.rest(x) == vec(x)
+            @test pointer(Base.rest(x)) != pointer(x)
+            a, b... = x
+            @test b == Vec(2, 3, 4)
+
+            a, b... = Vec(1)
+            @test b == []
+            @test b isa Vec{0}
+        end
+    end
+end