bounds.rs

/*
 * Copyright (c) godot-rust; Bromeon and contributors.
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */

//! Different ways how bounds of a `GodotClass` can be checked.
//!
//! This module contains multiple traits that can be used to check the characteristics of a `GodotClass` type:
//!
//! 1. [`Declarer`] tells you whether the class is provided by the engine or user-defined.
//!    - [`DeclEngine`] is used for all classes provided by the engine (e.g. `Node3D`).
//!    - [`DeclUser`] is used for all classes defined by the user, typically through `#[derive(GodotClass)]`.<br><br>
//!
//! 2. [`Memory`] is used to check the memory strategy of the **static** type.
//!
//!    This is useful when you operate on associated functions of `Gd<T>` or `T`, e.g. for construction.
//!    - [`MemRefCounted`] is used for `RefCounted` classes and derived.
//!    - [`MemManual`] is used for `Object` and all inherited classes, which are not `RefCounted` (e.g. `Node`).<br><br>
//!
// FIXME excluded because broken; see below.
// 3. [`DynMemory`] is used to check the memory strategy of the **dynamic** type.
//
//    When you operate on methods of `T` or `Gd<T>` and are interested in instances, you can use this.
//    Most of the time, this is not what you want -- just use `Memory` if you want to know if a type is manually managed or ref-counted.
//    - [`MemRefCounted`] is used for `RefCounted` classes and derived. These are **always** reference-counted.
//    - [`MemManual`] is used instances inheriting `Object`, which are not `RefCounted` (e.g. `Node`). Excludes `Object` itself. These are
//      **always** manually managed.
//    - [`MemDynamic`] is used for `Object` instances. `Gd<Object>` can point to objects of any possible class, so whether we are dealing with
//      a ref-counted or manually-managed object is determined only at runtime.
//!
//!
//! # Example
//!
//! Declare a custom smart pointer which wraps `Gd<T>` pointers, but only accepts `T` objects that are manually managed.
//! ```
//! use godot::prelude::*;
//! use godot::obj::{bounds, Bounds};
//!
//! struct MyGd<T>
//! where T: GodotClass + Bounds<Memory = bounds::MemManual>
//! {
//!    inner: Gd<T>,
//! }
//! ```
//!
// Note that depending on if you want to exclude `Object`, you should use `DynMemory` instead of `Memory`.

use private::Sealed;

use crate::obj::cap::GodotDefault;
use crate::obj::{Bounds, Gd, GodotClass, RawGd};
use crate::storage::{InstanceCache, Storage};
use crate::sys;

// ----------------------------------------------------------------------------------------------------------------------------------------------
// Sealed trait

pub(super) mod private {
    use super::{Declarer, DynMemory, Exportable, Memory};

    // Bounds trait declared here for code locality; re-exported in crate::obj.

    /// Library-implemented trait to check bounds on `GodotClass` types.
    ///
    /// See [`bounds`](crate::obj::bounds) module for how to use this for bounds checking.
    ///
    /// # No manual `impl`
    ///
    /// <div class="warning">
    /// <strong>Never</strong> implement this trait manually.
    /// </div>
    ///
    /// Most of the time, this trait is covered by [`#[derive(GodotClass)]`](../register/derive.GodotClass.html).
    /// If you implement `GodotClass` manually, use the [`implement_godot_bounds!`][crate::implement_godot_bounds] macro.
    ///
    /// There are two reasons to avoid a handwritten `impl Bounds`:
    /// - The trait is `unsafe` and it is very easy to get internal bounds wrong. This will lead to immediate UB.
    /// - Apart from the documented members, the trait may have undocumented items that may be broken at any time and stand under no SemVer
    ///   guarantees.
    ///
    /// # Safety
    ///
    /// Internal. The library implements this trait and ensures safety.
    pub unsafe trait Bounds {
        /// Defines the memory strategy of the static type.
        type Memory: Memory;

        // FIXME: this is broken as a bound: one cannot use T: Bounds<DynMemory = MemRefCounted> to include Object AND RefCounted,
        // since Object itself has DynMemory = MemDynamic. Needs to either use traits like in gdnative, or more types to account for
        // different combinations (as only positive ones can be expressed, not T: Bounds<Memory != MemManual>).
        #[doc(hidden)]
        /// Defines the memory strategy of the instance (at runtime).
        type DynMemory: DynMemory;

