uconvert rrule

src/UnitfulChainRules.jl

@@ -1,7 +1,7 @@
 module UnitfulChainRules
 
 using Unitful
-using Unitful: Quantity, Units, NoDims
+using Unitful: Quantity, Units, NoDims, FreeUnits
 using ChainRulesCore: NoTangent, @scalar_rule
 import ChainRulesCore: rrule, frule, ProjectTo
 

src/extras.jl

@@ -11,3 +11,15 @@ function rrule(::typeof(ustrip), x::Quantity{T,D,U}) where {T,D,U}
 
     return ustripped, ustrip_pb
 end
+
+function rrule(::typeof(uconvert), u::FreeUnits{N,D,A}, x::TX) where {N,D,A,TX}
+    x_convert = uconvert(u, x)
+    conversion = uconvert(u, oneunit(x)) / oneunit(x)
+    project_x = ProjectTo(x)
+
+    function uconvert_pb(Δ)
+        return (NoTangent(), NoTangent(), project_x(Δ * conversion))
+    end
+
+    return x_convert, uconvert_pb
+end

test/extras.jl

@@ -8,4 +8,12 @@ using ChainRulesCore
 
 	@test Ω == 5.0
 	@test last(pb(2.0)) == 2.0/u"m"
+end
+
+@testset "uconvert" begin
+	x = 30.0u"°"
+	Ω, pb = rrule(uconvert, u"rad", x)
+
+	@test Ω ≈ (π/6)u"rad"
+	@test last(pb(1.0)) ≈ π*u"rad"/180u"°"
 end