Allow bevy_utils in no_std Contexts (#15279)

# Objective

- Adjust `bevy_utils` to make it `no_std` compatible
- Partially replaces #6581
- Contributes to #8161
- Contributes to #6370

## Solution

Added `alloc` and `std` features to `bevy_utils` (`std` is enabled by
default), allowing the crate's use in `no_std` contexts.

## Testing

- CI passed locally.
- Used `bevy_utils` in a `no_std` crate as an experiment and compiled
successfully.

## Migration Guide

If you were importing `bevy_utils` and setting `default_features` to
`false`, but relying on elements which are now gated behind the `std` or
`alloc` features, include the relevant feature in your `Cargo.toml`.

## Notes

- Bevy already includes a single `no_std` crate, `bevy_ptr`, so there is
precedent for this change.
- As `bevy_utils` is widely used across the rest of Bevy, further work
to make Bevy `no_std` compatible would be blocked on this crate, if such
work was to be undertaken.
- Most of the changes in this PR are just the removal of an unnecessary
call to `to_string()` within unit tests.

crates/bevy_utils/Cargo.toml

@@ -9,11 +9,16 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
+default = ["std"]
+std = ["alloc", "tracing/std", "ahash/std"]
+alloc = []
 detailed_trace = []
 
 [dependencies]
-ahash = "0.8.7"
-tracing = { version = "0.1", default-features = false, features = ["std"] }
+ahash = { version = "0.8.7", default-features = false, features = [
+  "runtime-rng",
+] }
+tracing = { version = "0.1", default-features = false }
 web-time = { version = "1.1" }
 hashbrown = { version = "0.14.2", features = ["serde"] }
 bevy_utils_proc_macros = { version = "0.15.0-dev", path = "macros" }

crates/bevy_utils/src/futures.rs

@@ -1,5 +1,5 @@
 //! Utilities for working with [`Future`]s.
-use std::{
+use core::{
     future::Future,
     pin::Pin,
     task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
@@ -44,7 +44,7 @@ fn noop(_data: *const ()) {}
 const NOOP_WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(noop_clone, noop, noop, noop);
 
 fn noop_raw_waker() -> RawWaker {
-    RawWaker::new(std::ptr::null(), &NOOP_WAKER_VTABLE)
+    RawWaker::new(core::ptr::null(), &NOOP_WAKER_VTABLE)
 }
 
 fn noop_waker() -> Waker {

crates/bevy_utils/src/lib.rs

@@ -4,12 +4,16 @@
     html_logo_url = "https://bevyengine.org/assets/icon.png",
     html_favicon_url = "https://bevyengine.org/assets/icon.png"
 )]
+#![cfg_attr(not(feature = "std"), no_std)]
 
 //! General utilities for first-party [Bevy] engine crates.
 //!
 //! [Bevy]: https://bevyengine.org/
 //!
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
 /// The utilities prelude.
 ///
 /// This includes the most common types in this crate, re-exported for your convenience.
@@ -18,7 +22,9 @@ pub mod prelude {
 }
 
 pub mod futures;
+#[cfg(feature = "alloc")]
 mod short_names;
+#[cfg(feature = "alloc")]
 pub use short_names::get_short_name;
 pub mod synccell;
 pub mod syncunsafecell;
@@ -37,8 +43,10 @@ pub use parallel_queue::*;
 pub use tracing;
 pub use web_time::{Duration, Instant, SystemTime, SystemTimeError, TryFromFloatSecsError};
 
-use hashbrown::hash_map::RawEntryMut;
-use std::{
+#[cfg(feature = "alloc")]
+use alloc::boxed::Box;
+
+use core::{
     any::TypeId,
     fmt::Debug,
     hash::{BuildHasher, BuildHasherDefault, Hash, Hasher},
@@ -46,6 +54,7 @@ use std::{
     mem::ManuallyDrop,
     ops::Deref,
 };
+use hashbrown::hash_map::RawEntryMut;
 
 #[cfg(not(target_arch = "wasm32"))]
 mod conditional_send {
@@ -66,11 +75,12 @@ pub use conditional_send::*;
 
 /// Use [`ConditionalSendFuture`] for a future with an optional Send trait bound, as on certain platforms (eg. Wasm),
 /// futures aren't Send.
-pub trait ConditionalSendFuture: std::future::Future + ConditionalSend {}
-impl<T: std::future::Future + ConditionalSend> ConditionalSendFuture for T {}
+pub trait ConditionalSendFuture: core::future::Future + ConditionalSend {}
+impl<T: core::future::Future + ConditionalSend> ConditionalSendFuture for T {}
 
 /// An owned and dynamically typed Future used when you can't statically type your result or need to add some indirection.
-pub type BoxedFuture<'a, T> = std::pin::Pin<Box<dyn ConditionalSendFuture<Output = T> + 'a>>;
+#[cfg(feature = "alloc")]
+pub type BoxedFuture<'a, T> = core::pin::Pin<Box<dyn ConditionalSendFuture<Output = T> + 'a>>;
 
 /// A shortcut alias for [`hashbrown::hash_map::Entry`].
 pub type Entry<'a, K, V, S = BuildHasherDefault<AHasher>> = hashbrown::hash_map::Entry<'a, K, V, S>;
@@ -192,7 +202,7 @@ impl<V: PartialEq, H> PartialEq for Hashed<V, H> {
 }
 
 impl<V: Debug, H> Debug for Hashed<V, H> {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         f.debug_struct("Hashed")
             .field("hash", &self.hash)
             .field("value", &self.value)
@@ -417,7 +427,7 @@ mod tests {
     fn fast_typeid_hash() {
         struct Hasher;
 
