How I learned to love the struct and stop worrying (part II)...

NoBoxStruct.fs

// Use the Test solution configuration and now test cases will fail if they are using '=' on structs.

#if STRUCT_EQ_CHECK
[<AutoOpen>]
#endif
module StructEqCheck =

    type StructEqualityException() =
        inherit exn()
        
    let inline (=) (a: 'T) (b: 'T) =
        let ty = typeof<'T>

        if
            ty.IsValueType // look for structs
            && (not ty.IsPrimitive) // except primitives
            && (not ty.IsEnum) // except enums
            && ty.IsAssignableTo(typeof<IEquatable<'T>>) // must be IEquatable<'T> otherwise fast track not possible
        then
            raise (new StructEqualityException())

        // TODO also check types with nested structs

        a = b

NoBoxStruct.fsproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    <AssemblyName>Foo</AssemblyName>
    <Configurations>Debug;Release;Test</Configurations>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)'=='Test'">
    <DefineConstants>STRUCT_EQ_CHECK</DefineConstants>
  </PropertyGroup>
  
</Project>

Resources

• The performance of Comparing and Ordering things #9348
• Devirtualize Equality #6175
• Equality operator causes boxing on value types #526
• Add generic IStructuralEquatable<T> and IStructuralComparable<T> #37482
• (=) uses non-generic equality on custom types #9398
• Satisfy the "comparison" constraint with IComparable<'T> #816
• Optimize DateTime comparison/equality #9224 (merged)