Add methods for silencing system-order ambiguity warnings (#6158)

# Background

Incremental implementation of #4299. The code is heavily borrowed from that PR.

# Objective

The execution order ambiguity checker often emits false positives, since bevy is not aware of invariants upheld by the user.

## Solution

Title

---

## Changelog

+ Added methods `SystemDescriptor::ignore_all_ambiguities` and `::ambiguous_with`. These allow you to silence warnings for specific system-order ambiguities.

## Migration Guide

***Note for maintainers**: This should replace the migration guide for #5916*

Ambiguity sets have been replaced with a simpler API.

```rust
// These systems technically conflict, but we don't care which order they run in.
fn jump_on_click(mouse: Res<Input<MouseButton>>, mut transforms: Query<&mut Transform>) { ... }
fn jump_on_spacebar(keys: Res<Input<KeyCode>>, mut transforms: Query<&mut Transform>) { ... }

//
// Before

#[derive(AmbiguitySetLabel)]
struct JumpSystems;

app
  .add_system(jump_on_click.in_ambiguity_set(JumpSystems))
  .add_system(jump_on_spacebar.in_ambiguity_set(JumpSystems));

//
// After

app
  .add_system(jump_on_click.ambiguous_with(jump_on_spacebar))
  .add_system(jump_on_spacebar);

```

crates/bevy_ecs/src/schedule/ambiguity_detection.rs

@@ -2,9 +2,11 @@ use bevy_utils::tracing::info;
 use fixedbitset::FixedBitSet;
 
 use crate::component::ComponentId;
-use crate::schedule::{SystemContainer, SystemStage};
+use crate::schedule::{AmbiguityDetection, GraphNode, SystemContainer, SystemStage};
 use crate::world::World;
 
+use super::SystemLabelId;
+
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 struct SystemOrderAmbiguity {
     segment: SystemStageSegment,
@@ -194,6 +196,24 @@ impl SystemStage {
 /// along with specific components that have triggered the warning.
 /// Systems must be topologically sorted beforehand.
 fn find_ambiguities(systems: &[SystemContainer]) -> Vec<(usize, usize, Vec<ComponentId>)> {
+    // Check if we should ignore ambiguities between `system_a` and `system_b`.
+    fn should_ignore(system_a: &SystemContainer, system_b: &SystemContainer) -> bool {
+        fn should_ignore_inner(
+            system_a_detection: &AmbiguityDetection,
+            system_b_labels: &[SystemLabelId],
+        ) -> bool {
+            match system_a_detection {
+                AmbiguityDetection::Check => false,
+                AmbiguityDetection::IgnoreAll => true,
+                AmbiguityDetection::IgnoreWithLabel(labels) => {
+                    labels.iter().any(|l| system_b_labels.contains(l))
+                }
+            }
+        }
+        should_ignore_inner(&system_a.ambiguity_detection, system_b.labels())
+            || should_ignore_inner(&system_b.ambiguity_detection, system_a.labels())
+    }
+
     let mut all_dependencies = Vec::<FixedBitSet>::with_capacity(systems.len());
     let mut all_dependants = Vec::<FixedBitSet>::with_capacity(systems.len());
     for (index, container) in systems.iter().enumerate() {
@@ -235,7 +255,8 @@ fn find_ambiguities(systems: &[SystemContainer]) -> Vec<(usize, usize, Vec<Compo
         for index_b in full_bitset.difference(&relations)
         // .take(index_a)
         {
-            if !processed.contains(index_b) {
+            if !processed.contains(index_b) && !should_ignore(&systems[index_a], &systems[index_b])
+            {
                 let system_a = &systems[index_a];
                 let system_b = &systems[index_b];
                 if system_a.is_exclusive() || system_b.is_exclusive() {
@@ -471,4 +492,117 @@ mod tests {
 