-        impl std::hash::Hasher for Hasher {
+        impl core::hash::Hasher for Hasher {
             fn finish(&self) -> u64 {
                 0
             }
@@ -430,8 +440,11 @@ mod tests {
         Hash::hash(&TypeId::of::<()>(), &mut Hasher);
     }
 
+    #[cfg(feature = "alloc")]
     #[test]
     fn stable_hash_within_same_program_execution() {
+        use alloc::vec::Vec;
+
         let mut map_1 = HashMap::new();
         let mut map_2 = HashMap::new();
         for i in 1..10 {

crates/bevy_utils/src/parallel_queue.rs

@@ -1,4 +1,7 @@
-use std::{cell::RefCell, ops::DerefMut};
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+use alloc::vec::Vec;
+
+use core::{cell::RefCell, ops::DerefMut};
 use thread_local::ThreadLocal;
 
 /// A cohesive set of thread-local values of a given type.
@@ -56,6 +59,7 @@ where
     }
 }
 
+#[cfg(feature = "alloc")]
 impl<T: Send> Parallel<Vec<T>> {
     /// Collect all enqueued items from all threads and appends them to the end of a
     /// single Vec.

crates/bevy_utils/src/short_names.rs

@@ -1,3 +1,5 @@
+use alloc::string::String;
+
 /// Shortens a type name to remove all module paths.
 ///
 /// The short name of a type is its full name as returned by
@@ -88,43 +90,37 @@ mod name_formatting_tests {
     fn path_separated() {
         assert_eq!(
             get_short_name("bevy_prelude::make_fun_game"),
-            "make_fun_game".to_string()
+            "make_fun_game"
         );
     }
 
     #[test]
     fn tuple_type() {
-        assert_eq!(
-            get_short_name("(String, String)"),
-            "(String, String)".to_string()
-        );
+        assert_eq!(get_short_name("(String, String)"), "(String, String)");
     }
 
     #[test]
     fn array_type() {
-        assert_eq!(get_short_name("[i32; 3]"), "[i32; 3]".to_string());
+        assert_eq!(get_short_name("[i32; 3]"), "[i32; 3]");
     }
 
     #[test]
     fn trivial_generics() {
-        assert_eq!(get_short_name("a<B>"), "a<B>".to_string());
+        assert_eq!(get_short_name("a<B>"), "a<B>");
     }
 
     #[test]
     fn multiple_type_parameters() {
-        assert_eq!(get_short_name("a<B, C>"), "a<B, C>".to_string());
+        assert_eq!(get_short_name("a<B, C>"), "a<B, C>");
     }
 
     #[test]
     fn enums() {
-        assert_eq!(get_short_name("Option::None"), "Option::None".to_string());
-        assert_eq!(
-            get_short_name("Option::Some(2)"),
-            "Option::Some(2)".to_string()
-        );
+        assert_eq!(get_short_name("Option::None"), "Option::None");
+        assert_eq!(get_short_name("Option::Some(2)"), "Option::Some(2)");
         assert_eq!(
             get_short_name("bevy_render::RenderSet::Prepare"),
-            "RenderSet::Prepare".to_string()
+            "RenderSet::Prepare"
         );
     }
 
@@ -132,7 +128,7 @@ mod name_formatting_tests {
     fn generics() {
         assert_eq!(
             get_short_name("bevy_render::camera::camera::extract_cameras<bevy_render::camera::bundle::Camera3d>"),
-            "extract_cameras<Camera3d>".to_string()
+            "extract_cameras<Camera3d>"
         );
     }
 
@@ -140,7 +136,7 @@ mod name_formatting_tests {
     fn nested_generics() {
         assert_eq!(
             get_short_name("bevy::mad_science::do_mad_science<mad_science::Test<mad_science::Tube>, bavy::TypeSystemAbuse>"),
-            "do_mad_science<Test<Tube>, TypeSystemAbuse>".to_string()
+            "do_mad_science<Test<Tube>, TypeSystemAbuse>"
         );
     }
 
@@ -148,15 +144,12 @@ mod name_formatting_tests {
     fn sub_path_after_closing_bracket() {
         assert_eq!(
             get_short_name("bevy_asset::assets::Assets<bevy_scene::dynamic_scene::DynamicScene>::asset_event_system"),
-            "Assets<DynamicScene>::asset_event_system".to_string()
+            "Assets<DynamicScene>::asset_event_system"
         );
         assert_eq!(
             get_short_name("(String, String)::default"),
-            "(String, String)::default".to_string()
-        );
-        assert_eq!(
-            get_short_name("[i32; 16]::default"),
-            "[i32; 16]::default".to_string()
+            "(String, String)::default"
         );
+        assert_eq!(get_short_name("[i32; 16]::default"), "[i32; 16]::default");
     }
 }

crates/bevy_utils/src/synccell.rs

@@ -2,7 +2,7 @@
 //!
 //! [`std::sync::Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
 
-use std::ptr;
+use core::ptr;
 
 /// See [`Exclusive`](https://github.com/rust-lang/rust/issues/98407) for stdlib's upcoming implementation,
 /// which should replace this one entirely.