Swap out num_cpus for std:🧵:available_parallelism (#4970)

# Objective
As of Rust 1.59, `std:🧵:available_parallelism` has been stabilized. As of Rust 1.61, the API matches `num_cpus::get` by properly handling Linux's cgroups and other sandboxing mechanisms.

As bevy does not have an established MSRV, we can replace `num_cpus` in `bevy_tasks` and reduce our dependency tree by one dep.

## Solution
Replace `num_cpus` with `std:🧵:available_parallelism`. Wrap it to have a fallback in the case it errors out and have it operate in the same manner as `num_cpus` did.

This however removes `physical_core_count` from the API, though we are currently not using it in any way in first-party crates.

---

## Changelog
Changed: `bevy_tasks::logical_core_count` -> `bevy_tasks::available_parallelism`.
Removed: `bevy_tasks::physical_core_count`.

## Migration Guide
`bevy_tasks::logical_core_count` and `bevy_tasks::physical_core_count` have been removed. `logical_core_count` has been replaced with `bevy_tasks::available_parallelism`, which works identically. If `bevy_tasks::physical_core_count` is required, the `num_cpus` crate can be used directly, as these two were just aliases for `num_cpus` APIs.

crates/bevy_core/src/task_pool_options.rs

@@ -94,8 +94,8 @@ impl DefaultTaskPoolOptions {
 
     /// Inserts the default thread pools into the given resource map based on the configured values
     pub fn create_default_pools(&self) {
-        let total_threads =
-            bevy_tasks::logical_core_count().clamp(self.min_total_threads, self.max_total_threads);
+        let total_threads = bevy_tasks::available_parallelism()
+            .clamp(self.min_total_threads, self.max_total_threads);
         trace!("Assigning {} cores to default task pools", total_threads);
 
         let mut remaining_threads = total_threads;

crates/bevy_tasks/Cargo.toml

@@ -12,7 +12,6 @@ keywords = ["bevy"]
 futures-lite = "1.4.0"
 async-executor = "1.3.0"
 async-channel = "1.4.2"
-num_cpus = "1"
 once_cell = "1.7"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]

crates/bevy_tasks/src/lib.rs

@@ -33,5 +33,16 @@ pub mod prelude {
     };
 }
 
-pub use num_cpus::get as logical_core_count;
-pub use num_cpus::get_physical as physical_core_count;
+use std::num::NonZeroUsize;
+
+/// Gets the logical CPU core count available to the current process.
+///
+/// This is identical to [`std::thread::available_parallelism`], except
+/// it will return a default value of 1 if it internally errors out.
+///
+/// This will always return at least 1.
+pub fn available_parallelism() -> usize {
+    std::thread::available_parallelism()
+        .map(NonZeroUsize::get)
+        .unwrap_or(1)
+}

crates/bevy_tasks/src/task_pool.rs

@@ -95,7 +95,7 @@ impl TaskPool {
 
         let executor = Arc::new(async_executor::Executor::new());
 
-        let num_threads = num_threads.unwrap_or_else(num_cpus::get);
+        let num_threads = num_threads.unwrap_or_else(crate::available_parallelism);
 
         let threads = (0..num_threads)
             .map(|i| {