Create bevy_platform_support Crate

[bushrat011899]

Objective

• Contributes to Reorganise Platform Support Dependencies #16877

Solution

• Initial creation of bevy_platform_support crate.
• Moved bevy_utils::Instant into new bevy_platform_support crate.
• Moved portable-atomic, portable-atomic-util, and critical-section into new bevy_platform_support crate.

Testing

• CI

---

Showcase

Instead of needing code like this to import an Arc:

#[cfg(feature = "portable-atomic")]
use portable_atomic_util::Arc;

#[cfg(not(feature = "portable-atomic"))]
use alloc::sync::Arc;

We can now use:

use bevy_platform_support::sync::Arc;

This applies to many other types, but the goal is overall the same: allowing crates to use std-like types without the boilerplate of conditional compilation and platform-dependencies.

Migration Guide

• Replace imports of bevy_utils::Instant with bevy_platform_support::time::Instant
• Replace imports of bevy::utils::Instant with bevy::platform_support::time::Instant

Notes

• bevy_platform_support hasn't been reserved on crates.io
• bevy_platform_support is not re-exported from bevy at this time. It may be worthwhile exporting this crate, but I am unsure of a reasonable name to export it under (platform_support may be a bit wordy for user-facing).
• I've included an implementation of Instant which is suitable for no_std platforms that are not Wasm for the sake of eliminating feature gates around its use. It may be a controversial inclusion, so I'm happy to remove it if required.
• There are many other items (spin, bevy_utils::Sync(Unsafe)Cell, etc.) which should be added to this crate. I have kept the initial scope small to demonstrate utility without making this too unwieldy.

[alice-i-cecile]

bevy_platform_support is not re-exported from bevy at this time. It may be worthwhile exporting this crate, but I am unsure of a reasonable name to export it under (platform_support may be a bit wordy for user-facing).

Just re-export it as is, the name is fine and this is less surprising.

[bushrat011899]

Just re-export it as is, the name is fine and this is less surprising.

Done!

[alice-i-cecile]

Marking as blessed due to this comment by @cart. I agree with it too!

[TimJentzsch]

I don't really understand the fallback implementation for Instant, but the rest looks good to me

[bushrat011899]

I don't really understand the fallback implementation for Instant, but the rest looks good to me

I should update the documentation to be more clear on how this works. Basically it's a straight copy of the Instant from std, but it includes two key changes:

1. Instant::set_elapsed is a new method that lets you tell Instant how to know how much time has elapsed. Since we don't have an OS, we can't use the typical syscalls. A simpler alternative would be to just provide Instant with the elapsed time directly, rather than giving it a function to determine elapsed time. But the benefit here is now Instant can update itself only when it needs to, instead of the user needing to eagerly update it (e.g., in the mainloop)
2. Instant::now calls the method provided to Instant::set_elapsed to determine how much time has elapsed, instead of using a syscall directly like in std.

For the sake of convenience, I've included an unset_getter method, which is the default function called by Instant::now if you haven't set one yourself. If you're on x86 or aarch64, we have access to a built-in instruction for getting a system tick in nanoseconds, so we can actually provide a working default implementation on those platforms. For all others, this method panics, explaining that you need to call Instant::set_elapsed before Instant::now is called.

[BenjaminBrienen]

Pretty straightforward and well done!

[Bleachfuel]

Why not just call it bevy_platform? It is already reserved on crates.io

[bushrat011899]

Why not just call it bevy_platform? It is already reserved on crates.io

It could be called that, but that's a pretty prime crate name that'd have a lot of uses. platform_support, while verbose, is pretty specific to the idea of providing cross-platform support utilities.

[mockersf]

could you add the new crate (in the correct order by depdendency) in https://github.com/bevyengine/bevy/blob/main/tools/publish.sh?

[bushrat011899]

could you add the new crate (in the correct order by depdendency) in https://github.com/bevyengine/bevy/blob/main/tools/publish.sh?

Done! bevy_platform_support is required by bevy_utils, so I've placed it at the very top of the list directly above it.

[BD103]

• bevy_platform_support hasn't been reserved on crates.io

Once this PR is merged, we should add the crate to bevy-crate-reservations.

cc @cart :)

[cart]

Reserved!