Fix no_std CI Warnings and WASM Compatibility (#17049)

# Objective

- Resolve several warnings encountered when compiling for `no_std`
around `dead_code`
- Fix compatibility with `wasm32-unknown-unknown` when using `no_std`
(identified by Sachymetsu on
[Discord](https://discord.com/channels/691052431525675048/692572690833473578/1323365426901549097))

## Solution

- Removed some unused imports
- Added `allow(dead_code)` for certain private items when compiling on
`no_std`
- Fixed `bevy_app` and `bevy_tasks` compatibility with WASM when
compiling without `std` by appropriately importing `Box` and feature
gating panic unwinding

## Testing

- CI

crates/bevy_app/src/schedule_runner.rs

@@ -10,7 +10,7 @@ use bevy_utils::Instant;
 
 #[cfg(target_arch = "wasm32")]
 use {
-    alloc::rc::Rc,
+    alloc::{boxed::Box, rc::Rc},
     core::cell::RefCell,
     wasm_bindgen::{prelude::*, JsCast},
 };

crates/bevy_math/src/primitives/polygon.rs

@@ -1,12 +1,17 @@
 #[cfg(feature = "alloc")]
-use alloc::{collections::BTreeMap, vec::Vec};
+use {
+    super::{Measured2d, Triangle2d},
+    alloc::{collections::BTreeMap, vec::Vec},
+};
 
 use core::cmp::Ordering;
 
 use crate::Vec2;
 
-use super::{Measured2d, Triangle2d};
-
+#[cfg_attr(
+    not(feature = "alloc"),
+    expect(dead_code, reason = "this type is only used with the alloc feature")
+)]
 #[derive(Debug, Clone, Copy)]
 enum Endpoint {
     Left,
@@ -20,12 +25,20 @@ enum Endpoint {
 ///
 /// This is the order expected by the [`SweepLine`].
 #[derive(Debug, Clone, Copy)]
+#[cfg_attr(
+    not(feature = "alloc"),
+    allow(dead_code, reason = "this type is only used with the alloc feature")
+)]
 struct SweepLineEvent {
     segment: Segment,
     /// Type of the vertex (left or right)
     endpoint: Endpoint,
 }
 impl SweepLineEvent {
+    #[cfg_attr(
+        not(feature = "alloc"),
+        allow(dead_code, reason = "this type is only used with the alloc feature")
+    )]
     fn position(&self) -> Vec2 {
         match self.endpoint {
             Endpoint::Left => self.segment.left,
@@ -51,6 +64,10 @@ impl Ord for SweepLineEvent {
 }
 
 /// Orders 2D points according to the order expected by the sweep line and event queue from -X to +X and then -Y to Y.
+#[cfg_attr(
+    not(feature = "alloc"),
+    allow(dead_code, reason = "this type is only used with the alloc feature")
+)]
 fn xy_order(a: Vec2, b: Vec2) -> Ordering {
     a.x.total_cmp(&b.x).then_with(|| a.y.total_cmp(&b.y))
 }
@@ -134,6 +151,10 @@ impl Ord for Segment {
 }
 
 /// Holds information about which segment is above and which is below a given [`Segment`]
+#[cfg_attr(
+    not(feature = "alloc"),
+    expect(dead_code, reason = "this type is only used with the alloc feature")
+)]
 #[derive(Debug, Clone, Copy)]
 struct SegmentOrder {
     above: Option<usize>,
@@ -241,6 +262,13 @@ impl<'a> SweepLine<'a> {
 /// Test what side of the line through `p1` and `p2` `q` is.
 ///
 /// The result will be `0` if the `q` is on the segment, negative for one side and positive for the other.
+#[cfg_attr(
+    not(feature = "alloc"),
+    expect(
+        dead_code,
+        reason = "this function is only used with the alloc feature"
+    )
+)]
 #[inline(always)]
 fn point_side(p1: Vec2, p2: Vec2, q: Vec2) -> f32 {
     (p2.x - p1.x) * (q.y - p1.y) - (q.x - p1.x) * (p2.y - p1.y)

crates/bevy_reflect/src/impls/glam.rs

@@ -4,9 +4,6 @@ use assert_type_match::assert_type_match;
 use bevy_reflect_derive::{impl_reflect, impl_reflect_opaque};
 use glam::*;
 
-#[cfg(not(feature = "std"))]
-use alloc::format;
-
 /// Reflects the given foreign type as an enum and asserts that the variants/fields match up.
 macro_rules! reflect_enum {
     ($(#[$meta:meta])* enum $ident:ident { $($ty:tt)* } ) => {
@@ -381,6 +378,7 @@ impl_reflect_opaque!(::glam::BVec4A(Debug, Default, Deserialize, Serialize));
 
