# Objective
- Acts on certain elements of #18799
- Closes#1615
- New baseline for #18170
## Solution
- Created a new `cfg` module in `bevy_platform` which contains two
macros to aid in working with features like `web`, `std`, and `alloc`.
- `switch` is a stable implementation of
[`cfg_match`](https://doc.rust-lang.org/std/macro.cfg_match.html), which
itself is a `core` alternative to [`cfg_if`](https://docs.rs/cfg-if).
- `define_alias` is a `build.rs`-free alternative to
[`cfg_aliases`](https://docs.rs/cfg_aliases) with the ability to share
feature information between crates.
- Switched to these macros within `bevy_platform` to demonstrate usage.
## Testing
- CI
---
## Showcase
Consider the typical `std` feature as an example of a "virality". With
just `bevy_platform`, `bevy_utils`, and `bevy_ecs`, we have 3 crates in
a chain where activating `std` in any of them should really activate it
everywhere. The status-quo for this is for each crate to define its own
`std` feature, and ensure it includes the `std` feature of every
dependency in that feature. For crates which don't even interact with
`std` directly, this can be quite cumbersome. Especially considering
that Bevy has a fundamental crate, `bevy_platform`, which is a
dependency for effectively every crate.
Instead, we can use `define_alias` to create a macro which will
conditionally compile code if and only if the specified configuration
condition is met _in the defining crate_.
```rust
// In `bevy_platform`
define_alias! {
#[cfg(feature = "std")] => {
/// Indicates the `std` crate is available and can be used.
std
}
#[cfg(all(target_arch = "wasm32", feature = "web"))] => {
/// Indicates that this target has access to browser APIs.
web
}
}
```
The above `web` and `std` macros will either no-op the provided code if
the conditions are not met, or pass it unmodified if it is met. Since it
is evaluated in the context of the defining crate, `bevy_platform/std`
can be used to conditionally compile code in `bevy_utils` and `bevy_ecs`
_without_ those crates including their own `std` features.
```rust
// In `bevy_utils`
use bevy_platform::cfg;
// If `bevy_platform` has `std`, then we can too!
cfg::std! {
extern crate std;
}
```
To aid in more complex configurations, `switch` is provided to provide a
`cfg_if` alternative that is compatible with `define_alias`:
```rust
use bevy_platform::cfg;
cfg::switch! {
#[cfg(feature = "foo")] => { /* use the foo API */ }
cfg::web => { /* use browser API */ }
cfg::std => { /* use std */ }
_ => { /* use a fallback implementation */ }
}
```
This paradigm would allow Bevy's sub-crates to avoid re-exporting viral
features, and also enable functionality in response to availability in
their dependencies, rather than from explicit features (bottom-up
instead of top-down). I imagine that a "full rollout" of this paradigm
would remove most viral features from Bevy's crates, leaving only
`bevy_platform`, `bevy_internal`, and `bevy` (since `bevy`/`_internal`
are explicitly re-exports of all of Bevy's crates).
This bottom-up approach may be useful in other areas of Bevy's features
too. For example, `bevy_core_pipeline/tonemapping_luts` requires:
- bevy_render/ktx2
- bevy_image/ktx2
- bevy_image/zstd
If `define_alias` was used in `bevy_image`, `bevy_render` would not need
to re-export the `ktx2` feature, and `bevy_core_pipeline` could directly
probe `bevy_image` for the status of `ktx2` and `zstd` features to
determine if it should compile the `tonemapping_luts` functionality,
rather than having an explicitly feature. Of course, an explicit feature
is still important for _features_, so this may not be the best example,
but it highlights that with this paradigm crates can reactively provide
functionality, rather than needing to proactively declare feature
combinations up-front and hope the user enables them.
---------
Co-authored-by: BD103 <59022059+BD103@users.noreply.github.com>
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