Add method hints for non-nullable return types and nullable arguments

[Calinou]

See also #162 and #2242.

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

Currently, GDNative bindings must assume that all non-primitive return types (such as Node) may be null. This is problematic for safety-oriented languages such as Rust and Swift as it leads to uglier and unsafe code.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

• Add method hints to ClassDB to signal that a return value will never be null. This hint should only be added to methods that return Object-derived types, not primitive types such as int or Vector2 as these are non-nullable by design. If this hint is not present, the method is assumed to have a nullable return type (for Object-derived types only).
• Add a way to hint specific method arguments as being nullable. In other words, it means that the method will behave correctly if the specified arguments are null. If this hint is not present, the argument is assumed to be non-nullable.

This information can also be displayed in the generated class reference to improve documentation.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

• Non-null return type hinting could be implemented as a ClassDB method flag such as METHOD_FLAG_RETURN_NON_NULL.
• Nullable argument hinting could be implemented using argument metadata feature (GodotTypeInfo::Metadata).

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, as this is core engine functionality.

Is there a reason why this should be core and not an add-on in the asset library?

This functionality is part of the engine itself.

[migueldeicaza]

Couple of observations:

• These annotations could be used to improve C#, Rust and Swift bindings - as they could all make the parameters explicitly nullable or not-nullable.

• The annotations could help both the arguments, as well as the return values. In languages like Rust and Swift, nil return values must be tested for, you you end up writing code like:

if let window = object.getWindow {
    // window is guaranteed to not be nil
   window.title = "Hello"
} else {
    /// window was nil
}

So it can become quite cumbersome in scenarios where we would always know that no nil can ever be produced.

To achieve this, we would need to do an audit of every API entry point, to annotate properly which arguments can be NULL, and which methods could return NULL. In the argument case, the good news is that there is extensive use of macros that indicate that this is an error condition (like ERR_FAIL_NULL, so at least part of the work has been done).

[CedNaru]

I am all for that addition.
My group and I are working on a Kotlin Script implementation for Godot and the lack of nullable information is a struggle so far because we have to assume that any Object parameter/return type can be null for now. As nullable/non nullable types are part of the language, it makes the users add extra null check code when it's not necessary.

[dsnopek]

Copying some discussion from the #gdextension channel on RocketChat:

@akien-mga wrote:

And for this I see two possible options:

• Implement Optional<T> in C++, and refactor the whole API to use it. Then it will be included automatically in ClassDB with compile time checks, like BitField. That will require quite some discussion and work, probably spanning multiple release cycles -- if it can be done without breaking compat and forcing us to release 5.0.
• Add it manually as a piece of metadata in _bind_methods for the methods which have nullable parameters or returns. That doesn't require changing the core API so it's easier to get done within a reasonable timespan, but that's going to be very tedious and error prone. Can also be done the other way around with some way to flag parameters/returns which cannot be null, which might be less common than stuff which may be null.

And I wrote:

I worry that if this is just metadata, and isn't supported by GDScript or C++, that folks would forget to set it when necessary, or set it incorrectly, since most aren't actively using GDExtension. Something more like the Optional<T> suggestion would be better, since you'd need to interact with it right away in C++. But, yeah, that'd be a pretty big undertaking. But like Remi suggests for the 2nd point, maybe it'd be easier to add it iteratively via a Required<T> that represents a pointer/ref that cannot be null?

This would be good to discuss at a (or several) GDExtension meetings! I've added it to the agenda.

[dsnopek]

We discussed this at the GDExtension meeting earlier (sorry, I forgot to comment during the meeting) and came up with the following next steps:

• The GDScript team is working on a "unified type system" proposal that will likely address nullability in some way. So, we'll wait until that's posted, and see if we can add our needs to that proposal. It would be nice to address the type system holistically, rather than just trying to squeeze in this bit we need on its own.
• I'm going to make a PR with an experimental implementation of Required<T> so that we can look at the specifics and discuss. There's a couple of tricky questions with multiple answers that have different pro's/con's.