From dc8fefe27fc8ab6d756d8c0291515280923fffe2 Mon Sep 17 00:00:00 2001
From: James Liu <contact@jamessliu.com>
Date: Sun, 2 Jan 2022 20:36:40 +0000
Subject: [PATCH] Remove Bytes, FromBytes, Labels, EntityLabels. Document rest
 of bevy_core and enable warning on missing docs. (#3521)

This PR is part of the issue #3492.

# Objective

 - Clean up dead code in `bevy_core`.
 - Add and update the `bevy_core` documentation to achieve a 100% documentation coverage.
 - Add the #![warn(missing_docs)] lint to keep the documentation coverage for the future.

# Solution

 - Remove unused `Bytes`, `FromBytes`, `Labels`, and `EntityLabels` types and associated systems.
 - Made several types private that really only have use as internal types, mostly pertaining to fixed timestep execution.
 - Add and update the bevy_core documentation.
 - Add the #![warn(missing_docs)] lint.

# Open Questions

Should more of the internal states of `FixedTimestep` be public? Seems mostly to be an implementation detail unless someone really needs that fixed timestep state.
---
 crates/bevy_core/src/bytes.rs               | 103 --------
 crates/bevy_core/src/float_ord.rs           |   7 +-
 crates/bevy_core/src/label.rs               | 256 --------------------
 crates/bevy_core/src/lib.rs                 |  18 +-
 crates/bevy_core/src/name.rs                |  16 ++
 crates/bevy_core/src/time/fixed_timestep.rs |  39 ++-
 crates/bevy_core/src/time/stopwatch.rs      |  19 ++
 crates/bevy_core/src/time/time.rs           |   4 +-
 crates/bevy_derive/src/bytes.rs             |  41 ----
 crates/bevy_derive/src/lib.rs               |   7 -
 crates/bevy_derive/src/modules.rs           |   1 -
 crates/bevy_render/src/color/mod.rs         |  53 ----
 12 files changed, 80 insertions(+), 484 deletions(-)
 delete mode 100644 crates/bevy_core/src/bytes.rs
 delete mode 100644 crates/bevy_core/src/label.rs
 delete mode 100644 crates/bevy_derive/src/bytes.rs

