Add rules needed for mutation

Project.toml

@@ -20,7 +20,7 @@ SuiteSparse = "4607b0f0-06f3-5cda-b6b1-a6196a1729e9"
 
 [compat]
 Adapt = "3.4.0, 4"
-ChainRulesCore = "1.15.3"
+ChainRulesCore = "1.20"
 ChainRulesTestUtils = "1.5"
 Compat = "3.46, 4.2"
 Distributed = "1"

src/rulesets/Base/array.jl

@@ -86,7 +86,13 @@ _instantiate_zeros(ẋs::AbstractArray{<:AbstractArray}, xs) = ẋs
 #####
 
 function frule((_, ẏ, ẋ), ::typeof(copyto!), y::AbstractArray, x)
-    return copyto!(y, x), copyto!(ẏ, ẋ)
+    if ẏ isa AbstractZero
+        # it's allowed to have an imutable zero tangent for ẏ as long as ẋ is zero
+        @assert iszero(ẋ)
+    else
+        copyto!(ẏ, ẋ)
+    end
+    return copyto!(y, x), ẏ
 end
 
 function frule((_, ẏ, _, ẋ), ::typeof(copyto!), y::AbstractArray, i::Integer, x, js::Integer...)

src/rulesets/Base/base.jl

@@ -26,6 +26,14 @@ function rrule(::typeof(one), x)
     return (one(x), one_pullback)
 end
 
+
+function ChainRulesCore.frule((_, ȯbj, _, ẋ), ::typeof(setfield!), obj, field, x)
+    ȯbj::MutableTangent
+    y = setfield!(obj, field, x)
+    ẏ = setproperty!(ȯbj, field, ẋ)
+    return y, ẏ
+end
+
 # `adjoint`
 
 frule((_, Δz), ::typeof(adjoint), z::Number) = (z', Δz')

test/rulesets/Base/base.jl

@@ -1,3 +1,7 @@
+mutable struct MDemo
+    x::Float64
+end
+
 @testset "base.jl" begin
     @testset "zero/one" begin
         for f in [zero, one]
@@ -18,6 +22,11 @@
             end
         end
     end
+
+    @testset "setfield!" begin
+        test_frule(setfield!, MDemo(3.5) ⊢ MutableTangent{MDemo}(; x=2.0), :x, 5.0)
+        test_frule(setfield!, MDemo(3.5) ⊢ MutableTangent{MDemo}(; x=2.0), 1, 5.0)
+    end
 
     @testset "Trig" begin
         @testset "Basics" for x = (Float64(π)-0.01, Complex(π, π/2))