cant create constant array

[ogyrec-o]

#[godot_api]
impl test
{
  #[constant]
  const dd: VariantArray = varray![42_i64, "hello", true];
}

err

error[E0277]: the trait bound `godot::prelude::Array<Variant>: Copy` is not satisfied   --> src\lib.rs:114:6
    |
113 |   #[godot_api]
    |   ------------ required by a bound introduced by this call
114 |   impl Chunk2
    |  ______^
115 | | {
116 | |   #[constant]
117 | |   const dd: VariantArray = varray![42_i64, "hello", true];
    | |__________^ the trait `Copy` is not implemented for `godot::prelude::Array<Variant>`
    |
error[E0015]: cannot call non-const fn `<bool as godot::prelude::ToGodot>::to_variant` in constants
   --> src\lib.rs:117:28
    |
117 |   const dd: VariantArray = varray![42_i64, "hello", true];
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `varray` (in Nightly builds, run with -Z macro-backtrace for more info)

[miruji]

This is a bug. The creation works with #[var], but does not work through #[constant]

[Bromeon]

GDExtension only allows to register integral constants:
https://github.com/godotengine/godot/blob/b15b24b087e792335d919fd83055f50f276fbe22/core/extension/gdextension_interface.h#L2918-L2935

So this isn't a bug, it's a feature that's not supported by the Godot engine. You could maybe open a proposal there.

---

Now, even if Godot did support it, we cannot use Rust const for values that aren't known at compile time (such as Array). So we'd need to change how to represent this in Rust code, as const in impl isn't suitable. Rough ideas:

• use fields instead (not per-instance though)
• abuse functions (similar to #[signal]), but that's also weird
• find a wrapper that allows const-compatible expressions that can then be converted to Array etc at runtime.

Anyway, for now there's nothing we can do on godot-rust side, so I'm closing this. Let me know in case I missed a GDExtension API 🙂

[ogyrec-o]

GDExtension only allows to register integral constants: https://github.com/godotengine/godot/blob/b15b24b087e792335d919fd83055f50f276fbe22/core/extension/gdextension_interface.h#L2918-L2935

So this isn't a bug, it's a feature that's not supported by the Godot engine. You could maybe open a proposal there.

Now, even if Godot did support it, we cannot use Rust const for values that aren't known at compile time (such as Array). So we'd need to change how to represent this in Rust code, as const in impl isn't suitable. Rough ideas:

• use fields instead (not per-instance though)
• abuse functions (similar to #[signal]), but that's also weird
• find a wrapper that allows const-compatible expressions that can then be converted to Array etc at runtime.

Anyway, for now there's nothing we can do on godot-rust side, so I'm closing this. Let me know in case I missed a GDExtension API 🙂

godotengine/godot-proposals#11636