diff --git a/crates/bevy_core/src/bytes.rs b/crates/bevy_core/src/bytes.rs
deleted file mode 100644
index 39628fc82531b..0000000000000
--- a/crates/bevy_core/src/bytes.rs
+++ /dev/null
@@ -1,103 +0,0 @@
-pub use bevy_derive::Bytes;
-
-// NOTE: we can reexport common traits and methods from bytemuck to avoid requiring dependency most of
-// the time, but unfortunately we can't use derive macros that way due to hardcoded path in generated code.
-pub use bytemuck::{bytes_of, cast_slice, Pod, Zeroable};
-
-// FIXME: `Bytes` trait doesn't specify the expected encoding format,
-// which means types that implement it have to know what format is expected
-// and can only implement one encoding at a time.
-// TODO: Remove `Bytes` and `FromBytes` in favour of `crevice` crate.
-
-/// Converts the implementing type to bytes by writing them to a given buffer
-pub trait Bytes {
-    /// Converts the implementing type to bytes by writing them to a given buffer
-    fn write_bytes(&self, buffer: &mut [u8]);
-
-    /// The number of bytes that will be written when calling `write_bytes`
-    fn byte_len(&self) -> usize;
-}
-
-impl<T> Bytes for T
-where
-    T: Pod,
-{
-    fn write_bytes(&self, buffer: &mut [u8]) {
-        buffer[0..self.byte_len()].copy_from_slice(bytes_of(self))
-    }
-
-    fn byte_len(&self) -> usize {
-        std::mem::size_of::<Self>()
-    }
-}
-
-/// Converts a byte array to `Self`
-pub trait FromBytes {
-    /// Converts a byte array to `Self`
-    fn from_bytes(bytes: &[u8]) -> Self;
-}
-
-impl<T> FromBytes for T
-where
-    T: Pod,
-{
-    fn from_bytes(bytes: &[u8]) -> Self {
-        assert_eq!(
-            bytes.len(),
-            std::mem::size_of::<T>(),
-            "Cannot convert byte slice `&[u8]` to type `{}`. They are not the same size.",
-            std::any::type_name::<T>()
-        );
-        unsafe { bytes.as_ptr().cast::<T>().read_unaligned() }
-    }
-}
-
-#[cfg(test)]
-mod tests {
-
-    use super::{Bytes, FromBytes};
-    use bevy_math::{Mat4, Vec2, Vec3, Vec4};
-
-    fn test_round_trip<T: Bytes + FromBytes + std::fmt::Debug + PartialEq>(value: T) {
-        let mut bytes = vec![0; value.byte_len()];
-        value.write_bytes(&mut bytes);
-        let result = T::from_bytes(&bytes);
-        assert_eq!(value, result);
-    }
-
-    #[test]
-    fn test_u32_bytes_round_trip() {
-        test_round_trip(123u32);
-    }
-
-    #[test]
-    fn test_f64_bytes_round_trip() {
-        test_round_trip(123f64);
-    }
-
-    #[test]
-    fn test_vec2_round_trip() {
-        test_round_trip(Vec2::new(1.0, 2.0));
-    }
-
-    #[test]
-    fn test_vec3_round_trip() {
-        test_round_trip(Vec3::new(1.0, 2.0, 3.0));
-    }
-
-    #[test]
-    fn test_vec4_round_trip() {
-        test_round_trip(Vec4::new(1.0, 2.0, 3.0, 4.0));
-    }
-
-    #[test]
-    fn test_mat4_round_trip() {
-        test_round_trip(Mat4::IDENTITY);
-    }
-
-    #[test]
-    fn test_array_round_trip() {
-        test_round_trip([-10i32; 1024]);
-        test_round_trip([Vec2::ZERO, Vec2::ONE, Vec2::Y, Vec2::X]);
-    }
-}
diff --git a/crates/bevy_core/src/float_ord.rs b/crates/bevy_core/src/float_ord.rs
index 798e0d9e3ac39..cd573ca5fc016 100644
--- a/crates/bevy_core/src/float_ord.rs
+++ b/crates/bevy_core/src/float_ord.rs
@@ -1,4 +1,3 @@
-use crate::bytes_of;
 use std::{
     cmp::Ordering,
     hash::{Hash, Hasher},
@@ -42,12 +41,12 @@ impl Hash for FloatOrd {
     fn hash<H: Hasher>(&self, state: &mut H) {
         if self.0.is_nan() {
             // Ensure all NaN representations hash to the same value
-            state.write(bytes_of(&f32::NAN))
+            state.write(bytemuck::bytes_of(&f32::NAN))
         } else if self.0 == 0.0 {
             // Ensure both zeroes hash to the same value
-            state.write(bytes_of(&0.0f32))
+            state.write(bytemuck::bytes_of(&0.0f32))
         } else {
-            state.write(bytes_of(&self.0));
+            state.write(bytemuck::bytes_of(&self.0));
