Enable nullable field backing properties for value types

[weitzhandler]

Please enable the following:

Current

public Guid? _Id;
public Guid Id
{
    get
    {
        if (!_Id.HasValue)
        {
            _Id = ComputeIdOrDefault();
        }

        return _Id.Value;
    }
}

Suggested

[field: MaybeNull]
public Guid Id
{
    get
    {
        if (!field.HasValue)
        {
            field = ComputeIdOrDefault();
        }

        return field.Value;
    }
}

[jnm2]

It would have to be some other attribute or marker, since the existing code already has meaning and it would be a breaking change to change memory layouts based on changing the language version. Also, [MaybeNull] appears in many other places in the language where by design it has no effect on non-nullable value types, so it would be a significant inconsistency for it to start doing so here.

[jnm2]

An alternative is #133, a feature which solves a wider class of issues including this one (e.g. wanting the type Lazy<T> as the backing field, etc), where you would write:

public Guid Id
{
    Guid? field;
    get
    {
        if (!field.HasValue)
        {
            field = ComputeIdOrDefault();
        }

        return field.Value;
    }
}

Or simplified:

public Guid Id { Guid? field; get => field ??= ComputeIdOrDefault(); }

[TahirAhmadov]

Yeah it's definitely better to have a general solution than a very narrow one; other than nullable and Lazy, one could also have a separate property-scoped "locker" field (be it Monitor-based object or something else).

[TahirAhmadov]

Ideally, static fields would be allowed in property scope, and maybe even #832 is championed, but one can only hope.