#[rpc] annotation for functions in #[godot_api] inherent impl blocks.

The style is similar to GDScript's @rpc annotation, the macro can be used as follows:

godot-rust#1 - Separate arguments:
```rust
#[rpc(any_peer, reliable)]
fn some_rpc(&mut self) {
    //..
}
```

Providing overlapping arguments generates a compile error.

Any omitted arguments are set to their default values.

godot-rust#2 - Provide an expression:
```rust
const CONFIG: RpcArgs = RpcArgs {
    mode: RpcMode::Authority,
    ..RpcArgs::default()
};

#[rpc(config = CONFIG_EXPR)]
fn some_rpc(&mut self) {
    //..
}
```

Number godot-rust#2 is useful in case you want to reuse the configuration on multiple functions.

Number godot-rust#2 is mutually exclusive with number godot-rust#1.
---

The generated macro code works as follows:
- Caches the configuration in a `ClassPlugin`.
- On `__before_ready()`, searches for the configuration in the plugin, registering them with Node::rpc_config().

godot-core/src/meta/mod.rs

@@ -40,13 +40,15 @@ mod godot_convert;
 mod method_info;
 mod property_info;
 mod ref_arg;
+mod rpc_args;
 mod sealed;
 mod signature;
 mod traits;
 
 pub mod error;
 pub use class_name::ClassName;
 pub use godot_convert::{FromGodot, GodotConvert, ToGodot};
+pub use rpc_args::RpcArgs;
 pub use traits::{ArrayElement, GodotType, PackedArrayElement};
 
 pub(crate) use crate::impl_godot_as_self;

godot-core/src/meta/rpc_args.rs

@@ -0,0 +1,50 @@
+/*
+ * 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/.
+ */
+
+use crate::builtin::{Dictionary, StringName};
+use crate::classes::multiplayer_api::RpcMode;
+use crate::classes::multiplayer_peer::TransferMode;
+use crate::classes::Node;
+use crate::dict;
+use crate::meta::ToGodot;
+
+/// See [Godot documentation](https://docs.godotengine.org/en/stable/tutorials/networking/high_level_multiplayer.html#remote-procedure-calls)
+#[derive(Debug, Clone, Copy)]
+pub struct RpcArgs {
+    pub mode: RpcMode,
+    pub transfer_mode: TransferMode,
+    pub call_local: bool,
+    pub transfer_channel: u32,
+}
+
+impl Default for RpcArgs {
+    fn default() -> Self {
+        Self {
+            mode: RpcMode::AUTHORITY,
+            transfer_mode: TransferMode::UNRELIABLE,
+            call_local: false,
+            transfer_channel: 0,
+        }
+    }
+}
+
+impl RpcArgs {
+    /// Register `method` as a remote procedure call on `node`.
+    pub fn register(self, node: &mut Node, method: impl Into<StringName>) {
+        node.rpc_config(method.into(), &self.into_dictionary().to_variant());
+    }
+
+    /// Returns a [`Dictionary`] populated with the values required for a call to [`Node::rpc_config`].
+    pub fn into_dictionary(self) -> Dictionary {
+        dict! {
+            "mode": self.mode,
+            "transfer_mode": self.transfer_mode,
+            "call_local": self.call_local,
+            "transfer_channel": self.transfer_channel,
+        }
+    }
+}

godot-core/src/obj/traits.rs

@@ -455,6 +455,7 @@ pub mod cap {
     use super::*;
     use crate::builtin::{StringName, Variant};
     use crate::obj::{Base, Bounds, Gd};
+    use std::any::Any;
 
     /// Trait for all classes that are default-constructible from the Godot engine.
     ///
@@ -558,6 +559,8 @@ pub mod cap {
         fn __register_methods();
         #[doc(hidden)]
         fn __register_constants();
+        #[doc(hidden)]
+        fn __register_rpcs(_: &mut dyn Any) {}
     }
 
     pub trait ImplementsGodotExports: GodotClass {

godot-core/src/private.rs

@@ -8,7 +8,9 @@
 pub use crate::gen::classes::class_macros;
 pub use crate::obj::rtti::ObjectRtti;
 pub use crate::registry::callbacks;
-pub use crate::registry::plugin::{ClassPlugin, ErasedRegisterFn, PluginItem};
+pub use crate::registry::plugin::{
+    ClassPlugin, ErasedRegisterFn, ErasedRegisterRpcsFn, PluginItem,
+};
 pub use crate::storage::{as_storage, Storage};
 pub use sys::out;
 

godot-core/src/registry/callbacks.rs

@@ -354,3 +354,7 @@ pub fn register_user_methods_constants<T: cap::ImplementsGodotApi>(_class_builde
     T::__register_methods();
     T::__register_constants();
 }
+
+pub fn register_user_rpcs<T: cap::ImplementsGodotApi>(object: &mut dyn Any) {
+    T::__register_rpcs(object);
+}