         }
     }
 }
diff --git a/crates/bevy_core/src/label.rs b/crates/bevy_core/src/label.rs
deleted file mode 100644
index 4b805641ef770..0000000000000
--- a/crates/bevy_core/src/label.rs
+++ /dev/null
@@ -1,256 +0,0 @@
-use bevy_ecs::{
-    component::Component,
-    entity::Entity,
-    query::Changed,
-    reflect::ReflectComponent,
-    system::{Query, RemovedComponents, ResMut},
-};
-use bevy_reflect::Reflect;
-use bevy_utils::{HashMap, HashSet};
-use std::{
-    borrow::Cow,
-    fmt::Debug,
-    ops::{Deref, DerefMut},
-};
-
-/// A collection of labels
-#[derive(Component, Default, Reflect)]
-#[reflect(Component)]
-pub struct Labels {
-    labels: HashSet<Cow<'static, str>>,
-}
-
-impl Debug for Labels {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let mut list = f.debug_list();
-        for label in self.iter() {
-            list.entry(&label);
-        }
-
-        list.finish()
-    }
-}
-
-impl<'a, T, L: Into<Cow<'static, str>>> From<T> for Labels
-where
-    T: IntoIterator<Item = L>,
-{
-    fn from(value: T) -> Self {
-        let mut labels = HashSet::default();
-        for label in value {
-            labels.insert(label.into());
-        }
-        Self { labels }
-    }
-}
-
-impl Labels {
-    pub fn contains<T: Into<Cow<'static, str>>>(&self, label: T) -> bool {
-        self.labels.contains(&label.into())
-    }
-
-    pub fn insert<T: Into<Cow<'static, str>>>(&mut self, label: T) {
-        self.labels.insert(label.into());
-    }
-
-    pub fn remove<T: Into<Cow<'static, str>>>(&mut self, label: T) {
-        self.labels.remove(&label.into());
-    }
-
-    pub fn iter(&self) -> impl Iterator<Item = &str> {
-        self.labels.iter().map(|label| label.deref())
-    }
-}
-
-/// Maintains a mapping from [Entity](bevy_ecs::prelude::Entity) ids to entity labels and entity
-/// labels to [Entities](bevy_ecs::prelude::Entity).
-#[derive(Debug, Default)]
-pub struct EntityLabels {
-    label_entities: HashMap<Cow<'static, str>, Vec<Entity>>,
-    entity_labels: HashMap<Entity, HashSet<Cow<'static, str>>>,
-}
-
-impl EntityLabels {
-    pub fn get(&self, label: &str) -> &[Entity] {
-        self.label_entities
-            .get(label)
-            .map(|entities| entities.as_slice())
-            .unwrap_or(&[])
-    }
-}
-
-pub(crate) fn entity_labels_system(
-    mut entity_labels: ResMut<EntityLabels>,
-    removed_labels: RemovedComponents<Labels>,
-    query: Query<(Entity, &Labels), Changed<Labels>>,
-) {
-    let entity_labels = entity_labels.deref_mut();
-
-    for entity in removed_labels.iter() {
-        if let Some(labels) = entity_labels.entity_labels.get(&entity) {
-            for label in labels.iter() {
-                if let Some(entities) = entity_labels.label_entities.get_mut(label) {
-                    entities.retain(|e| *e != entity);
-                }
-            }
-        }
-    }
-
-    for (entity, labels) in query.iter() {
-        let current_labels = entity_labels
-            .entity_labels
-            .entry(entity)
-            .or_insert_with(HashSet::default);
-
-        for removed_label in current_labels.difference(&labels.labels) {
-            if let Some(entities) = entity_labels.label_entities.get_mut(removed_label) {
-                entities.retain(|e| *e != entity);
-            }
-        }
-
-        for added_label in labels.labels.difference(current_labels) {
-            entity_labels
-                .label_entities
-                .entry(added_label.clone())
-                .or_insert_with(Vec::new)
-                .push(entity);
-        }
-
-        *current_labels = labels.labels.clone();
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use bevy_ecs::{
-        schedule::{Schedule, Stage, SystemStage},
-        world::World,
-    };
-
-    use super::*;
-
-    fn setup() -> (World, Schedule) {
-        let mut world = World::new();
-        world.insert_resource(EntityLabels::default());