         assert_eq!(test_stage.ambiguity_count(&world), 0);
     }
+
+    #[test]
+    fn ignore_all_ambiguities() {
+        let mut world = World::new();
+        world.insert_resource(R);
+
+        let mut test_stage = SystemStage::parallel();
+        test_stage
+            .add_system(resmut_system.ignore_all_ambiguities())
+            .add_system(res_system)
+            .add_system(nonsend_system);
+
+        test_stage.run(&mut world);
+
+        assert_eq!(test_stage.ambiguity_count(&world), 0);
+    }
+
+    #[test]
+    fn ambiguous_with_label() {
+        let mut world = World::new();
+        world.insert_resource(R);
+
+        #[derive(SystemLabel)]
+        struct IgnoreMe;
+
+        let mut test_stage = SystemStage::parallel();
+        test_stage
+            .add_system(resmut_system.ambiguous_with(IgnoreMe))
+            .add_system(res_system.label(IgnoreMe))
+            .add_system(nonsend_system.label(IgnoreMe));
+
+        test_stage.run(&mut world);
+
+        assert_eq!(test_stage.ambiguity_count(&world), 0);
+    }
+
+    #[test]
+    fn ambiguous_with_system() {
+        let mut world = World::new();
+
+        let mut test_stage = SystemStage::parallel();
+        test_stage
+            .add_system(write_component_system.ambiguous_with(read_component_system))
+            .add_system(read_component_system);
+
+        test_stage.run(&mut world);
+
+        assert_eq!(test_stage.ambiguity_count(&world), 0);
+    }
+
+    fn system_a(_res: ResMut<R>) {}
+    fn system_b(_res: ResMut<R>) {}
+    fn system_c(_res: ResMut<R>) {}
+    fn system_d(_res: ResMut<R>) {}
+    fn system_e(_res: ResMut<R>) {}
+
+    // Tests that the correct ambiguities were reported in the correct order.
+    #[test]
+    fn correct_ambiguities() {
+        use super::*;
+
+        let mut world = World::new();
+        world.insert_resource(R);
+
+        let mut test_stage = SystemStage::parallel();
+        test_stage
+            .add_system(system_a)
+            .add_system(system_b)
+            .add_system(system_c.ignore_all_ambiguities())
+            .add_system(system_d.ambiguous_with(system_b))
+            .add_system(system_e.after(system_a));
+
+        test_stage.run(&mut world);
+
+        let ambiguities = test_stage.ambiguities(&world);
+        assert_eq!(
+            ambiguities,
+            vec![
+                SystemOrderAmbiguity {
+                    system_names: [
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_a".to_string(),
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_b".to_string()
+                    ],
+                    conflicts: vec!["bevy_ecs::schedule::ambiguity_detection::tests::R".to_string()],
+                    segment: SystemStageSegment::Parallel,
+                },
+                SystemOrderAmbiguity {
+                    system_names: [
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_a".to_string(),
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_d".to_string()
+                    ],
+                    conflicts: vec!["bevy_ecs::schedule::ambiguity_detection::tests::R".to_string()],
+                    segment: SystemStageSegment::Parallel,
+                },
+                SystemOrderAmbiguity {
+                    system_names: [
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_b".to_string(),
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_e".to_string()
+                    ],
+                    conflicts: vec!["bevy_ecs::schedule::ambiguity_detection::tests::R".to_string()],
+                    segment: SystemStageSegment::Parallel,
+                },
+                SystemOrderAmbiguity {
+                    system_names: [
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_d".to_string(),
+                        "bevy_ecs::schedule::ambiguity_detection::tests::system_e".to_string()
+                    ],
+                    conflicts: vec!["bevy_ecs::schedule::ambiguity_detection::tests::R".to_string()],
+                    segment: SystemStageSegment::Parallel,
+                },
+            ]
+        );
+    }
 }

crates/bevy_ecs/src/schedule/system_container.rs

@@ -1,7 +1,9 @@
 use crate::{
     component::ComponentId,
     query::Access,
-    schedule::{GraphNode, RunCriteriaLabelId, SystemDescriptor, SystemLabelId},
+    schedule::{
+        AmbiguityDetection, GraphNode, RunCriteriaLabelId, SystemDescriptor, SystemLabelId,
+    },
     system::System,
 };
 use std::borrow::Cow;
@@ -16,6 +18,7 @@ pub struct SystemContainer {
     labels: Vec<SystemLabelId>,
     before: Vec<SystemLabelId>,
     after: Vec<SystemLabelId>,
+    pub(crate) ambiguity_detection: AmbiguityDetection,
 }
 
 impl SystemContainer {
@@ -29,6 +32,7 @@ impl SystemContainer {
             labels: descriptor.labels,
             before: descriptor.before,
             after: descriptor.after,
+            ambiguity_detection: descriptor.ambiguity_detection,
             is_exclusive: descriptor.exclusive_insertion_point.is_some(),
         }
     }

crates/bevy_ecs/src/schedule/system_descriptor.rs

@@ -3,6 +3,16 @@ use crate::{
     system::{AsSystemLabel, BoxedSystem, IntoSystem},
 };
 
