Upgrade to Rust Edition 2024 (#17967)

# Objective

- Fixes #17960

## Solution

- Followed the [edition upgrade
guide](https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html)

## Testing

- CI

---

## Summary of Changes

### Documentation Indentation

When using lists in documentation, proper indentation is now linted for.
This means subsequent lines within the same list item must start at the
same indentation level as the item.

```rust
/* Valid */
/// - Item 1
///   Run-on sentence.
/// - Item 2
struct Foo;

/* Invalid */
/// - Item 1
///     Run-on sentence.
/// - Item 2
struct Foo;
```

### Implicit `!` to `()` Conversion

`!` (the never return type, returned by `panic!`, etc.) no longer
implicitly converts to `()`. This is particularly painful for systems
with `todo!` or `panic!` statements, as they will no longer be functions
returning `()` (or `Result<()>`), making them invalid systems for
functions like `add_systems`. The ideal fix would be to accept functions
returning `!` (or rather, _not_ returning), but this is blocked on the
[stabilisation of the `!` type
itself](https://doc.rust-lang.org/std/primitive.never.html), which is
not done.

The "simple" fix would be to add an explicit `-> ()` to system
signatures (e.g., `|| { todo!() }` becomes `|| -> () { todo!() }`).
However, this is _also_ banned, as there is an existing lint which (IMO,
incorrectly) marks this as an unnecessary annotation.

So, the "fix" (read: workaround) is to put these kinds of `|| -> ! { ...
}` closuers into variables and give the variable an explicit type (e.g.,
`fn()`).

```rust
// Valid
let system: fn() = || todo!("Not implemented yet!");
app.add_systems(..., system);

// Invalid
app.add_systems(..., || todo!("Not implemented yet!"));
```

### Temporary Variable Lifetimes

The order in which temporary variables are dropped has changed. The
simple fix here is _usually_ to just assign temporaries to a named
variable before use.

### `gen` is a keyword

We can no longer use the name `gen` as it is reserved for a future
generator syntax. This involved replacing uses of the name `gen` with
`r#gen` (the raw-identifier syntax).

### Formatting has changed

Use statements have had the order of imports changed, causing a
substantial +/-3,000 diff when applied. For now, I have opted-out of
this change by amending `rustfmt.toml`

```toml
style_edition = "2021"
```

This preserves the original formatting for now, reducing the size of
this PR. It would be a simple followup to update this to 2024 and run
`cargo fmt`.

### New `use<>` Opt-Out Syntax

Lifetimes are now implicitly included in RPIT types. There was a handful
of instances where it needed to be added to satisfy the borrow checker,
but there may be more cases where it _should_ be added to avoid
breakages in user code.

### `MyUnitStruct { .. }` is an invalid pattern

Previously, you could match against unit structs (and unit enum
variants) with a `{ .. }` destructuring. This is no longer valid.

### Pretty much every use of `ref` and `mut` are gone

Pattern binding has changed to the point where these terms are largely
unused now. They still serve a purpose, but it is far more niche now.

### `iter::repeat(...).take(...)` is bad

New lint recommends using the more explicit `iter::repeat_n(..., ...)`
instead.

## Migration Guide

The lifetimes of functions using return-position impl-trait (RPIT) are
likely _more_ conservative than they had been previously. If you
encounter lifetime issues with such a function, please create an issue
to investigate the addition of `+ use<...>`.

## Notes

- Check the individual commits for a clearer breakdown for what
_actually_ changed.

---------

Co-authored-by: François Mockers <francois.mockers@vleue.com>

.github/workflows/ci.yml

@@ -335,6 +335,7 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
       - name: check for missing metadata
         id: missing-metadata
         run: cargo run -p build-templated-pages -- check-missing examples
@@ -369,6 +370,7 @@ jobs:
     needs: check-missing-examples-in-docs
     steps:
       - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
       - name: check for missing features
         id: missing-features
         run: cargo run -p build-templated-pages -- check-missing features
@@ -412,6 +414,7 @@ jobs:
             ~/.cargo/git/db/
             target/
           key: ${{ runner.os }}-cargo-msrv-${{ hashFiles('**/Cargo.toml') }}
+      - uses: dtolnay/rust-toolchain@stable
       - name: get MSRV
         id: msrv
         run: |

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "bevy"
 version = "0.16.0-dev"
-edition = "2021"
+edition = "2024"
 categories = ["game-engines", "graphics", "gui", "rendering"]
 description = "A refreshingly simple data-driven game engine and app framework"
 exclude = ["assets/", "tools/", ".github/", "crates/", "examples/wasm/assets/"]
@@ -10,7 +10,7 @@ keywords = ["game", "engine", "gamedev", "graphics", "bevy"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/bevyengine/bevy"
 documentation = "https://docs.rs/bevy"
-rust-version = "1.83.0"
+rust-version = "1.85.0"
 
