Implement SpacetimeType for Result<T, E>

[mamcx]

Description of Changes

Closes #3673

NOTE: C++ part will be in another PR

Expected complexity level and risk

2

Adding a new type touch everywhere

Testing

• Adding smoke and unit test

[JasonAtClockwork]

I'll build out an attached PR to add the new functionality for the Unreal SDK.

[JasonAtClockwork]

There is updates to the /modules/sdk-test-cs but this won't be enough testing for the C# side. We'll need to add tests into the /sdks/csharp/examples~/regression-tests like for UUID to make sure this works end-to-end.

@rekhoff - I'll also want your eyes on this one when you can for the C# side

[gefjon]

I'm happy with the Rust parts of this PR. We'll still need review from the other languages' maintainers.

[rekhoff]

The C# side looks good. I did some local testing of end-to-end behavior, and things look good.

[coolreader18]

Seems reasonable to me, except for my one comment.

[coolreader18]

Shouldn't this be -> Result<Result<AlgebraicType, AlgebraicType>, Self>?

[mamcx]

Mm, this is like the option variant:

/// Converts `self` into an `Option<AlgebraicValue>`, if applicable.
    pub fn into_option(self) -> Result<Option<Self>, Self> {
        match self {
            AlgebraicValue::Sum(sum_value) => match sum_value.tag {
                0 => Ok(Some(*sum_value.value)),
                1 => Ok(None),
                _ => Err(AlgebraicValue::Sum(sum_value)),
            },
            _ => Err(self),
        }
    }

[gefjon]

I think the equivalent would be:

pub fn into_result(self) -> Result<Result<Self, Self>, Self> {
  match self {
    AlgebraicValue::Sum(sum_value) => match sum_value.tag {
      0 => Ok(Ok(*sum_value.value)),
      1 => Ok(Err(*sum_value.value)),
      _ => Err(self),
    },
    _ => Err(self),
  }
}

This way, it's possible to distinguish between the case where self is a Result::Err (so you get Ok(Err(val)), and the case where self is not a Result at all (so you get Err(self)). Your version sends both Result::Err and non-result values to Err.

[JasonAtClockwork]

I've added a couple comments but otherwise this looks like a solid fix for tests on the C# side!

[JasonAtClockwork]

We'll either want to have required keyword added to the generated use of SpacetimeDB.Result<> or add a default in the empty constructor like:

this.Value = default!

@rekhoff - Do you have any preference, without this devs will have ugly warnings using Result<> in C#?

[JasonAtClockwork]

This is the warning devs will see:
warning CS8618: Non-nullable field 'Msg' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

[rekhoff]

No strong preference here, but I think I lean towards the default empty constructor. If a developer doesn't actually care about the results, marking it as required could be kind of annoying to them.