add and test zero_tangent

src/ChainRulesCore.jl

@@ -11,7 +11,7 @@ export RuleConfig, HasReverseMode, NoReverseMode, HasForwardsMode, NoForwardsMod
 export frule_via_ad, rrule_via_ad
 # definition helper macros
 export @non_differentiable, @opt_out, @scalar_rule, @thunk, @not_implemented
-export ProjectTo, canonicalize, unthunk  # tangent operations
+export ProjectTo, canonicalize, unthunk, zero_tangent  # tangent operations
 export add!!, is_inplaceable_destination  # gradient accumulation operations
 export ignore_derivatives, @ignore_derivatives
 # tangents

src/tangent_types/abstract_zero.jl

@@ -87,3 +87,32 @@ arguments.
 ```
 """
 struct NoTangent <: AbstractZero end
+
+"""
+    zero_tangent(primal)
+
+This returns an appropriate zero tangent suitable for accumulating tangents of the primal.
+For mutable composites types this is a structural []`MutableTangent`](@ref)
+For `Array`s, it is applied recursively for each element.
+For immutable types, this is simply [`ZeroTangent()`](@ref) as accumulation is default out-of-place for contexts where mutation does not apply.
+(Where mutation is not to be supported even for mutable types, then [`ZeroTangent()`](@ref) should be used for everything)
+
+!!! warning Exprimental
+    `zero_tangent`is an experimental feature, and is part of the mutation support featureset.
+    While this notice remains it may have changes in behavour, and interface in any _minor_ version of ChainRulesCore.
+    Exactly how it should be used (e.g. is it forward-mode only?)
+"""
+function zero_tangent end
+zero_tangent(::AbstractString) = ZeroTangent()
+# zero_tangent(::Number) = zero(x) # TODO: do we want this?
+zero_tangent(primal::Array{<:Number}) = zero(primal)  # TODO: do we want this?
+zero_tangent(primal::Array) = map(zero_tangent, primal)
+@generated function zero_tangent(primal)
+    has_mutable_tangent(primal) || return ZeroTangent()  # note this takes care of tuples
+    zfield_exprs = map(fieldnames(primal)) do fname
+        fval = Expr(:call, zero_tangent, Expr(:call, getfield, :primal, QuoteNode(fname)))
+        Expr(:kw, fname, fval)
+    end
+    backing_expr = Expr(:tuple, Expr(:parameters, zfield_exprs...))
+    return :($MutableTangent{$primal}($backing_expr))
+end

src/tangent_types/structural_tangent.jl

@@ -22,7 +22,7 @@ function StructuralTangent{P}(nt::NamedTuple) where {P}
     end
 end
 
-has_mutable_tangent(::Type{P}) where P = ismutabletype(P) && (!isabstracttype(P) && fieldcount(T) > 0)
+has_mutable_tangent(::Type{P}) where P = ismutabletype(P) && (!isabstracttype(P) && fieldcount(P) > 0)
 
 
 StructuralTangent{P}(tup::Tuple) where P = Tangent{P,typeof(tup)}(tup)

test/tangent_types/abstract_zero.jl

@@ -154,3 +154,17 @@
         @test isempty(detect_ambiguities(M))
     end
 end
+
+@testset "zero_tangent" begin
+    mutable struct MutDemo
+        x::Float64
+    end
+    @test zero_tangent(MutDemo(1.5)) isa MutableTangent{MutDemo}
+    @test iszero(zero_tangent(MutDemo(1.5)))
+
+    @test zero_tangent((;a=1)) isa ZeroTangent
+
+    @test zero_tangent([1.0, 2.0]) == [0.0, 0.0]
+    @test zero_tangent([[1.0, 2.0], [3.0]]) == [[0.0, 0.0], [0.0]]
+end
+