 [workspace]
 resolver = "2"

benches/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "benches"
-edition = "2021"
+edition = "2024"
 description = "Benchmarks that test Bevy's performance"
 publish = false
 license = "MIT OR Apache-2.0"

benches/benches/bevy_ecs/change_detection.rs

@@ -95,7 +95,7 @@ fn all_added_detection_generic<T: Component + Default>(group: &mut BenchGroup, e
                     let query = generic_filter_query::<Added<T>>(&mut world);
                     (world, query)
                 },
-                |(ref mut world, ref mut query)| {
+                |(world, query)| {
                     let mut count = 0;
                     for entity in query.iter(world) {
                         black_box(entity);
@@ -143,7 +143,7 @@ fn all_changed_detection_generic<T: Component<Mutability = Mutable> + Default +
                     let query = generic_filter_query::<Changed<T>>(&mut world);
                     (world, query)
                 },
-                |(ref mut world, ref mut query)| {
+                |(world, query)| {
                     let mut count = 0;
                     for entity in query.iter(world) {
                         black_box(entity);
@@ -196,7 +196,7 @@ fn few_changed_detection_generic<T: Component<Mutability = Mutable> + Default +
                     let query = generic_filter_query::<Changed<T>>(&mut world);
                     (world, query)
                 },
-                |(ref mut world, ref mut query)| {
+                |(world, query)| {
                     for entity in query.iter(world) {
                         black_box(entity);
                     }
@@ -237,7 +237,7 @@ fn none_changed_detection_generic<T: Component<Mutability = Mutable> + Default>(
                     let query = generic_filter_query::<Changed<T>>(&mut world);
                     (world, query)
                 },
-                |(ref mut world, ref mut query)| {
+                |(world, query)| {
                     let mut count = 0;
                     for entity in query.iter(world) {
                         black_box(entity);
@@ -343,7 +343,7 @@ fn multiple_archetype_none_changed_detection_generic<
                     let query = generic_filter_query::<Changed<T>>(&mut world);
                     (world, query)
                 },
-                |(ref mut world, ref mut query)| {
+                |(world, query)| {
                     let mut count = 0;
                     for entity in query.iter(world) {
                         black_box(entity);

benches/benches/bevy_ecs/components/add_remove.rs

@@ -12,7 +12,7 @@ impl Benchmark {
         let mut world = World::default();
 
         let entities = world
-            .spawn_batch(core::iter::repeat(A(0.)).take(10000))
+            .spawn_batch(core::iter::repeat_n(A(0.), 10_000))
             .collect();
         Self(world, entities)
     }

benches/benches/bevy_ecs/iteration/iter_simple.rs

@@ -19,15 +19,15 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Position(Vec3::X),
                 Rotation(Vec3::X),
                 Velocity(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query::<(&Velocity, &mut Position)>();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_foreach.rs

@@ -19,15 +19,15 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Position(Vec3::X),
                 Rotation(Vec3::X),
                 Velocity(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query::<(&Velocity, &mut Position)>();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_foreach_sparse_set.rs

@@ -21,15 +21,15 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Position(Vec3::X),
                 Rotation(Vec3::X),
                 Velocity(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query::<(&Velocity, &mut Position)>();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_foreach_wide.rs

@@ -33,8 +33,8 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Rotation(Vec3::X),
                 Position::<0>(Vec3::X),
@@ -47,9 +47,9 @@ impl<'w> Benchmark<'w> {
                 Velocity::<3>(Vec3::X),
                 Position::<4>(Vec3::X),
                 Velocity::<4>(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_foreach_wide_sparse_set.rs

@@ -35,8 +35,8 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Rotation(Vec3::X),
                 Position::<0>(Vec3::X),
@@ -49,9 +49,9 @@ impl<'w> Benchmark<'w> {
                 Velocity::<3>(Vec3::X),
                 Position::<4>(Vec3::X),
                 Velocity::<4>(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_sparse_set.rs

@@ -21,15 +21,15 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Position(Vec3::X),
                 Rotation(Vec3::X),
                 Velocity(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query::<(&Velocity, &mut Position)>();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_system.rs

@@ -19,15 +19,15 @@ impl Benchmark {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Position(Vec3::X),
                 Rotation(Vec3::X),
                 Velocity(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         fn query_system(mut query: Query<(&Velocity, &mut Position)>) {
             for (velocity, mut position) in &mut query {

benches/benches/bevy_ecs/iteration/iter_simple_wide.rs

@@ -33,8 +33,8 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Rotation(Vec3::X),
                 Position::<0>(Vec3::X),
@@ -47,9 +47,9 @@ impl<'w> Benchmark<'w> {
                 Velocity::<3>(Vec3::X),
                 Position::<4>(Vec3::X),
                 Velocity::<4>(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query();
         Self(world, query)

benches/benches/bevy_ecs/iteration/iter_simple_wide_sparse_set.rs

@@ -35,8 +35,8 @@ impl<'w> Benchmark<'w> {
     pub fn new() -> Self {
         let mut world = World::new();
 
