Adding Debug implementations for App, Stage, Schedule, Query, QueryState, etc. (#6214)

# Objective

- Adding Debug implementations for App, Stage, Schedule, Query, QueryState.
- Fixes #1130.

## Solution

- Implemented std::fmt::Debug for a number of structures.

---

## Changelog

Also added Debug implementations for ParallelSystemExecutor, SingleThreadedExecutor, various RunCriteria structures, SystemContainer, and SystemDescriptor.

Opinions are sure to differ as to what information to provide in a Debug implementation.  Best guess was taken for this initial version for these structures.


Co-authored-by: targrub <62773321+targrub@users.noreply.github.com>

crates/bevy_app/src/app.rs

@@ -70,12 +70,32 @@ pub struct App {
     sub_apps: HashMap<AppLabelId, SubApp>,
 }
 
+impl Debug for App {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "App {{ sub_apps: ")?;
+        f.debug_map()
+            .entries(self.sub_apps.iter().map(|(k, v)| (k, v)))
+            .finish()?;
+        write!(f, "}}")
+    }
+}
+
 /// Each `SubApp` has its own [`Schedule`] and [`World`], enabling a separation of concerns.
 struct SubApp {
     app: App,
     runner: Box<dyn Fn(&mut World, &mut App)>,
 }
 
+impl Debug for SubApp {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "SubApp {{ app: ")?;
+        f.debug_map()
+            .entries(self.app.sub_apps.iter().map(|(k, v)| (k, v)))
+            .finish()?;
+        write!(f, "}}")
+    }
+}
+
 impl Default for App {
     fn default() -> Self {
         let mut app = App::empty();

crates/bevy_ecs/src/query/state.rs

@@ -35,6 +35,17 @@ pub struct QueryState<Q: WorldQuery, F: ReadOnlyWorldQuery = ()> {
     pub(crate) filter_state: F::State,
 }
 
+impl<Q: WorldQuery, F: ReadOnlyWorldQuery> std::fmt::Debug for QueryState<Q, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "QueryState<Q, F> matched_table_ids: {} matched_archetype_ids: {}",
+            self.matched_table_ids.len(),
+            self.matched_archetype_ids.len()
+        )
+    }
+}
+
 impl<Q: WorldQuery, F: ReadOnlyWorldQuery> FromWorld for QueryState<Q, F> {
     fn from_world(world: &mut World) -> Self {
         world.query_filtered()

crates/bevy_ecs/src/schedule/executor.rs

@@ -1,4 +1,5 @@
 use crate::{schedule::SystemContainer, world::World};
+use core::fmt::Debug;
 use downcast_rs::{impl_downcast, Downcast};
 
 pub trait ParallelSystemExecutor: Downcast + Send + Sync {
@@ -8,9 +9,15 @@ pub trait ParallelSystemExecutor: Downcast + Send + Sync {
     fn run_systems(&mut self, systems: &mut [SystemContainer], world: &mut World);
 }
 
+impl Debug for dyn ParallelSystemExecutor {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "dyn ParallelSystemExecutor")
+    }
+}
+
 impl_downcast!(ParallelSystemExecutor);
 
-#[derive(Default)]
+#[derive(Debug, Default)]
 pub struct SingleThreadedExecutor;
 
 impl ParallelSystemExecutor for SingleThreadedExecutor {

crates/bevy_ecs/src/schedule/mod.rs

@@ -37,7 +37,7 @@ use bevy_utils::HashMap;
 /// In this way, the properties of the child schedule can be set differently from the parent.
 /// For example, it can be set to run only once during app execution, while the parent schedule
 /// runs indefinitely.
-#[derive(Default)]
+#[derive(Debug, Default)]
 pub struct Schedule {
     stages: HashMap<StageLabelId, Box<dyn Stage>>,
     stage_order: Vec<StageLabelId>,

crates/bevy_ecs/src/schedule/run_criteria.rs

@@ -1,8 +1,10 @@
 use crate::{
+    prelude::System,
     schedule::{GraphNode, RunCriteriaLabel, RunCriteriaLabelId},
     system::{BoxedSystem, IntoSystem, Local},
     world::World,
 };
+use core::fmt::Debug;
 use std::borrow::Cow;
 
 /// Determines whether a system should be executed or not, and how many times it should be ran each
@@ -66,7 +68,7 @@ impl From<bool> for ShouldRun {
     }
 }
 
-#[derive(Default)]
+#[derive(Debug, Default)]
 pub(crate) struct BoxedRunCriteria {
     criteria_system: Option<BoxedSystem<(), ShouldRun>>,
     initialized: bool,
@@ -93,6 +95,7 @@ impl BoxedRunCriteria {
     }
 }
 
