Add Metal.jl support for lu_instance

- Add ArrayInterfaceMetalExt extension with proper lu_instance implementation
- Fix issue where MtlArray lu_instance failed due to 0x0 matrix creation
- Add Metal.jl to weakdeps and extensions in Project.toml
- Add Metal.jl test to verify lu_instance works properly
- Follow same pattern as CUDA extension for GPU array support

Fixes #467

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -14,6 +14,7 @@ CUDSS = "45b445bb-4962-46a0-9369-b4df9d0f772e"
 ChainRules = "082447d4-558c-5d27-93f4-14fc19e9eca2"
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 GPUArraysCore = "46192b85-c4d5-4398-a991-12ede77f4527"
+Metal = "dde4c033-4e86-420c-a63e-0dd931031962"
 ReverseDiff = "37e2e3b7-166d-5795-8a7a-e32c996b4267"
 SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 StaticArraysCore = "1e83bf80-4336-4d27-bf5d-d5a4f845583c"
@@ -27,6 +28,7 @@ ArrayInterfaceCUDSSExt = "CUDSS"
 ArrayInterfaceChainRulesCoreExt = "ChainRulesCore"
 ArrayInterfaceChainRulesExt = "ChainRules"
 ArrayInterfaceGPUArraysCoreExt = "GPUArraysCore"
+ArrayInterfaceMetalExt = "Metal"
 ArrayInterfaceReverseDiffExt = "ReverseDiff"
 ArrayInterfaceSparseArraysExt = "SparseArrays"
 ArrayInterfaceStaticArraysCoreExt = "StaticArraysCore"
@@ -43,6 +45,7 @@ ChainRulesCore = "1"
 ChainRulesTestUtils = "1"
 GPUArraysCore = "0.1, 0.2"
 LinearAlgebra = "1.10"
+Metal = "1"
 ReverseDiff = "1"
 SparseArrays = "1.10"
 StaticArraysCore = "1"
@@ -59,6 +62,8 @@ ChainRulesTestUtils = "cdddcdb0-9152-4a09-a978-84456f9df70a"
 ComponentArrays = "b0b7db55-cfe3-40fc-9ded-d10e2dbeff66"
 FillArrays = "1a297f60-69ca-5386-bcde-b61e274b549b"
 GPUArraysCore = "46192b85-c4d5-4398-a991-12ede77f4527"
+JuliaFormatter = "98e50ef6-434e-11e9-1051-2b60c6c9e899"
+Metal = "dde4c033-4e86-420c-a63e-0dd931031962"
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 ReverseDiff = "37e2e3b7-166d-5795-8a7a-e32c996b4267"

ext/ArrayInterfaceMetalExt.jl

@@ -0,0 +1,15 @@
+module ArrayInterfaceMetalExt
+
+using ArrayInterface
+using Metal
+using LinearAlgebra
+
+function ArrayInterface.lu_instance(A::MtlMatrix{T}) where {T}
+    ipiv = MtlVector{Int32}(undef, 0)
+    info = zero(Int)
+    return LinearAlgebra.LU(similar(A, 0, 0), ipiv, info)
+end
+
+ArrayInterface.device(::Type{<:Metal.MtlArray}) = ArrayInterface.GPU()
+
+end # module

test/gpu/Project.toml

@@ -1,2 +1,3 @@
 [deps]
-CUDA = "052768ef-5323-5732-b1bb-66c8b64840ba"
+CUDA = "052768ef-5323-5732-b1bb-66c8b64840ba"
+Metal = "dde4c033-4e86-420c-a63e-0dd931031962"

test/gpu/metal.jl

@@ -0,0 +1,7 @@
+using Metal
+using ArrayInterface
+
+using Test
+
+# Test whether lu_instance throws an error when invoked with a Metal.jl gpu array
+@test !isa(try ArrayInterface.lu_instance(MtlArray([1.f0 1.f0; 1.f0 1.f0])) catch ex ex end, Exception) 

test/runtests.jl

@@ -21,5 +21,6 @@ end
     if GROUP == "GPU"
         activate_gpu_env()
         @time @safetestset "CUDA" begin include("gpu/cuda.jl") end
+        @time @safetestset "Metal" begin include("gpu/metal.jl") end
     end
 end