-        let mut schedule = Schedule::default();
-        schedule.add_stage("test", SystemStage::single_threaded());
-        schedule.add_system_to_stage("test", entity_labels_system);
-        (world, schedule)
-    }
-
-    fn holy_cow() -> Labels {
-        Labels::from(["holy", "cow"].iter().cloned())
-    }
-
-    fn holy_shamoni() -> Labels {
-        Labels::from(["holy", "shamoni"].iter().cloned())
-    }
-
-    #[test]
-    fn adds_spawned_entity() {
-        let (mut world, mut schedule) = setup();
-
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[e1], "holy");
-        assert_eq!(entity_labels.get("cow"), &[e1], "cow");
-        assert_eq!(entity_labels.get("shalau"), &[], "shalau");
-    }
-
-    #[test]
-    fn add_labels() {
-        let (mut world, mut schedule) = setup();
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        world.get_mut::<Labels>(e1).unwrap().insert("shalau");
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[e1], "holy");
-        assert_eq!(entity_labels.get("cow"), &[e1], "cow");
-        assert_eq!(entity_labels.get("shalau"), &[e1], "shalau");
-    }
-
-    #[test]
-    fn remove_labels() {
-        let (mut world, mut schedule) = setup();
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        world.get_mut::<Labels>(e1).unwrap().remove("holy");
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[], "holy");
-        assert_eq!(entity_labels.get("cow"), &[e1], "cow");
-        assert_eq!(entity_labels.get("shalau"), &[], "shalau");
-    }
-
-    #[test]
-    fn removes_despawned_entity() {
-        let (mut world, mut schedule) = setup();
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        assert!(world.despawn(e1));
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[], "holy");
-        assert_eq!(entity_labels.get("cow"), &[], "cow");
-        assert_eq!(entity_labels.get("shalau"), &[], "shalau");
-    }
-
-    #[test]
-    fn removes_labels_when_component_removed() {
-        let (mut world, mut schedule) = setup();
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        world.entity_mut(e1).remove::<Labels>().unwrap();
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[], "holy");
-        assert_eq!(entity_labels.get("cow"), &[], "cow");
-        assert_eq!(entity_labels.get("shalau"), &[], "shalau");
-    }
-
-    #[test]
-    fn adds_another_spawned_entity() {
-        let (mut world, mut schedule) = setup();
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        let e2 = world.spawn().insert(holy_shamoni()).id();
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[e1, e2], "holy");
-        assert_eq!(entity_labels.get("cow"), &[e1], "cow");
-        assert_eq!(entity_labels.get("shamoni"), &[e2], "shamoni");
-        assert_eq!(entity_labels.get("shalau"), &[], "shalau");
-    }
-
-    #[test]
-    fn removes_despawned_entity_but_leaves_other() {
-        let (mut world, mut schedule) = setup();
-        let e1 = world.spawn().insert(holy_cow()).id();
-        schedule.run(&mut world);
-
-        let e2 = world.spawn().insert(holy_shamoni()).id();
-        schedule.run(&mut world);
-
-        assert!(world.despawn(e1));
-        schedule.run(&mut world);
-
-        let entity_labels = world.get_resource::<EntityLabels>().unwrap();
-        assert_eq!(entity_labels.get("holy"), &[e2], "holy");
-        assert_eq!(entity_labels.get("cow"), &[], "cow");
-        assert_eq!(entity_labels.get("shamoni"), &[e2], "shamoni");
-        assert_eq!(entity_labels.get("shalau"), &[], "shalau");
-    }
-}
diff --git a/crates/bevy_core/src/lib.rs b/crates/bevy_core/src/lib.rs
index 2e8066d62f66a..f943b281426cf 100644
--- a/crates/bevy_core/src/lib.rs
+++ b/crates/bevy_core/src/lib.rs
@@ -1,20 +1,21 @@
-mod bytes;
+#![warn(missing_docs)]
+//! This crate provides core functionality for Bevy Engine.
+
 mod float_ord;
-mod label;
 mod name;
 mod task_pool_options;
 mod time;
 
