Champion "blittable"/"unmanaged" constraint (15.7)

[gafter]

The blittable feature will give language enforcement to the class of types known as "unmanaged types" in the C# language spec. This is defined in section 18.2 as a type which is not a reference type and doesn't contain reference type fields at any level of nesting.

Allows where T : unmanaged

• Proposal added (https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/blittable.md)
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

[iam3yal]

Is this a proposal to improve existing blittable types or something? what's this about? :)

[vbcodec]

@eyalsk It is [Redacted] ::)

[ufcpp]

Is this related to dotnet/roslyn#2209 ?

[jaredpar]

Sorry for the confusion. Will reopen when we have the proposal ready.

[jaredpar]

Sorry there isn't more details here. This is being used as a reminder for me to add more details.

[jaredpar]

Proposal added here #206

[ufcpp]

This is good idea! but I have some questions

• How is this related to Blittable Types in Interop Marshaling? The same? or similar but bit different?
• Can IntPtr and UIntPtr be blittable? They are blittable in the Interop Marshaling and unmanaged in the current C# spec.

[iam3yal]

@jaredpar Just a thought.

Why can't it be an attribute?

[Blittable]
struct Point 
{
    public int X;
    public int Y;
}

And instead of adding a new blittable constraint such in the following case:

void Hash<T>(T value) where T : blittable struct
{
    using (T* p = &value) { 
        ...
    }
}

Allow attributes to be used as a generic constraint? so something like this.

void Hash<T>(T value) where T : struct, Blittable
{
    using (T* p = &value) { 
        ...
    }
}

This seems like a more general feature that can be used here and elsewhere in the framework. :)

Alternatively, use a marker interface?

[iam3yal]

@bbarry Why the downvote? what's the point of adding a new keyword? and then adding a special constraint when existing features can actually do the job or/and improved? not to mention that blittable struct is at odds with existing constraints, for example, why can't we check that the generic parameter is abstract class? what makes blittable any special?

Maybe I'm overlooking something so if you can, please do elaborate. :)