Unify system state (#19506)

# Objective

- A preparation for the 'system as entities'
- The current system has a series of states such as `is_send`,
`is_exclusive`, `has_defered`, As `system as entites` landed, it may
have more states. Using Bitflags to unify all states is a more concise
and performant approach

## Solution

- Using Bitflags to  unify system state.

crates/bevy_ecs/src/schedule/executor/mod.rs

@@ -20,7 +20,7 @@ use crate::{
     prelude::{IntoSystemSet, SystemSet},
     query::{Access, FilteredAccessSet},
     schedule::{BoxedCondition, InternedSystemSet, NodeId, SystemTypeSet},
-    system::{ScheduleSystem, System, SystemIn, SystemParamValidationError},
+    system::{ScheduleSystem, System, SystemIn, SystemParamValidationError, SystemStateFlags},
     world::{unsafe_world_cell::UnsafeWorldCell, DeferredWorld, World},
 };
 
@@ -171,26 +171,9 @@ impl System for ApplyDeferred {
         const { &FilteredAccessSet::new() }
     }
 
-    fn is_send(&self) -> bool {
+    fn flags(&self) -> SystemStateFlags {
-        // Although this system itself does nothing on its own, the system
+        // non-send , exclusive , no deferred
-        // executor uses it to apply deferred commands. Commands must be allowed
+        SystemStateFlags::NON_SEND | SystemStateFlags::EXCLUSIVE
-        // to access non-send resources, so this system must be non-send for
-        // scheduling purposes.
-        false
-    }
-
-    fn is_exclusive(&self) -> bool {
-        // This system is labeled exclusive because it is used by the system
-        // executor to find places where deferred commands should be applied,
-        // and commands can only be applied with exclusive access to the world.
-        true
-    }
-
-    fn has_deferred(&self) -> bool {
-        // This system itself doesn't have any commands to apply, but when it
-        // is pulled from the schedule to be ran, the executor will apply
-        // deferred commands from other systems.
-        false
     }
 
     unsafe fn run_unsafe(

crates/bevy_ecs/src/system/adapter_system.rs

@@ -137,16 +137,9 @@ where
         self.system.component_access_set()
     }
 
-    fn is_send(&self) -> bool {
+    #[inline]
-        self.system.is_send()
+    fn flags(&self) -> super::SystemStateFlags {
-    }
+        self.system.flags()
-
-    fn is_exclusive(&self) -> bool {
-        self.system.is_exclusive()
-    }
-
-    fn has_deferred(&self) -> bool {
-        self.system.has_deferred()
     }
 
     #[inline]

crates/bevy_ecs/src/system/combinator.rs

@@ -152,16 +152,9 @@ where
         &self.component_access_set
     }
 
-    fn is_send(&self) -> bool {
+    #[inline]
-        self.a.is_send() && self.b.is_send()
+    fn flags(&self) -> super::SystemStateFlags {
-    }
+        self.a.flags() | self.b.flags()
-
-    fn is_exclusive(&self) -> bool {
-        self.a.is_exclusive() || self.b.is_exclusive()
-    }
-
-    fn has_deferred(&self) -> bool {
-        self.a.has_deferred() || self.b.has_deferred()
     }
 
     unsafe fn run_unsafe(
@@ -378,16 +371,9 @@ where
         &self.component_access_set
     }
 
-    fn is_send(&self) -> bool {
+    #[inline]
-        self.a.is_send() && self.b.is_send()
+    fn flags(&self) -> super::SystemStateFlags {
-    }
+        self.a.flags() | self.b.flags()
-
-    fn is_exclusive(&self) -> bool {
-        self.a.is_exclusive() || self.b.is_exclusive()
-    }
-
-    fn has_deferred(&self) -> bool {
-        self.a.has_deferred() || self.b.has_deferred()
     }
 
     unsafe fn run_unsafe(

crates/bevy_ecs/src/system/exclusive_function_system.rs

@@ -13,7 +13,7 @@ use alloc::{borrow::Cow, vec, vec::Vec};
 use core::marker::PhantomData;
 use variadics_please::all_tuples;
 
-use super::SystemParamValidationError;
+use super::{SystemParamValidationError, SystemStateFlags};
 
 /// A function system that runs with exclusive [`World`] access.
 ///
@@ -98,22 +98,12 @@ where
     }
 
     #[inline]
-    fn is_send(&self) -> bool {
+    fn flags(&self) -> SystemStateFlags {
-        // exclusive systems should have access to non-send resources
+        // non-send , exclusive , no deferred
         // the executor runs exclusive systems on the main thread, so this
         // field reflects that constraint
-        false
-    }
-
-    #[inline]
-    fn is_exclusive(&self) -> bool {
-        true
-    }
-
-    #[inline]
-    fn has_deferred(&self) -> bool {
         // exclusive systems have no deferred system params
-        false
+        SystemStateFlags::NON_SEND | SystemStateFlags::EXCLUSIVE
     }
 