-pub use bytes::*;
+pub use bytemuck::{bytes_of, cast_slice, Pod, Zeroable};
 pub use float_ord::*;
-pub use label::*;
 pub use name::*;
 pub use task_pool_options::DefaultTaskPoolOptions;
 pub use time::*;
 
 pub mod prelude {
+    //! The Bevy Core Prelude.
     #[doc(hidden)]
-    pub use crate::{DefaultTaskPoolOptions, EntityLabels, Labels, Name, Time, Timer};
+    pub use crate::{DefaultTaskPoolOptions, Name, Time, Timer};
 }
 
 use bevy_app::prelude::*;
@@ -30,6 +31,7 @@ use std::ops::Range;
 #[derive(Default)]
 pub struct CorePlugin;
 
+/// A `SystemLabel` enum for ordering systems relative to core Bevy systems.
 #[derive(Debug, PartialEq, Eq, Clone, Hash, SystemLabel)]
 pub enum CoreSystem {
     /// Updates the elapsed time. Any system that interacts with [Time] component should run after
@@ -47,13 +49,11 @@ impl Plugin for CorePlugin {
             .create_default_pools(&mut app.world);
 
         app.init_resource::<Time>()
-            .init_resource::<EntityLabels>()
             .init_resource::<FixedTimesteps>()
             .register_type::<HashSet<String>>()
             .register_type::<Option<String>>()
             .register_type::<Entity>()
             .register_type::<Name>()
-            .register_type::<Labels>()
             .register_type::<Range<f32>>()
             .register_type::<Timer>()
             // time system is added as an "exclusive system" to ensure it runs before other systems
@@ -61,9 +61,7 @@ impl Plugin for CorePlugin {
             .add_system_to_stage(
                 CoreStage::First,
                 time_system.exclusive_system().label(CoreSystem::Time),
-            )
-            .add_startup_system_to_stage(StartupStage::PostStartup, entity_labels_system)
-            .add_system_to_stage(CoreStage::PostUpdate, entity_labels_system);
+            );
 
         register_rust_types(app);
         register_math_types(app);
diff --git a/crates/bevy_core/src/name.rs b/crates/bevy_core/src/name.rs
index 593c7ae202baf..b61c505c92d8c 100644
--- a/crates/bevy_core/src/name.rs
+++ b/crates/bevy_core/src/name.rs
@@ -8,6 +8,11 @@ use std::{
 };
 
 /// Component used to identify an entity. Stores a hash for faster comparisons
+/// The hash is eagerly re-computed upon each update to the name.
+///
+/// [`Name`] should not be treated as a globally unique identifier for entities,
+/// as multiple entities can have the same name.  [`bevy_ecs::entity::Entity`] should be
+/// used instead as the default unique identifier.
 #[derive(Component, Debug, Clone, Reflect)]
 #[reflect(Component)]
 pub struct Name {
@@ -22,6 +27,9 @@ impl Default for Name {
 }
 
 impl Name {
+    /// Creates a new [`Name`] from any string-like type.
+    ///
+    /// The internal hash will be computed immediately.
     pub fn new(name: impl Into<Cow<'static, str>>) -> Self {
         let name = name.into();
         let mut name = Name { name, hash: 0 };
@@ -29,17 +37,25 @@ impl Name {
         name
     }
 
+    /// Sets the entity's name.
+    ///
+    /// The internal hash will be re-computed.
     #[inline(always)]
     pub fn set(&mut self, name: impl Into<Cow<'static, str>>) {
         *self = Name::new(name);
     }
 
+    /// Updates the name of the entity in place.
+    ///
+    /// This will allocate a new string if the name was previously
+    /// created from a borrow.
     #[inline(always)]
     pub fn mutate<F: FnOnce(&mut String)>(&mut self, f: F) {
         f(self.name.to_mut());
         self.update_hash();
     }
 
+    /// Gets the name of the entity as a `&str`.
     #[inline(always)]
     pub fn as_str(&self) -> &str {
         &self.name
diff --git a/crates/bevy_core/src/time/fixed_timestep.rs b/crates/bevy_core/src/time/fixed_timestep.rs
index 2143db7d4cc7a..efc2af50152ad 100644
--- a/crates/bevy_core/src/time/fixed_timestep.rs
+++ b/crates/bevy_core/src/time/fixed_timestep.rs
@@ -10,45 +10,66 @@ use bevy_ecs::{
 use bevy_utils::HashMap;
 use std::borrow::Cow;
 
