Make the SystemParam derive macro more flexible (#6694)

# Objective

Currently, the `SystemParam` derive forces you to declare the lifetime parameters `<'w, 's>`, even if you don't use them.
If you don't follow this structure, the error message is quite nasty.

### Example (before):

```rust
#[derive(SystemParam)]
pub struct EventWriter<'w, 's, E: Event> {
    events: ResMut<'w, Events<E>>,
    // The derive forces us to declare the `'s` lifetime even though we don't use it,
    // so we have to add this `PhantomData` to please rustc.
    #[system_param(ignore)]
    _marker: PhantomData<&'s ()>,
}
```


## Solution

* Allow the user to omit either lifetime.
* Emit a descriptive error if any lifetimes used are invalid.

### Example (after):

```rust
#[derive(SystemParam)]
pub struct EventWriter<'w, E: Event> {
    events: ResMut<'w, Events<E>>,
}
```

---

## Changelog

* The `SystemParam` derive is now more flexible, allowing you to omit unused lifetime parameters.

crates/bevy_ecs/macros/src/lib.rs

@@ -373,7 +373,25 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
     }
 
     let generics = ast.generics;
-    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    // Emit an error if there's any unrecognized lifetime names.
+    for lt in generics.lifetimes() {
+        let ident = &lt.lifetime.ident;
+        let w = format_ident!("w");
+        let s = format_ident!("s");
+        if ident != &w && ident != &s {
+            return syn::Error::new_spanned(
+                lt,
+                r#"invalid lifetime name: expected `'w` or `'s`
+ 'w -- refers to data stored in the World.
+ 's -- refers to data stored in the SystemParam's state.'"#,
+            )
+            .into_compile_error()
+            .into();
+        }
+    }
+
+    let (_impl_generics, ty_generics, where_clause) = generics.split_for_impl();
 
     let lifetimeless_generics: Vec<_> = generics
         .params
@@ -404,10 +422,16 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
         // The struct can still be accessed via SystemParam::Fetch, e.g. EventReaderState can be accessed via
         // <EventReader<'static, 'static, T> as SystemParam>::Fetch
         const _: () = {
-            impl #impl_generics #path::system::SystemParam for #struct_name #ty_generics #where_clause {
-                type Fetch = FetchState <(#(<#field_types as #path::system::SystemParam>::Fetch,)*), #punctuated_generic_idents>;
+            impl<'w, 's, #punctuated_generics> #path::system::SystemParam for #struct_name #ty_generics #where_clause {
+                type Fetch = State<'w, 's, #punctuated_generic_idents>;
             }
 
+            #[doc(hidden)]
+            type State<'w, 's, #punctuated_generic_idents> = FetchState<
+                (#(<#field_types as #path::system::SystemParam>::Fetch,)*),
+                #punctuated_generic_idents
+            >;
+
             #[doc(hidden)]
             #fetch_struct_visibility struct FetchState <TSystemParamState, #punctuated_generic_idents> {
                 state: TSystemParamState,
@@ -431,7 +455,7 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
                 }
             }
 
-            impl #impl_generics #path::system::SystemParamFetch<'w, 's> for FetchState <(#(<#field_types as #path::system::SystemParam>::Fetch,)*), #punctuated_generic_idents> #where_clause {
+            impl<'w, 's, #punctuated_generics> #path::system::SystemParamFetch<'w, 's> for State<'w, 's, #punctuated_generic_idents> #where_clause {
                 type Item = #struct_name #ty_generics;
                 unsafe fn get_param(
                     state: &'s mut Self,

crates/bevy_ecs/src/event.rs

@@ -295,13 +295,11 @@ impl<'w, 's, E: Event> EventReader<'w, 's, E> {
 /// ```
 /// Note that this is considered *non-idiomatic*, and should only be used when `EventWriter` will not work.
 #[derive(SystemParam)]
-pub struct EventWriter<'w, 's, E: Event> {
+pub struct EventWriter<'w, E: Event> {
     events: ResMut<'w, Events<E>>,
-    #[system_param(ignore)]
-    marker: PhantomData<&'s usize>,
 }
 