+#[derive(Debug)]
 pub(crate) enum RunCriteriaInner {
     Single(BoxedSystem<(), ShouldRun>),
     Piped {
@@ -101,6 +104,7 @@ pub(crate) enum RunCriteriaInner {
     },
 }
 
+#[derive(Debug)]
 pub(crate) struct RunCriteriaContainer {
     pub(crate) should_run: ShouldRun,
     pub(crate) inner: RunCriteriaInner,
@@ -165,17 +169,19 @@ impl GraphNode for RunCriteriaContainer {
     }
 }
 
+#[derive(Debug)]
 pub enum RunCriteriaDescriptorOrLabel {
     Descriptor(RunCriteriaDescriptor),
     Label(RunCriteriaLabelId),
 }
 
-#[derive(Clone, Copy)]
+#[derive(Debug, Clone, Copy)]
 pub(crate) enum DuplicateLabelStrategy {
     Panic,
     Discard,
 }
 
+#[derive(Debug)]
 pub struct RunCriteriaDescriptor {
     pub(crate) system: RunCriteriaSystem,
     pub(crate) label: Option<RunCriteriaLabelId>,
@@ -184,6 +190,7 @@ pub struct RunCriteriaDescriptor {
     pub(crate) after: Vec<RunCriteriaLabelId>,
 }
 
+#[derive(Debug)]
 pub(crate) enum RunCriteriaSystem {
     Single(BoxedSystem<(), ShouldRun>),
     Piped(BoxedSystem<ShouldRun, ShouldRun>),
@@ -326,6 +333,7 @@ where
     }
 }
 
+#[derive(Debug)]
 pub struct RunCriteria {
     label: RunCriteriaLabelId,
 }
@@ -346,3 +354,49 @@ impl RunCriteria {
         }
     }
 }
+
+impl Debug for dyn System<In = (), Out = ShouldRun> + 'static {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "System {} with In=(), Out=ShouldRun: {{{}}}",
+            self.name(),
+            {
+                if self.is_send() {
+                    if self.is_exclusive() {
+                        "is_send is_exclusive"
+                    } else {
+                        "is_send"
+                    }
+                } else if self.is_exclusive() {
+                    "is_exclusive"
+                } else {
+                    ""
+                }
+            },
+        )
+    }
+}
+
+impl Debug for dyn System<In = ShouldRun, Out = ShouldRun> + 'static {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "System {} with In=ShouldRun, Out=ShouldRun: {{{}}}",
+            self.name(),
+            {
+                if self.is_send() {
+                    if self.is_exclusive() {
+                        "is_send is_exclusive"
+                    } else {
+                        "is_send"
+                    }
+                } else if self.is_exclusive() {
+                    "is_exclusive"
+                } else {
+                    ""
+                }
+            },
+        )
+    }
+}

crates/bevy_ecs/src/schedule/stage.rs

@@ -14,9 +14,10 @@ use crate::{
 };
 use bevy_ecs_macros::Resource;
 use bevy_utils::{tracing::warn, HashMap, HashSet};
+use core::fmt::Debug;
 use downcast_rs::{impl_downcast, Downcast};
 
-use super::IntoSystemDescriptor;
+use super::{IntoSystemDescriptor, Schedule};
 
 /// A type that can run as a step of a [`Schedule`](super::Schedule).
 pub trait Stage: Downcast + Send + Sync {
@@ -25,6 +26,18 @@ pub trait Stage: Downcast + Send + Sync {
     fn run(&mut self, world: &mut World);
 }
 
+impl Debug for dyn Stage {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        if let Some(as_systemstage) = self.as_any().downcast_ref::<SystemStage>() {
+            write!(f, "{:?}", as_systemstage)
+        } else if let Some(as_schedule) = self.as_any().downcast_ref::<Schedule>() {
+            write!(f, "{:?}", as_schedule)
+        } else {
+            write!(f, "Unknown dyn Stage")
+        }
+    }
+}
+
 impl_downcast!(Stage);
 