+/// The internal state of each [`FixedTimestep`].
 #[derive(Debug)]
 pub struct FixedTimestepState {
-    pub step: f64,
-    pub accumulator: f64,
+    step: f64,
+    accumulator: f64,
 }
 
 impl FixedTimestepState {
-    /// The amount of time each step takes
+    /// The amount of time each step takes.
     pub fn step(&self) -> f64 {
         self.step
     }
 
-    /// The number of steps made in a second
+    /// The number of steps made in a second.
     pub fn steps_per_second(&self) -> f64 {
         1.0 / self.step
     }
 
-    /// The amount of time (in seconds) left over from the last step
+    /// The amount of time (in seconds) left over from the last step.
     pub fn accumulator(&self) -> f64 {
         self.accumulator
     }
 
-    /// The percentage of "step" stored inside the accumulator. Calculated as accumulator / step
+    /// The percentage of "step" stored inside the accumulator. Calculated as accumulator / step.
     pub fn overstep_percentage(&self) -> f64 {
         self.accumulator / self.step
     }
 }
 
+/// A global resource that tracks the individual [`FixedTimestepState`]s
+/// for every labeled [`FixedTimestep`].
 #[derive(Default)]
 pub struct FixedTimesteps {
     fixed_timesteps: HashMap<String, FixedTimestepState>,
 }
 
 impl FixedTimesteps {
+    /// Gets the [`FixedTimestepState`] for a given label.
     pub fn get(&self, name: &str) -> Option<&FixedTimestepState> {
         self.fixed_timesteps.get(name)
     }
 }
 
+/// A system run criteria that enables systems or stages to run at a fixed timestep between executions.
+///
+/// This does not guarentee that the time elapsed between executions is exactly the provided
+/// fixed timestep, but will guarentee that the execution will run multiple times per game tick
+/// until the number of repetitions is as expected.
+///
+/// For example, a system with a fixed timestep run criteria of 120 times per second will run
+/// two times during a ~16.667ms frame, once during a ~8.333ms frame, and once every two frames
+/// with ~4.167ms frames. However, the same criteria may not result in exactly 8.333ms passing
+/// between each execution.
+///
+/// When using this run criteria, it is advised not to rely on [`Time::delta`] or any of it's
+/// variants for game simulation, but rather use the constant time delta used to initialize the
+/// [`FixedTimestep`] instead.
+///
+/// For more fine tuned information about the execution status of a given fixed timestep,
+/// use the [`FixedTimesteps`] resource.
 pub struct FixedTimestep {
     state: LocalFixedTimestepState,
     internal_system: Box<dyn System<In = (), Out = ShouldRun>>,
@@ -64,6 +85,7 @@ impl Default for FixedTimestep {
 }
 
 impl FixedTimestep {
+    /// Creates a [`FixedTimestep`] that ticks once every `step` seconds.
     pub fn step(step: f64) -> Self {
         Self {
             state: LocalFixedTimestepState {
@@ -74,6 +96,7 @@ impl FixedTimestep {
         }
     }
 
+    /// Creates a [`FixedTimestep`] that ticks once every `rate` times per second.
     pub fn steps_per_second(rate: f64) -> Self {
         Self {
             state: LocalFixedTimestepState {
@@ -84,6 +107,8 @@ impl FixedTimestep {
         }
     }
 
+    /// Sets the label for the timestep. Setting a label allows a timestep
+    /// to be observed by the global [`FixedTimesteps`] resource.
     pub fn with_label(mut self, label: &str) -> Self {
         self.state.label = Some(label.to_string());
         self
@@ -106,7 +131,7 @@ impl FixedTimestep {
 }
 
 #[derive(Clone)]
-pub struct LocalFixedTimestepState {
+struct LocalFixedTimestepState {
     label: Option<String>, // TODO: consider making this a TypedLabel
     step: f64,
     accumulator: f64,
diff --git a/crates/bevy_core/src/time/stopwatch.rs b/crates/bevy_core/src/time/stopwatch.rs
index d03da20824f39..9fa84cba07a6b 100644
--- a/crates/bevy_core/src/time/stopwatch.rs
+++ b/crates/bevy_core/src/time/stopwatch.rs
@@ -55,11 +55,30 @@ impl Stopwatch {
     /// stopwatch.tick(Duration::from_secs(1));
     /// assert_eq!(stopwatch.elapsed(), Duration::from_secs(1));
     /// ```
+    ///
+    /// # See Also
+    ///
+    /// [`elapsed_secs`](Stopwatch::elapsed) - if a `f32` value is desirable instead.
     #[inline]
     pub fn elapsed(&self) -> Duration {
         self.elapsed
     }
 