+/// Configures ambiguity detection for a single system.
+#[derive(Default)]
+pub(crate) enum AmbiguityDetection {
+    #[default]
+    Check,
+    IgnoreAll,
+    /// Ignore systems with any of these labels.
+    IgnoreWithLabel(Vec<SystemLabelId>),
+}
+
 /// Encapsulates a system and information on when it run in a `SystemStage`.
 ///
 /// Systems can be inserted into 4 different groups within the stage:
@@ -38,6 +48,7 @@ pub struct SystemDescriptor {
     pub(crate) labels: Vec<SystemLabelId>,
     pub(crate) before: Vec<SystemLabelId>,
     pub(crate) after: Vec<SystemLabelId>,
+    pub(crate) ambiguity_detection: AmbiguityDetection,
 }
 
 impl SystemDescriptor {
@@ -53,6 +64,7 @@ impl SystemDescriptor {
             run_criteria: None,
             before: Vec::new(),
             after: Vec::new(),
+            ambiguity_detection: Default::default(),
         }
     }
 }
@@ -75,6 +87,15 @@ pub trait IntoSystemDescriptor<Params> {
     /// Specifies that the system should run after systems with the given label.
     fn after<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor;
 
+    /// Marks this system as ambiguous with any system with the specified label.
+    /// This means that execution order between these systems does not matter,
+    /// which allows [some warnings](crate::schedule::ReportExecutionOrderAmbiguities) to be silenced.
+    fn ambiguous_with<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor;
+
+    /// Specifies that this system should opt out of
+    /// [execution order ambiguity detection](crate::schedule::ReportExecutionOrderAmbiguities).
+    fn ignore_all_ambiguities(self) -> SystemDescriptor;
+
     /// Specifies that the system should run with other exclusive systems at the start of stage.
     fn at_start(self) -> SystemDescriptor;
 
@@ -110,6 +131,26 @@ impl IntoSystemDescriptor<()> for SystemDescriptor {
         self
     }
 
+    fn ambiguous_with<Marker>(mut self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor {
+        match &mut self.ambiguity_detection {
+            detection @ AmbiguityDetection::Check => {
+                *detection =
+                    AmbiguityDetection::IgnoreWithLabel(vec![label.as_system_label().as_label()]);
+            }
+            AmbiguityDetection::IgnoreWithLabel(labels) => {
+                labels.push(label.as_system_label().as_label());
+            }
+            // This descriptor is already ambiguous with everything.
+            AmbiguityDetection::IgnoreAll => {}
+        }
+        self
+    }
+
+    fn ignore_all_ambiguities(mut self) -> SystemDescriptor {
+        self.ambiguity_detection = AmbiguityDetection::IgnoreAll;
+        self
+    }
+
     fn at_start(mut self) -> SystemDescriptor {
         self.exclusive_insertion_point = Some(ExclusiveInsertionPoint::AtStart);
         self
@@ -154,6 +195,14 @@ where
         SystemDescriptor::new(Box::new(IntoSystem::into_system(self))).after(label)
     }
 
+    fn ambiguous_with<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor {
+        SystemDescriptor::new(Box::new(IntoSystem::into_system(self))).ambiguous_with(label)
+    }
+
+    fn ignore_all_ambiguities(self) -> SystemDescriptor {
+        SystemDescriptor::new(Box::new(IntoSystem::into_system(self))).ignore_all_ambiguities()
+    }
+
     fn at_start(self) -> SystemDescriptor {
         SystemDescriptor::new(Box::new(IntoSystem::into_system(self))).at_start()
     }
@@ -191,6 +240,14 @@ impl IntoSystemDescriptor<()> for BoxedSystem<(), ()> {
         SystemDescriptor::new(self).after(label)
     }
 
+    fn ambiguous_with<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor {
+        SystemDescriptor::new(self).ambiguous_with(label)
+    }
+
+    fn ignore_all_ambiguities(self) -> SystemDescriptor {
+        SystemDescriptor::new(self).ignore_all_ambiguities()
+    }
+
     fn at_start(self) -> SystemDescriptor {
         SystemDescriptor::new(self).at_start()
     }