simple stack adjoints

src/lib/array.jl

@@ -5,6 +5,8 @@
 
 @adjoint Base.vect(xs...) = Base.vect(xs...), Δ -> (Δ...,)
 
+@adjoint copy(x::AbstractArray) = copy(x), ȳ -> (ȳ,)
+
 Base.zero(xs::AbstractArray{Any}) = fill!(similar(xs), nothing)
 
 @adjoint function getindex(xs::Array, i...)

src/lib/base.jl

@@ -2,7 +2,26 @@ using Base: @get!
 
 @nograd readline
 
-@adjoint copy(x::AbstractArray) = copy(x), ȳ -> (ȳ,)
+# Gradient of AD stacks
+
+grad_mut(::AbstractVector) = []
+
+@adjoint! function _push!(a::Vector, x)
+  _push!(a, x), function (y)
+    dstk = grad_mut(__context__, a)
+    return (nothing, pop!(dstk))
+  end
+end
+
+@adjoint! function pop!(stk::Stack)
+  pop!(stk), function (Δ)
+    dstk = grad_mut(__context__, stk.data)
+    push!(dstk, Δ)
+    return
+  end
+end
+
+# Dictionaries
 
 grad_mut(d::AbstractDict) = Dict()
 

test/features.jl

@@ -159,6 +159,7 @@ D(f, x) = grad(f, x)[1]
 if VERSION > v"1.2-"
   @test D(x -> x*D(y -> x+y, 1), 1) == 1
   @test D(x -> x*D(y -> x*y, 1), 4) == 8
+  @test_broken sin'''(1.0) == -cos(1.0)
 end
 
 f(x) = throw(DimensionMismatch("fubar"))
@@ -247,3 +248,10 @@ end
   push!([], x)
   return x
 end
+
+@test gradient(1) do x
+  stk = []
+  Zygote._push!(stk, x)
+  stk = Zygote.Stack(stk)
+  pop!(stk)
+end == (1,)