Add ChainRules Overloads to StaticArrays

Project.toml

@@ -1,6 +1,6 @@
 name = "StaticArrays"
 uuid = "90137ffa-7385-5640-81b9-e52037218182"
-version = "1.7.0"
+version = "1.8.0"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
@@ -10,25 +10,30 @@ StaticArraysCore = "1e83bf80-4336-4d27-bf5d-d5a4f845583c"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 
 [weakdeps]
+ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 
 [extensions]
+StaticArraysChainRulesCoreExt = "ChainRulesCore"
 StaticArraysStatisticsExt = "Statistics"
 
 [compat]
 Aqua = "0.7"
+ChainRulesCore = "1"
 PrecompileTools = "1"
 StaticArraysCore = "~1.4.0"
 julia = "1.6"
 
 [extras]
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
 BenchmarkTools = "6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf"
+ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
+ChainRulesTestUtils = "cdddcdb0-9152-4a09-a978-84456f9df70a"
 InteractiveUtils = "b77e0a4c-d291-57a0-90e8-8db25a27a240"
 OffsetArrays = "6fe1bfb0-de20-5000-8ca7-80f57d26f881"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
 
 [targets]
-test = ["InteractiveUtils", "Test", "BenchmarkTools", "OffsetArrays", "Statistics", "Unitful", "Aqua"]
+test = ["InteractiveUtils", "Test", "BenchmarkTools", "OffsetArrays", "Statistics", "Unitful", "Aqua", "ChainRulesTestUtils", "ChainRulesCore"]

ext/StaticArraysChainRulesCoreExt.jl

@@ -0,0 +1,32 @@
+module StaticArraysChainRulesCoreExt
+
+using StaticArrays
+# ChainRulesCore imports
+import ChainRulesCore: NoTangent, ProjectTo, Tangent, project_type, rrule
+import ChainRulesCore as CRC
+
+# Projecting a tuple to SMatrix leads to CRC._projection_mismatch by default, so
+# overloaded here
+function (project::ProjectTo{<:Tangent{<:Tuple}})(dx::StaticArraysCore.SArray)
+    dy = reshape(dx, axes(project.elements))
+    dz = ntuple(i -> project.elements[i](dy[i]), length(project.elements))
+    return project_type(project)(dz...)
+end
+
+# Project SArray to SArray
+function ProjectTo(x::SArray{S, T}) where {S, T}
+    return ProjectTo{SArray}(; element = CRC._eltype_projectto(T), axes = S)
+end
+
+function (project::ProjectTo{SArray})(dx::AbstractArray{S, M}) where {S, M}
+    return SArray{project.axes}(dx)
+end
+
+# Adjoint for SArray constructor
+function rrule(::Type{T}, x::Tuple) where {T <: SArray}
+    project_x = ProjectTo(x)
+    ∇Array(∂y) = (NoTangent(), project_x(∂y))
+    return T(x), ∇Array
+end
+
+end

test/chainrules.jl

@@ -0,0 +1,10 @@
+using StaticArrays, ChainRulesCore, ChainRulesTestUtils, Test
+
+@testset "Chain Rules Integration" begin
+    @testset "Projection" begin
+        test_rrule(SMatrix{1, 4}, (1.0, 1.0, 1.0, 1.0))
+        test_rrule(SMatrix{4, 1}, (1.0, 1.0, 1.0, 1.0))
+        test_rrule(SMatrix{2, 2}, (1.0, 1.0, 1.0, 1.0))
+        test_rrule(SVector{4}, (1.0, 1.0, 1.0, 1.0))
+    end
+end

test/runtests.jl

@@ -88,4 +88,9 @@ if TEST_GROUP ∈ ["", "all", "group-B"]
     addtests("io.jl")
     addtests("svd.jl")
     addtests("unitful.jl")
+
+    # chain rules integration via pkg extensions is available only in Julia 1.9+
+    if VERSION ≥ v"1.9-"
+        addtests("chainrules.jl")
+    end
 end