ref struct interfaces

Proposal

Questions

1. Move forward with ref struct interfaces out of preview
2. Do we want to introduce a warning in the DIM case?
3. Do we want to limit the constraint language?

Concrete Usages

Needed for ITensor<T>

The tensor team wants to ship the following interface that is meant

interface ITensor<T>
{
    [UnscopedRef]
    ReadOnlySpan<nint> Lengths { get; }
}

Comparer Interfaces

This is now used by the runtime in the comparer interfaces.

public static int BinarySearch<T, TComparer>(this System.ReadOnlySpan<T> span, T value, TComparer comparer)
    where TComparer : System.Collections.Generic.IComparer<T>, allows ref struct;

public static int BinarySearch<T, TComparable>(this System.ReadOnlySpan<T> span, TComparable comparable)
    where TComparable : System.IComparable<T>, allows ref struct;

Enumerator

Runtime wants to have existing ref struct based enumerators inherit IEnumerator<T>

Math interfaces

Runtime is considering them for the math related interfaces:

• IAdditionOperators
• IParsable
• ISpanParsable
• IUtf8SpanParsable

Customer Scenarios

• U8String project alows for unification
• Asset Ripper makes heavy use of this already

Warn on DIM case?

The runtime doesn't, and never will, support calling a default implemented member (DIM) when the receiver type is a ref struct. The compiler will require that a ref struct implement all members. Could also choose to warn at the point the code invites this problem if we wanted to.

interface I1
{
    // Virtual method with default implementation
    void M() { }
}

// Invocation of a virtual instance method with default implementation in a generic method that has the `allows ref struct`
// anti-constraint
void M<T>(T p)
    where T : allows ref struct, I1
{
    p.M(); // Warn?
}

Limit Constraint Language?

The language allows for us to do this today

void M<T>(T t) where T : IDisposable, allows ref struct { }

That is strange if there is no way T can be ref struct but not also implement interfaces