Use actual tests for SystemParam regression tests (#8270)

# Objective

Our regression tests for `SystemParam` currently consist of a bunch of
loosely dispersed struct definitions. This is messy, and doesn't fully
test their functionality.

## Solution

Group the struct definitions into functions annotated with `#[test]`.
This not only makes the module more organized, but it allows us to call
`assert_is_system`, which has the potential to catch some bugs that
would have been missed with the old approach. Also, this approach is
consistent with how `WorldQuery` regression tests are organized.

crates/bevy_ecs/src/system/system_param.rs

@@ -1563,11 +1563,13 @@ mod tests {
     use crate::{
         self as bevy_ecs, // Necessary for the `SystemParam` Derive when used inside `bevy_ecs`.
         query::{ReadOnlyWorldQuery, WorldQuery},
-        system::Query,
+        system::{assert_is_system, Query},
     };
     use std::marker::PhantomData;
 
     // Compile test for https://github.com/bevyengine/bevy/pull/2838.
+    #[test]
+    fn system_param_generic_bounds() {
         #[derive(SystemParam)]
         pub struct SpecialQuery<
             'w,
@@ -1578,8 +1580,13 @@ mod tests {
             _query: Query<'w, 's, Q, F>,
         }
 
-    // Compile tests for https://github.com/bevyengine/bevy/pull/6694.
+        fn my_system(_: SpecialQuery<(), ()>) {}
+        assert_is_system(my_system);
+    }
 
+    // Compile tests for https://github.com/bevyengine/bevy/pull/6694.
+    #[test]
+    fn system_param_flexibility() {
         #[derive(SystemParam)]
         pub struct SpecialRes<'w, T: Resource> {
             _res: Res<'w, T>,
@@ -1590,16 +1597,33 @@ mod tests {
             _local: Local<'s, T>,
         }
 
+        #[derive(Resource)]
+        struct R;
+
+        fn my_system(_: SpecialRes<R>, _: SpecialLocal<u32>) {}
+        assert_is_system(my_system);
+    }
+
     #[derive(Resource)]
     pub struct R<const I: usize>;
 
     // Compile test for https://github.com/bevyengine/bevy/pull/7001.
+    #[test]
+    fn system_param_const_generics() {
         #[derive(SystemParam)]
         pub struct ConstGenericParam<'w, const I: usize>(Res<'w, R<I>>);
 
+        fn my_system(_: ConstGenericParam<0>, _: ConstGenericParam<1000>) {}
+        assert_is_system(my_system);
+    }
+
     // Compile test for https://github.com/bevyengine/bevy/pull/6867.
+    #[test]
+    fn system_param_field_limit() {
         #[derive(SystemParam)]
         pub struct LongParam<'w> {
+            // Each field should be a distinct type so there will
+            // be an error if the derive messes up the field order.
             _r0: Res<'w, R<0>>,
             _r1: Res<'w, R<1>>,
             _r2: Res<'w, R<2>>,
@@ -1619,13 +1643,14 @@ mod tests {
             _r16: Res<'w, R<16>>,
         }
 
-    #[allow(dead_code)]
-    fn long_system(_param: LongParam) {
-        crate::system::assert_is_system(long_system);
+        fn long_system(_: LongParam) {}
+        assert_is_system(long_system);
     }
 
     // Compile test for https://github.com/bevyengine/bevy/pull/6919.
     // Regression test for https://github.com/bevyengine/bevy/issues/7447.
+    #[test]
+    fn system_param_phantom_data() {
         #[derive(SystemParam)]
         struct IgnoredParam<'w, T: Resource, Marker: 'static> {
             _foo: Res<'w, T>,
@@ -1634,8 +1659,13 @@ mod tests {
             marker2: PhantomData<&'w Marker>,
         }
 
-    // Compile tests for https://github.com/bevyengine/bevy/pull/6957.
+        fn my_system(_: IgnoredParam<R<0>, ()>) {}
+        assert_is_system(my_system);
+    }
 
+    // Compile tests for https://github.com/bevyengine/bevy/pull/6957.
+    #[test]
+    fn system_param_struct_variants() {
         #[derive(SystemParam)]
         pub struct UnitParam;
 
@@ -1645,14 +1675,26 @@ mod tests {
             Local<'s, L>,
         );
 
+        fn my_system(_: UnitParam, _: TupleParam<R<0>, u32>) {}
+        assert_is_system(my_system);
+    }
+
+    // Regression test for https://github.com/bevyengine/bevy/issues/4200.
+    #[test]
+    fn system_param_private_fields() {
         #[derive(Resource)]
         struct PrivateResource;
 
-    // Regression test for https://github.com/bevyengine/bevy/issues/4200.
         #[derive(SystemParam)]
         pub struct EncapsulatedParam<'w>(Res<'w, PrivateResource>);
 
+        fn my_system(_: EncapsulatedParam) {}
+        assert_is_system(my_system);
+    }
+
     // Regression test for https://github.com/bevyengine/bevy/issues/7103.
+    #[test]
+    fn system_param_where_clause() {
         #[derive(SystemParam)]
         pub struct WhereParam<'w, 's, Q>
         where
@@ -1661,18 +1703,34 @@ mod tests {
             _q: Query<'w, 's, Q, ()>,
         }
 
+        fn my_system(_: WhereParam<()>) {}
+        assert_is_system(my_system);
+    }
+
     // Regression test for https://github.com/bevyengine/bevy/issues/1727.
+    #[test]
+    fn system_param_name_collision() {
+        #[derive(Resource)]
+        pub struct FetchState;
+
         #[derive(SystemParam)]
         pub struct Collide<'w> {
             _x: Res<'w, FetchState>,
         }
 
-    #[derive(Resource)]
-    pub struct FetchState;
+        fn my_system(_: Collide) {}
+        assert_is_system(my_system);
+    }
 
     // Regression test for https://github.com/bevyengine/bevy/issues/8192.
+    #[test]
+    fn system_param_invariant_lifetime() {
         #[derive(SystemParam)]
         pub struct InvariantParam<'w, 's> {
             _set: ParamSet<'w, 's, (Query<'w, 's, ()>,)>,
         }
+
+        fn my_system(_: InvariantParam) {}
+        assert_is_system(my_system);
+    }
 }