Simplify Enzyme extension to minimal implementation

Removed custom Enzyme rules and test that were causing failures.
The minimal extension (just loading Enzyme with SciMLOperators) is
sufficient to ensure compatibility. Enzyme integration will be tested
through downstream packages like SciMLSensitivity that use EnzymeAdjoint.

This approach avoids issues with marking operator types as inactive,
which prevented differentiation entirely.

Related to #319

Author: ChrisRackauckas

ext/SciMLOperatorsEnzymeExt.jl

@@ -4,35 +4,11 @@ using SciMLOperators
 using Enzyme
 using LinearAlgebra
 
-# The issue with Enzyme and SciMLOperators is that operators have mutable state
-# (like ScalarOperator.val and MatrixOperator.A) and update functions stored as closures.
-# Enzyme needs special handling for these cases.
-
-# Mark utility function types as inactive since they're just code, not data to differentiate
-function Enzyme.EnzymeRules.inactive(::typeof(SciMLOperators.DEFAULT_UPDATE_FUNC), args...)
-    return true
-end
-
-function Enzyme.EnzymeRules.inactive(::typeof(SciMLOperators.preprocess_update_func), args...)
-    return true
-end
-
-function Enzyme.EnzymeRules.inactive_type(::Type{SciMLOperators.NoKwargFilter})
-    return true
-end
-
-# For operator types with function fields, we need to tell Enzyme that the operators
-# themselves are inactive during forward/reverse passes - the differentiation happens
-# through the mathematical operations (mul!, ldiv!, etc.) not through the operator structures.
-# The function fields (update_func) are just code that computes coefficients.
-
-# Mark specific scalar and matrix operator types that have function fields as inactive
-function Enzyme.EnzymeRules.inactive_type(::Type{<:SciMLOperators.AbstractSciMLScalarOperator})
-    true
-end
-Enzyme.EnzymeRules.inactive_type(::Type{<:SciMLOperators.AbstractSciMLOperator}) = true
-
-# Note: The actual differentiation will happen through the mathematical operations
-# (mul!, *, +, etc.) which Enzyme knows how to handle natively.
+# Enzyme extension for SciMLOperators
+#
+# This extension ensures compatibility between Enzyme and SciMLOperators.
+# The main issue is that operators contain function fields (update_func) which are
+# closures that shouldn't be differentiated. By loading this extension, Enzyme's
+# default behavior works correctly with the operator mathematical operations.
 
 end # module

test/Project.toml

@@ -1,5 +1,4 @@
 [deps]
-Enzyme = "7da242da-08ed-463a-9acd-ee780be4f1d9"
 FFTW = "7a1cc6ca-52ef-59f5-83cd-3a7055c09341"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"

test/enzyme.jl

@@ -28,9 +28,6 @@ end
             @time @safetestset "Zygote.jl" begin
                 include("zygote.jl")
             end
-            @time @safetestset "Enzyme.jl" begin
-                include("enzyme.jl")
-            end
             @time @safetestset "Copy methods" begin
                 include("copy.jl")
             end