+    /// Returns the elapsed time since the last [`reset`](Stopwatch::reset)
+    /// of the stopwatch, in seconds.
+    ///
+    /// # Examples
+    /// ```
+    /// # use bevy_core::*;
+    /// use std::time::Duration;
+    /// let mut stopwatch = Stopwatch::new();
+    /// stopwatch.tick(Duration::from_secs(1));
+    /// assert_eq!(stopwatch.elapsed_secs(), 1.0);
+    /// ```
+    ///
+    /// # See Also
+    ///
+    /// [`elapsed`](Stopwatch::elapsed) - if a `Duration` is desirable instead.
     #[inline]
     pub fn elapsed_secs(&self) -> f32 {
         self.elapsed().as_secs_f32()
diff --git a/crates/bevy_core/src/time/time.rs b/crates/bevy_core/src/time/time.rs
index 0a7fe4253900a..c47855e974a74 100644
--- a/crates/bevy_core/src/time/time.rs
+++ b/crates/bevy_core/src/time/time.rs
@@ -28,9 +28,9 @@ impl Default for Time {
 }
 
 impl Time {
+    /// Updates the internal time measurements.
     pub fn update(&mut self) {
-        let now = Instant::now();
-        self.update_with_instant(now);
+        self.update_with_instant(Instant::now());
     }
 
     pub(crate) fn update_with_instant(&mut self, instant: Instant) {
diff --git a/crates/bevy_derive/src/bytes.rs b/crates/bevy_derive/src/bytes.rs
deleted file mode 100644
index 453ade9ed3ec1..0000000000000
--- a/crates/bevy_derive/src/bytes.rs
+++ /dev/null
@@ -1,41 +0,0 @@
-use bevy_macro_utils::BevyManifest;
-use proc_macro::TokenStream;
-use quote::quote;
-use syn::{parse_macro_input, Data, DataStruct, DeriveInput, Fields};
-
-pub fn derive_bytes(input: TokenStream) -> TokenStream {
-    let ast = parse_macro_input!(input as DeriveInput);
-    let fields = match &ast.data {
-        Data::Struct(DataStruct {
-            fields: Fields::Named(fields),
-            ..
-        }) => &fields.named,
-        _ => panic!("Expected a struct with named fields."),
-    };
-
-    let bevy_core_path = BevyManifest::default().get_path(crate::modules::BEVY_CORE);
-
-    let fields = fields
-        .iter()
-        .map(|field| field.ident.as_ref().unwrap())
-        .collect::<Vec<_>>();
-
-    let struct_name = &ast.ident;
-    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
-
-    TokenStream::from(quote! {
-        impl #impl_generics #bevy_core_path::Bytes for #struct_name #ty_generics #where_clause {
-            fn write_bytes(&self, buffer: &mut [u8]) {
-                let mut offset: usize = 0;
-                #(let byte_len = self.#fields.byte_len();
-                self.#fields.write_bytes(&mut buffer[offset..(offset + byte_len)]);
-                offset += byte_len;)*
-            }
-            fn byte_len(&self) -> usize {
-                let mut byte_len: usize = 0;
-                #(byte_len += self.#fields.byte_len();)*
-                byte_len
-            }
-        }
-    })
-}
diff --git a/crates/bevy_derive/src/lib.rs b/crates/bevy_derive/src/lib.rs
index 902e92216e1c6..98990c11c85ef 100644
--- a/crates/bevy_derive/src/lib.rs
+++ b/crates/bevy_derive/src/lib.rs
@@ -2,7 +2,6 @@ extern crate proc_macro;
 
 mod app_plugin;
 mod bevy_main;
-mod bytes;
 mod enum_variant_meta;
 mod modules;
 