        /// Whether this class is a core Godot class provided by the engine, or declared by the user as a Rust struct.
        // TODO what about GDScript user classes?
        type Declarer: Declarer;

        /// True if *either* `T: Inherits<Node>` *or* `T: Inherits<Resource>` is fulfilled.
        ///
        /// Enables `#[export]` for those classes.
        #[doc(hidden)]
        type Exportable: Exportable;
    }

    /// Implements [`Bounds`] for a user-defined class.
    ///
    /// This is only necessary if you do not use the proc-macro API.
    ///
    /// Since `Bounds` is a supertrait of [`GodotClass`][crate::obj::GodotClass], you cannot accidentally forget to implement it.
    ///
    /// # Example
    /// ```no_run
    /// use godot::prelude::*;
    /// use godot::obj::bounds::implement_godot_bounds;
    /// use godot::meta::ClassId;
    ///
    /// struct MyClass {}
    ///
    /// impl GodotClass for MyClass {
    ///     type Base = Node;
    ///
    ///     fn class_id() -> ClassId {
    ///         ClassId::new_cached::<MyClass>(|| "MyClass".to_string())
    ///     }
    /// }
    ///
    /// implement_godot_bounds!(MyClass);
    #[macro_export]
    macro_rules! implement_godot_bounds {
        ($UserClass:ty) => {
            // SAFETY: bounds are library-defined, dependent on base. User has no influence in selecting them -> macro is safe.
            unsafe impl $crate::obj::Bounds for $UserClass {
                type Memory = <<$UserClass as $crate::obj::GodotClass>::Base as $crate::obj::Bounds>::Memory;
                type DynMemory = <<$UserClass as $crate::obj::GodotClass>::Base as $crate::obj::Bounds>::DynMemory;
                type Declarer = $crate::obj::bounds::DeclUser;
                type Exportable = <<$UserClass as $crate::obj::GodotClass>::Base as $crate::obj::Bounds>::Exportable;
            }
        };
    }

    pub trait Sealed {}
}

// ----------------------------------------------------------------------------------------------------------------------------------------------
// Macro re-exports

pub use crate::implement_godot_bounds;

// ----------------------------------------------------------------------------------------------------------------------------------------------
// Memory bounds

/// Specifies the memory strategy of the static type.
pub trait Memory: Sealed {
    /// True for everything inheriting `RefCounted`, false for `Object` and all other classes.
    #[doc(hidden)]
    const IS_REF_COUNTED: bool;
}

/// Specifies the memory strategy of the dynamic type.
///
/// For `Gd<Object>`, it is determined at runtime whether the instance is manually managed or ref-counted.
///
/// This trait only answers *whether* an object is ref-counted; the ref-counting operations themselves are the same in all cases and
/// live on [`RawGd`]. For `MemRefCounted`/`MemManual`, the answer is known at compile-time and can be optimized.
#[doc(hidden)]
pub trait DynMemory: Sealed {
    /// Check if ref-counted, return `None` if information is not available (dynamic and obj dead).
    #[doc(hidden)]
    fn is_ref_counted<T: GodotClass>(obj: &RawGd<T>) -> Option<bool>;

    /// Returns `true` if argument and return pointers are passed as `Ref<T>` pointers given this
    /// [`PtrcallType`].
    ///
    /// See [`PtrcallType::Virtual`] for information about `Ref<T>` objects.
    #[doc(hidden)]
    fn pass_as_ref(_call_type: sys::PtrcallType) -> bool {
        false
    }
}

/// Memory managed through Godot reference counter (always present).
/// This is used for `RefCounted` classes and derived.
pub struct MemRefCounted {}
impl Sealed for MemRefCounted {}
impl Memory for MemRefCounted {
    const IS_REF_COUNTED: bool = true;
}
impl DynMemory for MemRefCounted {
    fn is_ref_counted<T: GodotClass>(_obj: &RawGd<T>) -> Option<bool> {
        Some(true)
    }

    fn pass_as_ref(call_type: sys::PtrcallType) -> bool {
        matches!(call_type, sys::PtrcallType::Virtual)
    }
}