     #[inline]

crates/bevy_ecs/src/system/function_system.rs

@@ -17,7 +17,9 @@ use variadics_please::all_tuples;
 #[cfg(feature = "trace")]
 use tracing::{info_span, Span};
 
-use super::{IntoSystem, ReadOnlySystem, SystemParamBuilder, SystemParamValidationError};
+use super::{
+    IntoSystem, ReadOnlySystem, SystemParamBuilder, SystemParamValidationError, SystemStateFlags,
+};
 
 /// The metadata of a [`System`].
 #[derive(Clone)]
@@ -29,8 +31,7 @@ pub struct SystemMeta {
     pub(crate) component_access_set: FilteredAccessSet<ComponentId>,
     // NOTE: this must be kept private. making a SystemMeta non-send is irreversible to prevent
     // SystemParams from overriding each other
-    is_send: bool,
+    flags: SystemStateFlags,
-    has_deferred: bool,
     pub(crate) last_run: Tick,
     #[cfg(feature = "trace")]
     pub(crate) system_span: Span,
@@ -44,8 +45,7 @@ impl SystemMeta {
         Self {
             name: name.into(),
             component_access_set: FilteredAccessSet::default(),
-            is_send: true,
+            flags: SystemStateFlags::empty(),
-            has_deferred: false,
             last_run: Tick::new(0),
             #[cfg(feature = "trace")]
             system_span: info_span!("system", name = name),
@@ -78,7 +78,7 @@ impl SystemMeta {
     /// Returns true if the system is [`Send`].
     #[inline]
     pub fn is_send(&self) -> bool {
-        self.is_send
+        !self.flags.intersects(SystemStateFlags::NON_SEND)
     }
 
     /// Sets the system to be not [`Send`].
@@ -86,20 +86,20 @@ impl SystemMeta {
     /// This is irreversible.
     #[inline]
     pub fn set_non_send(&mut self) {
-        self.is_send = false;
+        self.flags |= SystemStateFlags::NON_SEND;
     }
 
     /// Returns true if the system has deferred [`SystemParam`]'s
     #[inline]
     pub fn has_deferred(&self) -> bool {
-        self.has_deferred
+        self.flags.intersects(SystemStateFlags::DEFERRED)
     }
 
     /// Marks the system as having deferred buffers like [`Commands`](`super::Commands`)
     /// This lets the scheduler insert [`ApplyDeferred`](`crate::prelude::ApplyDeferred`) systems automatically.
     #[inline]
     pub fn set_has_deferred(&mut self) {
-        self.has_deferred = true;
+        self.flags |= SystemStateFlags::DEFERRED;
     }
 
     /// Returns a reference to the [`FilteredAccessSet`] for [`ComponentId`].
@@ -631,18 +631,8 @@ where
     }
 
     #[inline]
-    fn is_send(&self) -> bool {
+    fn flags(&self) -> SystemStateFlags {
-        self.system_meta.is_send
+        self.system_meta.flags
-    }
-
-    #[inline]
-    fn is_exclusive(&self) -> bool {
-        false
-    }
-
-    #[inline]
-    fn has_deferred(&self) -> bool {
-        self.system_meta.has_deferred
     }
 
     #[inline]

crates/bevy_ecs/src/system/observer_system.rs

@@ -127,18 +127,8 @@ where
     }
 
     #[inline]
-    fn is_send(&self) -> bool {
+    fn flags(&self) -> super::SystemStateFlags {
-        self.observer.is_send()
+        self.observer.flags()
-    }
-
-    #[inline]
-    fn is_exclusive(&self) -> bool {
-        self.observer.is_exclusive()
-    }
-
-    #[inline]
-    fn has_deferred(&self) -> bool {
-        self.observer.has_deferred()
     }
 
     #[inline]

crates/bevy_ecs/src/system/schedule_system.rs

@@ -8,7 +8,7 @@ use crate::{
     world::{unsafe_world_cell::UnsafeWorldCell, DeferredWorld, FromWorld, World},
 };
 
-use super::{IntoSystem, SystemParamValidationError};
+use super::{IntoSystem, SystemParamValidationError, SystemStateFlags};
 
 /// A wrapper system to change a system that returns `()` to return `Ok(())` to make it into a [`ScheduleSystem`]
 pub struct InfallibleSystemWrapper<S: System<In = ()>>(S);
@@ -44,18 +44,8 @@ impl<S: System<In = ()>> System for InfallibleSystemWrapper<S> {
     }
 
