Add setfield frule

Project.toml

@@ -20,7 +20,7 @@ SuiteSparse = "4607b0f0-06f3-5cda-b6b1-a6196a1729e9"
 
 [compat]
 Adapt = "3.4.0"
-ChainRulesCore = "1.15.3"
+ChainRulesCore = "1.17"
 ChainRulesTestUtils = "1.5"
 Compat = "3.46, 4.2"
 Distributed = "1"

src/rulesets/Base/base.jl

@@ -7,6 +7,14 @@
 @scalar_rule zero(x) ZeroTangent()
 @scalar_rule transpose(x) true
 
+
+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

@@ -8,6 +8,15 @@
             end
         end
     end
+
+    @testset "setfield!" begin
+        mutable struct MDemo
+            x::Float64
+        end
+
+        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))