# Objective
Stumbled upon a `from <-> form` transposition while reviewing a PR,
thought it was interesting, and went down a bit of a rabbit hole.
## Solution
Fix em
# Background
In `no_std` compatible crates, there is often an `std` feature which
will allow access to the standard library. Currently, with the `std`
feature _enabled_, the
[`std::prelude`](https://doc.rust-lang.org/std/prelude/index.html) is
implicitly imported in all modules. With the feature _disabled_, instead
the [`core::prelude`](https://doc.rust-lang.org/core/prelude/index.html)
is implicitly imported. This creates a subtle and pervasive issue where
`alloc` items _may_ be implicitly included (if `std` is enabled), or
must be explicitly included (if `std` is not enabled).
# Objective
- Make the implicit imports for `no_std` crates consistent regardless of
what features are/not enabled.
## Solution
- Replace the `cfg_attr` "double negative" `no_std` attribute with
conditional compilation to _include_ `std` as an external crate.
```rust
// Before
#![cfg_attr(not(feature = "std"), no_std)]
// After
#![no_std]
#[cfg(feature = "std")]
extern crate std;
```
- Fix imports that are currently broken but are only now visible with
the above fix.
## Testing
- CI
## Notes
I had previously used the "double negative" version of `no_std` based on
general consensus that it was "cleaner" within the Rust embedded
community. However, this implicit prelude issue likely was considered
when forming this consensus. I believe the reason why is the items most
affected by this issue are provided by the `alloc` crate, which is
rarely used within embedded but extensively used within Bevy.
# Objective
Improve the code quality of the multithreaded executor.
## Solution
* Remove some unused variables.
* Use `Mutex::get_mut` where applicable instead of locking.
* Use a `startup_systems` FixedBitset to pre-compute the starting
systems instead of building it bit-by-bit on startup.
* Instead of using `FixedBitset::clear` and `FixedBitset::union_with`,
use `FixedBitset::clone_from` instead, which does only a single copy and
will not allocate if the target bitset has a large enough allocation.
* Replace the `Mutex` around `Conditions` with `SyncUnsafeCell`, and add
a `Context::try_lock` that forces it to be synchronized fetched
alongside the executor lock.
This might produce minimal performance gains, but the focus here is on
the code quality improvements.
# Objective
Fix typos throughout the project.
## Solution
[`typos`](https://github.com/crate-ci/typos) project was used for
scanning, but no automatic corrections were applied. I checked
everything by hand before fixing.
Most of the changes are documentation/comments corrections. Also, there
are few trivial changes to code (variable name, pub(crate) function name
and a few error/panic messages).
## Unsolved
`bevy_reflect_derive` has
[typo](1b51053f19/crates/bevy_reflect/bevy_reflect_derive/src/type_path.rs (L76))
in enum variant name that I didn't fix. Enum is `pub(crate)`, so there
shouldn't be any trouble if fixed. However, code is tightly coupled with
macro usage, so I decided to leave it for more experienced contributor
just in case.
# Objective
The function `SyncUnsafeCell::from_mut` returns `&SyncUnsafeCell<T>`,
even though it could return `&mut SyncUnsafeCell<T>`. This means it is
not possible to call `get_mut` on the returned value, so you need to use
unsafe code to get exclusive access back.
## Solution
Return `&mut Self` instead of `&Self` in `SyncUnsafeCell::from_mut`.
This is consistent with my proposal for `UnsafeCell::from_mut`:
https://github.com/rust-lang/libs-team/issues/198.
Replace an unsafe pointer dereference with a safe call to `get_mut`.
---
## Changelog
+ The function `bevy_utils::SyncUnsafeCell::get_mut` now returns a value
of type `&mut SyncUnsafeCell<T>`. Previously, this returned an immutable
reference.
## Migration Guide
The function `bevy_utils::SyncUnsafeCell::get_mut` now returns a value
of type `&mut SyncUnsafeCell<T>`. Previously, this returned an immutable
reference.
# Objective
Complete the first part of the migration detailed in bevyengine/rfcs#45.
## Solution
Add all the new stuff.
### TODO
- [x] Impl tuple methods.
- [x] Impl chaining.
- [x] Port ambiguity detection.
- [x] Write docs.
- [x] ~~Write more tests.~~(will do later)
- [ ] Write changelog and examples here?
- [x] ~~Replace `petgraph`.~~ (will do later)
Co-authored-by: james7132 <contact@jamessliu.com>
Co-authored-by: Michael Hsu <mike.hsu@gmail.com>
Co-authored-by: Mike Hsu <mike.hsu@gmail.com>