     #[inline]
-    fn is_send(&self) -> bool {
+    fn flags(&self) -> SystemStateFlags {
-        self.0.is_send()
+        self.0.flags()
-    }
-
-    #[inline]
-    fn is_exclusive(&self) -> bool {
-        self.0.is_exclusive()
-    }
-
-    #[inline]
-    fn has_deferred(&self) -> bool {
-        self.0.has_deferred()
     }
 
     #[inline]
@@ -172,16 +162,9 @@ where
         self.system.component_access_set()
     }
 
-    fn is_send(&self) -> bool {
+    #[inline]
-        self.system.is_send()
+    fn flags(&self) -> SystemStateFlags {
-    }
+        self.system.flags()
-
-    fn is_exclusive(&self) -> bool {
-        self.system.is_exclusive()
-    }
-
-    fn has_deferred(&self) -> bool {
-        self.system.has_deferred()
     }
 
     unsafe fn run_unsafe(
@@ -281,16 +264,9 @@ where
         self.system.component_access_set()
     }
 
-    fn is_send(&self) -> bool {
+    #[inline]
-        self.system.is_send()
+    fn flags(&self) -> SystemStateFlags {
-    }
+        self.system.flags()
-
-    fn is_exclusive(&self) -> bool {
-        self.system.is_exclusive()
-    }
-
-    fn has_deferred(&self) -> bool {
-        self.system.has_deferred()
     }
 
     unsafe fn run_unsafe(

crates/bevy_ecs/src/system/system.rs

@@ -2,6 +2,7 @@
     clippy::module_inception,
     reason = "This instance of module inception is being discussed; see #17353."
 )]
+use bitflags::bitflags;
 use core::fmt::Debug;
 use log::warn;
 use thiserror::Error;
@@ -19,6 +20,18 @@ use core::any::TypeId;
 
 use super::{IntoSystem, SystemParamValidationError};
 
+bitflags! {
+    /// Bitflags representing system states and requirements.
+    #[derive(Clone, Copy, PartialEq, Eq, Hash)]
+    pub struct SystemStateFlags: u8 {
+        /// Set if system cannot be sent across threads
+        const NON_SEND       = 1 << 0;
+        /// Set if system requires exclusive World access
+        const EXCLUSIVE      = 1 << 1;
+        /// Set if system has deferred buffers.
+        const DEFERRED       = 1 << 2;
+    }
+}
 /// An ECS system that can be added to a [`Schedule`](crate::schedule::Schedule)
 ///
 /// Systems are functions with all arguments implementing
@@ -50,14 +63,26 @@ pub trait System: Send + Sync + 'static {
     /// Returns the system's component [`FilteredAccessSet`].
     fn component_access_set(&self) -> &FilteredAccessSet<ComponentId>;
 
+    /// Returns the [`SystemStateFlags`] of the system.
+    fn flags(&self) -> SystemStateFlags;
+
     /// Returns true if the system is [`Send`].
-    fn is_send(&self) -> bool;
+    #[inline]
+    fn is_send(&self) -> bool {
+        !self.flags().intersects(SystemStateFlags::NON_SEND)
+    }
 
     /// Returns true if the system must be run exclusively.
-    fn is_exclusive(&self) -> bool;
+    #[inline]
+    fn is_exclusive(&self) -> bool {
+        self.flags().intersects(SystemStateFlags::EXCLUSIVE)
+    }
 
     /// Returns true if system has deferred buffers.
-    fn has_deferred(&self) -> bool;
+    #[inline]
+    fn has_deferred(&self) -> bool {
+        self.flags().intersects(SystemStateFlags::DEFERRED)
+    }
 
     /// Runs the system with the given input in the world. Unlike [`System::run`], this function
     /// can be called in parallel with other systems and may break Rust's aliasing rules

release-content/migration-guides/unified_system_state_flag.md

@@ -0,0 +1,43 @@
+---
+title: Unified system state flag
+pull_requests: [19506]
+---
+
+Now the system have a unified `SystemStateFlags` to represent its different states.
+
+If your code previously looked like this:
+
+```rust
+impl System for MyCustomSystem {
+    // ...
+
+    fn is_send(&self) -> bool {
+        false
+    }
+
+    fn is_exclusive(&self) -> bool {
+        true
+    }
+
+    fn has_deferred(&self) -> bool {
+        false
+    }
+
+    // ....
+}
+```
+
+You should migrate it to:
+
+```rust
+impl System for MyCustomSystem{
+  // ...
+
+  fn flags(&self) -> SystemStateFlags {
+    // non-send , exclusive , no deferred
+    SystemStateFlags::NON_SEND | SystemStateFlags::EXCLUSIVE
+  }
+
+  // ...
+}
+```