/// Memory managed through Godot reference counter, if present; otherwise manual.
/// This is used only for `Object` classes.
#[doc(hidden)]
pub struct MemDynamic {}
impl Sealed for MemDynamic {}
impl DynMemory for MemDynamic {
    fn is_ref_counted<T: GodotClass>(obj: &RawGd<T>) -> Option<bool> {
        // Return `None` if obj is dead. The instance ID carries a ref-countedness bit, so this needs no FFI call.
        obj.instance_id_unchecked().map(|id| id.is_ref_counted())
    }
}

/// No memory management, user responsible for not leaking.
/// This is used for all `Object` derivates, which are not `RefCounted`. `Object` itself is also excluded.
pub struct MemManual {}
impl Sealed for MemManual {}
impl Memory for MemManual {
    const IS_REF_COUNTED: bool = false;
}
impl DynMemory for MemManual {
    fn is_ref_counted<T: GodotClass>(_obj: &RawGd<T>) -> Option<bool> {
        Some(false)
    }
}

// ----------------------------------------------------------------------------------------------------------------------------------------------
// Declarer bounds

/// Trait that specifies who declares a given `GodotClass`.
pub trait Declarer: Sealed {
    /// The target type of a `Deref` operation on a `Gd<T>`.
    #[doc(hidden)]
    type DerefTarget<T: GodotClass>: GodotClass;

    /// Used as a field in `RawGd`; only set for user-defined classes.
    #[doc(hidden)]
    #[allow(private_bounds)]
    type InstanceCache: InstanceCache;

    /// Check if the object is a user object *and* currently locked by a `bind()` or `bind_mut()` guard.
    ///
    /// # Safety
    /// Object must be alive.
    #[doc(hidden)]
    unsafe fn is_currently_bound<T>(obj: &RawGd<T>) -> bool
    where
        T: GodotClass + Bounds<Declarer = Self>;

    #[doc(hidden)]
    fn create_gd<T>() -> Gd<T>
    where
        T: GodotDefault + Bounds<Declarer = Self>;
}

/// Expresses that a class is declared by the Godot engine.
pub enum DeclEngine {}
impl Sealed for DeclEngine {}
impl Declarer for DeclEngine {
    type DerefTarget<T: GodotClass> = T;
    type InstanceCache = ();

    unsafe fn is_currently_bound<T>(_obj: &RawGd<T>) -> bool
    where
        T: GodotClass + Bounds<Declarer = Self>,
    {
        false
    }

    fn create_gd<T>() -> Gd<T>
    where
        T: GodotDefault + Bounds<Declarer = Self>,
    {
        crate::classes::construct_engine_object()
    }
}

/// Expresses that a class is declared by the user.
pub enum DeclUser {}
impl Sealed for DeclUser {}
impl Declarer for DeclUser {
    type DerefTarget<T: GodotClass> = T::Base;
    type InstanceCache = std::cell::Cell<sys::GDExtensionClassInstancePtr>;

    unsafe fn is_currently_bound<T>(obj: &RawGd<T>) -> bool
    where
        T: GodotClass + Bounds<Declarer = Self>,
    {
        // `storage()` returns `None` for placeholder instances (runtime classes accessed in editor); they hold no Rust binding to be bound.
        // Treat as not currently bound -- callers like `Gd::free()` use this only to detect active `bind()` / `bind_mut()` guards.
        match obj.storage() {
            Some(storage) => storage.is_bound(),
            None => false,
        }
    }

    fn create_gd<T>() -> Gd<T>
    where
        T: GodotDefault + Bounds<Declarer = Self>,
    {
        Gd::default_instance()
    }
}

// ----------------------------------------------------------------------------------------------------------------------------------------------
// Exportable bounds (still hidden)

#[doc(hidden)]
pub trait Exportable: Sealed {}

#[doc(hidden)]
pub enum Yes {}
impl Sealed for Yes {}
impl Exportable for Yes {}

#[doc(hidden)]
pub enum No {}
impl Sealed for No {}
impl Exportable for No {}