[Proposal]: ref struct closures for lambdas

[agocke]

ref struct closures for lambdas

Spec: https://github.com/dotnet/csharplang/blob/main/proposals/ref-struct-closures.md
Discussion: #10210

Summary

The proposal is to allow C# to convert lambda expressions to generic type parameters constrained to specific IFunc/IAction interfaces. Furthermore, the type parameter should be marked allows ref struct and the backing closure should be a ref struct.

For instance, the following code

List<int> list = [1, 2, 3];

Console.Write(FirstOrNull(list, x => x % 2 == 0));

static int? FirstOrNull<TPred>(List<int> list, TPred pred)
    where TPred : allows ref struct, IFunc<int, bool>
{
    var filtered = new List<int>();
    foreach (var item in list)
    {
        if (pred.Invoke(item))
        {
            return item;
        }
    }
    return null;
}

would compile and produce the output "2". The generated code for the lambda would look equivalent to

ref struct __Closure : IFunc<int, bool>
{
    public bool Invoke(int x) => x % 2 == 0;
}