godot-core/src/registry/class.rs

@@ -11,7 +11,7 @@ use std::ptr;
 use crate::init::InitLevel;
 use crate::meta::ClassName;
 use crate::obj::{cap, GodotClass};
-use crate::private::{ClassPlugin, PluginItem};
+use crate::private::{ClassPlugin, PluginItem, __godot_rust_plugin___GODOT_PLUGIN_REGISTRY};
 use crate::registry::callbacks;
 use crate::registry::plugin::ErasedRegisterFn;
 use crate::{godot_error, sys};
@@ -200,6 +200,28 @@ pub fn unregister_classes(init_level: InitLevel) {
     }
 }
 
+pub fn auto_register_rpcs<T: GodotClass>(object: &mut T) {
+    let class_name = T::class_name();
+
+    // We do this manually instead of using `iterate_plugins()` because we want to break as soon as we find a match.
+    let plugins = __godot_rust_plugin___GODOT_PLUGIN_REGISTRY.lock().unwrap();
+
+    // Find the element that matches our class, and call the closure if it exists.
+    for elem in plugins.iter().filter(|elem| elem.class_name == class_name) {
+        if let PluginItem::InherentImpl {
+            register_rpcs_fn, ..
+        } = &elem.item
+        {
+            if let Some(closure) = register_rpcs_fn {
+                (closure.raw)(object);
+            }
+
+            // We found the element, break out of the loop regardless of the value of `register_rpcs_fn`.
+            break;
+        }
+    }
+}
+
 fn global_loaded_classes() -> GlobalGuard<'static, HashMap<InitLevel, Vec<LoadedClass>>> {
     match LOADED_CLASSES.try_lock() {
         Ok(it) => it,
@@ -283,6 +305,7 @@ fn fill_class_info(item: PluginItem, c: &mut ClassRegistrationInfo) {
 
         PluginItem::InherentImpl {
             register_methods_constants_fn,
+            register_rpcs_fn: _,
             #[cfg(all(since_api = "4.3", feature = "docs"))]
                 docs: _,
         } => {

godot-core/src/registry/plugin.rs

@@ -12,7 +12,6 @@ use crate::meta::ClassName;
 use crate::sys;
 use std::any::Any;
 use std::fmt;
-
 // TODO(bromeon): some information coming from the proc-macro API is deferred through PluginItem, while others is directly
 // translated to code. Consider moving more code to the PluginItem, which allows for more dynamic registration and will
 // be easier for a future builder API.
@@ -45,6 +44,17 @@ impl fmt::Debug for ErasedRegisterFn {
     }
 }
 
+#[derive(Copy, Clone)]
+pub struct ErasedRegisterRpcsFn {
+    pub raw: fn(&mut dyn Any),
+}
+
+impl fmt::Debug for ErasedRegisterRpcsFn {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "0x{:0>16x}", self.raw as usize)
+    }
+}
+
 /// Represents the data part of a [`ClassPlugin`] instance.
 ///
 /// Each enumerator represents a different item in Rust code, which is processed by an independent proc macro (for example,
@@ -107,6 +117,10 @@ pub enum PluginItem {
         ///
         /// Always present since that's the entire point of this `impl` block.
         register_methods_constants_fn: ErasedRegisterFn,
+        /// Callback to library-generated function which calls [`Node::rpc_config`](crate::classes::Node::rpc_config) for each function annotated with `#[rpc]` on the `impl` block.
+        ///
+        /// This function is called in [`UserClass::__before_ready()`](crate::obj::UserClass::__before_ready) definitions generated by the `#[derive(GodotClass)]` macro.
+        register_rpcs_fn: Option<ErasedRegisterRpcsFn>,
         #[cfg(all(since_api = "4.3", feature = "docs"))]
         docs: InherentImplDocs,
     },

godot-macros/src/class/data_models/field_var.rs

@@ -204,6 +204,7 @@ impl GetterSetterImpl {
                 external_attributes: Vec::new(),
                 rename: None,
                 is_script_virtual: false,
+                rpc_info: None,
             },
         );
 

godot-macros/src/class/data_models/func.rs

@@ -5,6 +5,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
+use crate::class::RpcInfo;
 use crate::util::{bail_fn, ident, safe_ident};
 use crate::{util, ParseResult};
 use proc_macro2::{Group, Ident, TokenStream, TokenTree};
@@ -19,6 +20,8 @@ pub struct FuncDefinition {
     /// The name the function will be exposed as in Godot. If `None`, the Rust function name is used.
     pub rename: Option<String>,
     pub is_script_virtual: bool,
+    /// Information about the RPC configuration, if provided.
+    pub rpc_info: Option<RpcInfo>,
 }
 
 /// Returns a C function which acts as the callback when a virtual method of this instance is invoked.