 /// When this resource is present in the `App`'s `Resources`,
@@ -584,6 +597,64 @@ impl SystemStage {
         }
         Ok(labels)
     }
+
+    pub fn vec_system_container_debug(
+        &self,
+        name: &str,
+        v: &Vec<SystemContainer>,
+        f: &mut std::fmt::Formatter<'_>,
+    ) -> std::fmt::Result {
+        write!(f, "{}: ", name)?;
+        if v.len() > 1 {
+            writeln!(f, "[")?;
+            for sc in v.iter() {
+                writeln!(f, "{:?},", sc)?;
+            }
+            write!(f, "], ")
+        } else {
+            write!(f, "{:?}, ", v)
+        }
+    }
+}
+
+impl std::fmt::Debug for SystemStage {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "SystemStage: {{ ")?;
+        write!(
+            f,
+            "world_id: {:?}, executor: {:?}, stage_run_criteria: {:?}, run_criteria: {:?}, ",
+            self.world_id, self.executor, self.stage_run_criteria, self.run_criteria
+        )?;
+        self.vec_system_container_debug("exclusive_at_start", &self.exclusive_at_start, f)?;
+        self.vec_system_container_debug(
+            "exclusive_before_commands",
+            &self.exclusive_before_commands,
+            f,
+        )?;
+        self.vec_system_container_debug("exclusive_at_end", &self.exclusive_at_end, f)?;
+        self.vec_system_container_debug("parallel", &self.parallel, f)?;
+        write!(
+            f,
+            "systems_modified: {:?}, uninitialized_run_criteria: {:?}, ",
+            self.systems_modified, self.uninitialized_run_criteria
+        )?;
+        write!(
+            f,
+            "uninitialized_at_start: {:?}, uninitialized_before_commands: {:?}, ",
+            self.uninitialized_at_start, self.uninitialized_before_commands
+        )?;
+        write!(
+            f,
+            "uninitialized_at_end: {:?}, uninitialized_parallel: {:?}, ",
+            self.uninitialized_at_end, self.uninitialized_parallel
+        )?;
+        write!(
+            f,
+            "last_tick_check: {:?}, apply_buffers: {:?}, ",
+            self.last_tick_check, self.apply_buffers
+        )?;
+        write!(f, "must_read_resource: {:?}}}", self.must_read_resource)
+    }
 }
 
 /// Sorts given system containers topologically, populates their resolved dependencies

crates/bevy_ecs/src/schedule/system_container.rs

@@ -6,6 +6,7 @@ use crate::{
     },
     system::System,
 };
+use core::fmt::Debug;
 use std::borrow::Cow;
 
 pub struct SystemContainer {
@@ -83,6 +84,12 @@ impl SystemContainer {
     }
 }
 
+impl Debug for SystemContainer {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{{{:?}}}", &self.system())
+    }
+}
+
 impl GraphNode for SystemContainer {
     type Label = SystemLabelId;
 

crates/bevy_ecs/src/schedule/system_descriptor.rs

@@ -4,7 +4,7 @@ use crate::{
 };
 
 /// Configures ambiguity detection for a single system.
-#[derive(Default)]
+#[derive(Debug, Default)]
 pub(crate) enum AmbiguityDetection {
     #[default]
     Check,
@@ -41,6 +41,7 @@ pub(crate) enum AmbiguityDetection {
 ///     .with_system(do_the_other_thing.after(Something))
 ///     .with_system(do_something_else.at_end());
 /// ```
+#[derive(Debug)]
 pub struct SystemDescriptor {
     pub(crate) system: BoxedSystem<(), ()>,
     pub(crate) exclusive_insertion_point: Option<ExclusiveInsertionPoint>,

crates/bevy_ecs/src/system/query.rs

@@ -280,6 +280,12 @@ pub struct Query<'world, 'state, Q: WorldQuery, F: ReadOnlyWorldQuery = ()> {
     pub(crate) change_tick: u32,
 }
 
+impl<'w, 's, Q: WorldQuery, F: ReadOnlyWorldQuery> std::fmt::Debug for Query<'w, 's, Q, F> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Query {{ matched entities: {}, world: {:?}, state: {:?}, last_change_tick: {}, change_tick: {} }}", self.iter().count(), self.world, self.state, self.last_change_tick, self.change_tick)
+    }
+}
+
 impl<'w, 's, Q: WorldQuery, F: ReadOnlyWorldQuery> Query<'w, 's, Q, F> {
     /// Creates a new query.
     ///

crates/bevy_ecs/src/system/system.rs

@@ -1,4 +1,5 @@
 use bevy_utils::tracing::warn;
+use core::fmt::Debug;
 
 use crate::{
     archetype::ArchetypeComponentId, change_detection::MAX_CHANGE_AGE, component::ComponentId,
@@ -95,3 +96,21 @@ pub(crate) fn check_system_change_tick(
         *last_change_tick = change_tick.wrapping_sub(MAX_CHANGE_AGE);
     }
 }
+
+impl Debug for dyn System<In = (), Out = ()> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "System {}: {{{}}}", self.name(), {
+            if self.is_send() {
+                if self.is_exclusive() {
+                    "is_send is_exclusive"
+                } else {
+                    "is_send"
+                }
+            } else if self.is_exclusive() {
+                "is_exclusive"
+            } else {
+                ""
+            }
+        },)
+    }
+}