 #[cfg(test)]
 mod tests {
+    use alloc::format;
     use ron::{
         ser::{to_string_pretty, PrettyConfig},
         Deserializer,

crates/bevy_reflect/src/impls/smallvec.rs

@@ -1,11 +1,8 @@
-use alloc::boxed::Box;
+use alloc::{boxed::Box, vec::Vec};
 use bevy_reflect_derive::impl_type_path;
 use core::any::Any;
 use smallvec::{Array as SmallArray, SmallVec};
 
-#[cfg(not(feature = "std"))]
-use alloc::{format, vec, vec::Vec};
-
 use crate::{
     self as bevy_reflect, utility::GenericTypeInfoCell, ApplyError, FromReflect, FromType,
     Generics, GetTypeRegistration, List, ListInfo, ListIter, MaybeTyped, PartialReflect, Reflect,

crates/bevy_tasks/src/executor.rs

@@ -8,7 +8,6 @@
 //! [`async-executor`]: https://crates.io/crates/async-executor
 //! [`edge-executor`]: https://crates.io/crates/edge-executor
 
-pub use async_task::Task;
 use core::{
     fmt,
     panic::{RefUnwindSafe, UnwindSafe},

crates/bevy_tasks/src/single_threaded_task_pool.rs

@@ -198,7 +198,7 @@ impl TaskPool {
     where
         T: 'static + MaybeSend + MaybeSync,
     {
-        #[cfg(all(target_arch = "wasm32", feature = "std"))]
+        #[cfg(target_arch = "wasm32")]
         return Task::wrap_future(future);
 
         #[cfg(all(not(target_arch = "wasm32"), feature = "std"))]
@@ -210,7 +210,7 @@ impl TaskPool {
             Task::new(task)
         });
 
-        #[cfg(not(feature = "std"))]
+        #[cfg(all(not(target_arch = "wasm32"), not(feature = "std")))]
         return {
             let task = LOCAL_EXECUTOR.spawn(future);
             // Loop until all tasks are done

crates/bevy_tasks/src/task.rs

@@ -14,11 +14,11 @@ use core::{
 /// Tasks that panic get immediately canceled. Awaiting a canceled task also causes a panic.
 #[derive(Debug)]
 #[must_use = "Tasks are canceled when dropped, use `.detach()` to run them in the background."]
-pub struct Task<T>(crate::executor::Task<T>);
+pub struct Task<T>(async_task::Task<T>);
 
 impl<T> Task<T> {
     /// Creates a new task from a given `async_executor::Task`
-    pub fn new(task: crate::executor::Task<T>) -> Self {
+    pub fn new(task: async_task::Task<T>) -> Self {
         Self(task)
     }
 

crates/bevy_tasks/src/thread_executor.rs

@@ -1,7 +1,8 @@
 use core::marker::PhantomData;
 use std::thread::{self, ThreadId};
 
-use crate::executor::{Executor, Task};
+use crate::executor::Executor;
+use async_task::Task;
 use futures_lite::Future;
 
 /// An executor that can only be ticked on the thread it was instantiated on. But

crates/bevy_tasks/src/wasm_task.rs

@@ -1,3 +1,4 @@
+use alloc::boxed::Box;
 use core::{
     any::Any,
     future::{Future, IntoFuture},
@@ -59,7 +60,12 @@ impl<T> Future for Task<T> {
             // NOTE: Propagating the panic here sorta has parity with the async_executor behavior.
             // For those tasks, polling them after a panic returns a `None` which gets `unwrap`ed, so
             // using `resume_unwind` here is essentially keeping the same behavior while adding more information.
+            #[cfg(feature = "std")]
             Poll::Ready(Ok(Err(panic))) => std::panic::resume_unwind(panic),
+            #[cfg(not(feature = "std"))]
+            Poll::Ready(Ok(Err(_panic))) => {
+                unreachable!("catching a panic is only possible with std")
+            }
             Poll::Ready(Err(_)) => panic!("Polled a task after it was cancelled"),
             Poll::Pending => Poll::Pending,
         }
@@ -74,6 +80,14 @@ struct CatchUnwind<F: UnwindSafe>(#[pin] F);
 impl<F: Future + UnwindSafe> Future for CatchUnwind<F> {
     type Output = Result<F::Output, Panic>;
     fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-        std::panic::catch_unwind(AssertUnwindSafe(|| self.project().0.poll(cx)))?.map(Ok)
+        let f = AssertUnwindSafe(|| self.project().0.poll(cx));
+
+        #[cfg(feature = "std")]
+        let result = std::panic::catch_unwind(f)?;
+
+        #[cfg(not(feature = "std"))]
+        let result = f();
+
+        result.map(Ok)
     }
 }