First pass at something that maybe works

Author: oxinabox

src/ChainRulesCore.jl

@@ -15,7 +15,7 @@ export ProjectTo, canonicalize, unthunk  # tangent operations
 export add!!, is_inplaceable_destination  # gradient accumulation operations
 export ignore_derivatives, @ignore_derivatives
 # tangents
-export Tangent, NoTangent, InplaceableThunk, Thunk, ZeroTangent, AbstractZero, AbstractThunk
+export Tangent, MutableTangent, NoTangent, InplaceableThunk, Thunk, ZeroTangent, AbstractZero, AbstractThunk
 
 include("debug_mode.jl")
 

src/tangent_types/structural_tangent.jl

@@ -418,12 +418,15 @@ It itself is also mutable.
     However, this requires that the tangent is aliased in turn and conversely that it is copied when the primal is).
     If you seperately alias the backing data, etc by using the internal `ChainRulesCore.backing` function you can break this.
 """
-mutable struct MutableTangent{P}
+mutable struct MutableTangent{P} <: StructuralTangent{P}
     #TODO: we may want to absolutely lock the type of this down
     backing::NamedTuple
 end
 
-Base.getproperty(tangent::MutableTangent, idx::Symbol) = unthunk(getfield(backing(tangent), idx))
+MutableTangent{P}(;kwargs...) where P = MutableTangent{P}(NamedTuple(kwargs))
+Base.getproperty(tangent::MutableTangent, idx::Symbol) = getfield(backing(tangent), idx)
 function Base.setproperty!(tangent::MutableTangent, name::Symbol, x)
     new_backing = Base.setindex(backing(tangent), x, name)
+    setfield!(tangent, :backing, new_backing)
+    return x
 end

test/tangent_types/structural_tangent.jl

@@ -421,3 +421,21 @@ end
         @test contains(sprint(show, tang), sprint(show, tang.x))  # inner piece appears whole
     end
 end
+
+@testset "MutableTangent" begin
+    mutable struct MDemo
+        x::Float64
+    end
+    function ChainRulesCore.frule((_, ȯbj, _, ẋ), ::typeof(setfield!), obj::MDemo, field, x)
+        y = setfield!(obj, field, x)
+        ẏ = setproperty!(ȯbj, field, ẋ)
+        return y, ẏ
+    end
+
+    obj = MDemo(99.0)
+    ∂obj = MutableTangent{MDemo}(;x=1.5)
+    frule((NoTangent(), ∂obj, NoTangent(), 10.0), setfield!, obj, :x, 95.0)
+
+    @test ∂obj.x == 10.0
+    @test obj.x == 95.0
+end