-impl<'w, 's, E: Event> EventWriter<'w, 's, E> {
+impl<'w, E: Event> EventWriter<'w, E> {
     /// Sends an `event`. [`EventReader`]s can then read the event.
     /// See [`Events`] for details.
     pub fn send(&mut self, event: E) {

crates/bevy_ecs/src/system/system_param.rs

@@ -33,19 +33,15 @@ use std::{
 /// See the *Generic `SystemParam`s* section for details and workarounds of the probable
 /// cause if this derive causes an error to be emitted.
 ///
-///
-/// The struct for which `SystemParam` is derived must (currently) have exactly
-/// two lifetime parameters.
-/// The first is the lifetime of the world, and the second the lifetime
-/// of the parameter's state.
+/// Derived `SystemParam` structs may have two lifetimes: `'w` for data stored in the [`World`],
+/// and `'s` for data stored in the parameter's state.
 ///
 /// ## Attributes
 ///
 /// `#[system_param(ignore)]`:
 /// Can be added to any field in the struct. Fields decorated with this attribute
 /// will be created with the default value upon realisation.
-/// This is most useful for `PhantomData` fields, to ensure that the required lifetimes are
-/// used, as shown in the example.
+/// This is most useful for `PhantomData` fields, such as markers for generic types.
 ///
 /// # Example
 ///
@@ -57,17 +53,17 @@ use std::{
 /// use bevy_ecs::system::SystemParam;
 ///
 /// #[derive(SystemParam)]
-/// struct MyParam<'w, 's> {
+/// struct MyParam<'w, Marker: 'static> {
 ///     foo: Res<'w, SomeResource>,
 ///     #[system_param(ignore)]
-///     marker: PhantomData<&'s ()>,
+///     marker: PhantomData<Marker>,
 /// }
 ///
-/// fn my_system(param: MyParam) {
+/// fn my_system<T: 'static>(param: MyParam<T>) {
 ///     // Access the resource through `param.foo`
 /// }
 ///
-/// # bevy_ecs::system::assert_is_system(my_system);
+/// # bevy_ecs::system::assert_is_system(my_system::<()>);
 /// ```
 ///
 /// # Generic `SystemParam`s
@@ -1567,7 +1563,7 @@ pub mod lifetimeless {
 /// struct GenericParam<'w,'s, T: SystemParam> {
 ///     field: T,
 ///     #[system_param(ignore)]
-///     // Use the lifetimes, as the `SystemParam` derive requires them
+///     // Use the lifetimes in this type, or they will be unbound.
 ///     phantom: core::marker::PhantomData<&'w &'s ()>
 /// }
 /// # fn check_always_is_system<T: SystemParam + 'static>(){
@@ -1651,7 +1647,7 @@ unsafe impl<S: SystemParamState, P: SystemParam + 'static> SystemParamState
 
 #[cfg(test)]
 mod tests {
-    use super::SystemParam;
+    use super::*;
     use crate::{
         self as bevy_ecs, // Necessary for the `SystemParam` Derive when used inside `bevy_ecs`.
         query::{ReadOnlyWorldQuery, WorldQuery},
@@ -1668,4 +1664,17 @@ mod tests {
     > {
         _query: Query<'w, 's, Q, F>,
     }
+
+    #[derive(SystemParam)]
+    pub struct SpecialRes<'w, T: Resource> {
+        _res: Res<'w, T>,
+    }
+
+    #[derive(SystemParam)]
+    pub struct SpecialLocal<'s, T: FromWorld + Send + 'static> {
+        _local: Local<'s, T>,
+    }
+
+    #[derive(SystemParam)]
+    pub struct UnitParam {}
 }