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# CFG Aliases
CFG Aliases is a tiny utility to help save you a lot of effort with long winded `#[cfg()]` checks. This crate provides a single [`cfg_aliases!`] macro that doesn't have any dependencies and specifically avoids pulling in `syn` or `quote` so that the impact on your comile times should be negligible.
You use the the [`cfg_aliases!`] macro in your `build.rs` script to define aliases such as `x11` that could then be used in the `cfg` attribute or macro for conditional compilation: `#[cfg(x11)]`.
## Example
**Cargo.toml:**
```toml
[build-dependencies]
cfg_aliases = "0.1.0"
```
**build.rs:**
```rust
use cfg_aliases::cfg_aliases;
fn main() {
// Setup cfg aliases
cfg_aliases! {
// Platforms
wasm: { target_arch = "wasm32" },
android: { target_os = "android" },
macos: { target_os = "macos" },
linux: { target_os = "linux" },
// Backends
surfman: { all(unix, feature = "surfman", not(wasm)) },
glutin: { all(feature = "glutin", not(wasm)) },
wgl: { all(windows, feature = "wgl", not(wasm)) },
dummy: { not(any(wasm, glutin, wgl, surfman)) },
}
}
```
Now that we have our aliases setup we can use them just like you would expect:
```rust
#[cfg(wasm)]
println!("This is running in WASM");
#[cfg(surfman)]
{
// Do stuff related to surfman
}
#[cfg(dummy)]
println!("We're in dummy mode, specify another feature if you want a smarter app!");
```
This greatly improves what would otherwise look like this without the aliases:
```rust
#[cfg(target_arch = "wasm32")]
println!("We're running in WASM");
#[cfg(all(unix, feature = "surfman", not(target_arch = "wasm32")))]
{
// Do stuff related to surfman
}
#[cfg(not(any(
target_arch = "wasm32",
all(unix, feature = "surfman", not(target_arch = "wasm32")),
all(windows, feature = "wgl", not(target_arch = "wasm32")),
all(feature = "glutin", not(target_arch = "wasm32")),
)))]
println!("We're in dummy mode, specify another feature if you want a smarter app!");
```
You can also use the `cfg!` macro or combine your aliases with other checks using `all()`, `not()`, and `any()`. Your aliases are genuine `cfg` flags now!
```rust
if cfg!(glutin) {
// use glutin
} else {
// Do something else
}
#[cfg(all(glutin, surfman))]
compile_error!("You cannot specify both `glutin` and `surfman` features");
```
## Syntax and Error Messages
The aliase names are restricted to the same rules as rust identifiers which, for one, means that they cannot have dashes ( `-` ) in them. Additionally, if you get certain syntax elements wrong, such as the alias name, the macro will error saying that the recursion limit was reached instead of giving a clear indication of what actually went wrong. This is due to a nuance with the macro parser and it might be fixed in a later release of this crate. It is also possible that aliases with dashes in the name might be supported in a later release. Open an issue if that is something that you would like implemented.
Finally, you can also induce an infinite recursion by having rules that both reference each-other, but this isn't a real limitation because that doesn't make logical sense anyway:
```rust,ignore
// This causes an error!
cfg_aliases! {
test1: { not(test2) },
test2: { not(test1) },
}
```
## Attribution and Thanks
- Thanks to my God and Father who led me through figuring this out and to whome I owe everything.
- Thanks to @Yandros on the Rust forum for [showing me][sm] some crazy macro hacks!
- Thanks to @sfackler for [pointing out][po] the way to make cargo add the cfg flags.
- Thanks to the authors of the [`tectonic_cfg_support::target_cfg`] macro from which most of the cfg attribute parsing logic is taken from. Also thanks to @ratmice for [bringing it up][bip] on the Rust forum.
[`tectonic_cfg_support::target_cfg`]: https://docs.rs/tectonic_cfg_support/0.0.1/src/tectonic_cfg_support/lib.rs.html#166-298