-        world.spawn_batch(
-            core::iter::repeat((
+        world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Rotation(Vec3::X),
                 Position::<0>(Vec3::X),
@@ -49,9 +49,9 @@ impl<'w> Benchmark<'w> {
                 Velocity::<3>(Vec3::X),
                 Position::<4>(Vec3::X),
                 Velocity::<4>(Vec3::X),
-            ))
-            .take(10_000),
-        );
+            ),
+            10_000,
+        ));
 
         let query = world.query();
         Self(world, query)

benches/benches/bevy_ecs/iteration/par_iter_simple.rs

@@ -30,15 +30,15 @@ impl<'w> Benchmark<'w> {
 
         let mut world = World::new();
 
-        let iter = world.spawn_batch(
-            core::iter::repeat((
+        let iter = world.spawn_batch(core::iter::repeat_n(
+            (
                 Transform(Mat4::from_scale(Vec3::ONE)),
                 Position(Vec3::X),
                 Rotation(Vec3::X),
                 Velocity(Vec3::X),
-            ))
-            .take(100_000),
-        );
+            ),
+            100_000,
+        ));
         let entities = iter.into_iter().collect::<Vec<Entity>>();
         for i in 0..fragment {
             let mut e = world.entity_mut(entities[i as usize]);

benches/benches/bevy_ecs/world/entity_hash.rs

@@ -11,16 +11,16 @@ fn make_entity(rng: &mut impl Rng, size: usize) -> Entity {
     // * For ids, half are in [0, size), half are unboundedly larger.
     // * For generations, half are in [1, 3), half are unboundedly larger.
 
-    let x: f64 = rng.gen();
+    let x: f64 = rng.r#gen();
     let id = -(1.0 - x).log2() * (size as f64);
-    let x: f64 = rng.gen();
-    let gen = 1.0 + -(1.0 - x).log2() * 2.0;
+    let x: f64 = rng.r#gen();
+    let generation = 1.0 + -(1.0 - x).log2() * 2.0;
 
     // this is not reliable, but we're internal so a hack is ok
-    let bits = ((gen as u64) << 32) | (id as u64);
+    let bits = ((generation as u64) << 32) | (id as u64);
     let e = Entity::from_bits(bits);
     assert_eq!(e.index(), id as u32);
-    assert_eq!(e.generation(), gen as u32);
+    assert_eq!(e.generation(), generation as u32);
     e
 }
 

benches/benches/bevy_reflect/list.rs

@@ -75,8 +75,8 @@ fn concrete_list_apply(criterion: &mut Criterion) {
     let mut group = create_group(criterion, bench!("concrete_list_apply"));
 
     let empty_base = |_: usize| Vec::<u64>::new;
-    let full_base = |size: usize| move || iter::repeat(0).take(size).collect::<Vec<u64>>();
-    let patch = |size: usize| iter::repeat(1).take(size).collect::<Vec<u64>>();
+    let full_base = |size: usize| move || iter::repeat_n(0, size).collect::<Vec<u64>>();
+    let patch = |size: usize| iter::repeat_n(1, size).collect::<Vec<u64>>();
 
     list_apply(&mut group, "empty_base_concrete_patch", empty_base, patch);
 
@@ -103,7 +103,7 @@ fn concrete_list_clone_dynamic(criterion: &mut Criterion) {
             BenchmarkId::from_parameter(size),
             &size,
             |bencher, &size| {
-                let v = iter::repeat(0).take(size).collect::<Vec<_>>();
+                let v = iter::repeat_n(0, size).collect::<Vec<_>>();
 
                 bencher.iter(|| black_box(&v).clone_dynamic());
             },
@@ -123,7 +123,7 @@ fn dynamic_list_push(criterion: &mut Criterion) {
             BenchmarkId::from_parameter(size),
             &size,
             |bencher, &size| {
-                let src = iter::repeat(()).take(size).collect::<Vec<_>>();
+                let src = iter::repeat_n((), size).collect::<Vec<_>>();
                 let dst = DynamicList::default();
 
                 bencher.iter_batched(
@@ -146,8 +146,8 @@ fn dynamic_list_apply(criterion: &mut Criterion) {
     let mut group = create_group(criterion, bench!("dynamic_list_apply"));
 
     let empty_base = |_: usize| || Vec::<u64>::new().clone_dynamic();
-    let full_base = |size: usize| move || iter::repeat(0).take(size).collect::<Vec<u64>>();
-    let patch = |size: usize| iter::repeat(1).take(size).collect::<Vec<u64>>();
+    let full_base = |size: usize| move || iter::repeat_n(0, size).collect::<Vec<u64>>();
+    let patch = |size: usize| iter::repeat_n(1, size).collect::<Vec<u64>>();
 
     list_apply(&mut group, "empty_base_concrete_patch", empty_base, patch);
 

benches/benches/bevy_reflect/map.rs

@@ -145,7 +145,7 @@ fn u64_to_n_byte_key(k: u64, n: usize) -> String {
     write!(&mut key, "{}", k).unwrap();
 
     // Pad key to n bytes.
-    key.extend(iter::repeat('\0').take(n - key.len()));
+    key.extend(iter::repeat_n('\0', n - key.len()));
     key
 }