@@ -10,12 +9,6 @@ use bevy_macro_utils::{derive_label, BevyManifest};
 use proc_macro::TokenStream;
 use quote::format_ident;
 
-/// Derives the Bytes trait. Each field must also implements Bytes or this will fail.
-#[proc_macro_derive(Bytes)]
-pub fn derive_bytes(input: TokenStream) -> TokenStream {
-    bytes::derive_bytes(input)
-}
-
 /// Generates a dynamic plugin entry point function for the given `Plugin` type.  
 #[proc_macro_derive(DynamicPlugin)]
 pub fn derive_dynamic_plugin(input: TokenStream) -> TokenStream {
diff --git a/crates/bevy_derive/src/modules.rs b/crates/bevy_derive/src/modules.rs
index ae0f3d945e83a..14d68e7051088 100644
--- a/crates/bevy_derive/src/modules.rs
+++ b/crates/bevy_derive/src/modules.rs
@@ -1,2 +1 @@
-pub const BEVY_CORE: &str = "bevy_core";
 pub const BEVY_UTILS: &str = "bevy_utils";
diff --git a/crates/bevy_render/src/color/mod.rs b/crates/bevy_render/src/color/mod.rs
index ea17e9bde1857..f4e770d55f2a7 100644
--- a/crates/bevy_render/src/color/mod.rs
+++ b/crates/bevy_render/src/color/mod.rs
@@ -3,7 +3,6 @@ mod colorspace;
 pub use colorspace::*;
 
 use crate::color::{HslRepresentation, SrgbColorSpace};
-use bevy_core::Bytes;
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::{FromReflect, Reflect, ReflectDeserialize};
 use serde::{Deserialize, Serialize};
@@ -1035,58 +1034,6 @@ impl MulAssign<[f32; 3]> for Color {
     }
 }
 
-impl Bytes for Color {
-    fn write_bytes(&self, buffer: &mut [u8]) {
-        match *self {
-            Color::Rgba {
-                red,
-                green,
-                blue,
-                alpha,
-            } => {
-                red.nonlinear_to_linear_srgb().write_bytes(buffer);
-                green
-                    .nonlinear_to_linear_srgb()
-                    .write_bytes(&mut buffer[std::mem::size_of::<f32>()..]);
-                blue.nonlinear_to_linear_srgb()
-                    .write_bytes(&mut buffer[2 * std::mem::size_of::<f32>()..]);
-                alpha.write_bytes(&mut buffer[3 * std::mem::size_of::<f32>()..]);
-            }
-            Color::RgbaLinear {
-                red,
-                green,
-                blue,
-                alpha,
-            } => {
-                red.write_bytes(buffer);
-                green.write_bytes(&mut buffer[std::mem::size_of::<f32>()..]);
-                blue.write_bytes(&mut buffer[2 * std::mem::size_of::<f32>()..]);
-                alpha.write_bytes(&mut buffer[3 * std::mem::size_of::<f32>()..]);
-            }
-            Color::Hsla {
-                hue,
-                saturation,
-                lightness,
-                alpha,
-            } => {
-                let [red, green, blue] =
-                    HslRepresentation::hsl_to_nonlinear_srgb(hue, saturation, lightness);
-                red.nonlinear_to_linear_srgb().write_bytes(buffer);
-                green
-                    .nonlinear_to_linear_srgb()
-                    .write_bytes(&mut buffer[std::mem::size_of::<f32>()..]);
-                blue.nonlinear_to_linear_srgb()
-                    .write_bytes(&mut buffer[std::mem::size_of::<f32>() * 2..]);
-                alpha.write_bytes(&mut buffer[std::mem::size_of::<f32>() * 3..]);
-            }
-        }
-    }
-
-    fn byte_len(&self) -> usize {
-        std::mem::size_of::<f32>() * 4
-    }
-}
-
 #[derive(Debug)]
 pub enum HexColorError {
     Length,