add getindex rrule

src/rulesets/Base/array.jl

@@ -100,3 +100,25 @@ function rrule(::typeof(fill), value::Any, dims::Int...)
     end
     return fill(value, dims), fill_pullback
 end
+
+#####
+##### getindex
+#####
+
+function rrule(::typeof(getindex), x::Array{<:Number}, inds::Union{Int, Vararg{Int}})
+    y = getindex(x, inds...)
+    function getindex_pullback(ȳ)
+        function getindex_add!(Δ)
+            Δ[inds...] .+= ȳ;
+            return Δ
+        end
+
+        x̄ = InplaceableThunk(
+            @thunk(getindex_add!(zeros(x))),
+            getindex_add!
+        )
+        return (NO_FIELDS, x̄, (DoesNotExist() for _ in inds)...)
+    end
+
+    return y, getindex_pullback
+end