Fallible param notes (#1729)

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

release-content/0.15/release-notes/15276_Reduce_runtime_panics_through_SystemParam_validation.md

@@ -1,4 +1,59 @@
 <!-- Reduce runtime panics through `SystemParam` validation -->
 <!-- https://github.com/bevyengine/bevy/pull/15276 -->
 
-<!-- TODO -->
+<!-- `QuerySingle` family of system params -->
+<!-- https://github.com/bevyengine/bevy/pull/15476 -->
+
+<!-- `Populated` (query) system param -->
+<!-- https://github.com/bevyengine/bevy/pull/15488 -->
+
+
+In Bevy 0.14 and prior, the following code would panic:
+```rust
+#[derive(Resource)]
+struct MyResource;
+
+fn my_system(my_resource: Res<MyResource>) {}
+
+fn main() {
+    let mut app = App::new();
+    app.add_plugins(DefaultPlugins)
+    app.add_systems(my_system);
+    // Panic here: `my_system` cannot fetch `MyResource`, because it was never added.
+    app.run();
+}
+```
+but in Bevy 0.15, `my_system` simply won't be executed.
+
+This works for all system-based features:
+- Systems & Observers - will be skipped,
+- Run conditions - will be skipped and return `false`.
+
+Compound systems, like `system_a.pipe(system_b)`, are currently skipped if any required data is missing.
+
+Pre-existing parameters which now benefit from this feature are: `Res` and `ResMut` as well as their siblings `NonSend` and `NonSendMut`.
+Parameters that build on top of other parameters: tuples, `DynSystemParam` and `ParamSet` are considered present if and only if all of their system parameters are present.
+
+Additionally, few new system params were introduced to simplify existing code:
+- `Single<D, F>` - Works like `Query<D, F>::single`, fails if query contains 0 or more than 1 match,
+- `Option<Single<D, F>>` - Works like `Query<D, F>::single`, fails if query contains more than 1 match,
+- `Populated<D, F>` - Works like a `Query<D, F>`, fails if query contains no matches.
+
+## Warnings
+
+Fallible system params come with a primitive warning mechanic.
+Currently, systems can behave in one of two ways:
+- (default) warn exactly once,
+- never warn.
+
+The default can be changed as following:
+```rust
+// For systems
+app.add_systems(my_system.never_param_warn());
+// For observers
+app.add_observer(my_observer.never_param_warn());
+// For run conditions
+app.add_systems(my_system.run_if(my_condition.never_param_warn()));
+```
+
+Let us know what other warning strategies you'd like!

release-content/0.15/release-notes/_release-notes.toml

@@ -244,10 +244,10 @@ prs = [14920]
 file_name = "14920_Add_cached_run_system_API.md"
 
 [[release_notes]]
-title = "Reduce runtime panics through `SystemParam` validation"
+title = "Fallible system parameters"
 authors = ["@MiniaczQ"]
-contributors = []
-prs = [15276]
+contributors = ["@alice-i-cecile"]
+prs = [15276, 15476, 15488]
 file_name = "15276_Reduce_runtime_panics_through_SystemParam_validation.md"
 
 [[release_notes]]
@@ -311,13 +311,6 @@ contributors = ["@cart"]
 prs = [12770]
 file_name = "12770_Implement_gamepads_as_entities.md"
 
-[[release_notes]]
-title = "`QuerySingle` family of system params"
-authors = ["@MiniaczQ"]
-contributors = []
-prs = [15476]
-file_name = "15476_QuerySingle_family_of_system_params.md"
-
 [[release_notes]]
 title = "Implement volumetric fog support for both point lights and spotlights"
 authors = ["@Soulghost"]
@@ -353,13 +346,6 @@ contributors = []
 prs = [14838]
 file_name = "14838_bevy_reflect_add_Type_type.md"
 
-[[release_notes]]
-title = "`Populated` (query) system param"
-authors = ["@MiniaczQ"]
-contributors = ["@alice-i-cecile"]
-prs = [15488]
-file_name = "15488_Populated_query_system_param.md"
-
 [[release_notes]]
 title = "Runtime required components"
 authors = ["@Jondolf"]