# Realia
This crate provides macros for conditional compilation based on various checks.
These macros are analogous to `#[cfg(...)]` and `#[cfg_attr(...)]`.
Realia is inspired by and heavily based on [rustversion](https://crates.io/crates/rustversion).
## Attributes
Primary:
* Environment variables:
* `#[realia::env("FOO")]`
* Checks if the `FOO` environment variable exists.
* `#[realia::env("FOO", "bar")]`
* Checks if the `FOO` environment variable has the value `bar`.
* Executables:
* `#[realia::cmd("foo")]`
* Checks if the executable `foo` exists in the `PATH` environment variable.
The above can be refined or augmented by these additional attributes:
* `#[realia::not(env("FOO"))]`
* Inverts the condition.
* `#[realia::any(env("FOO"), env("bar"))]`
* Checks if any of the conditions are met.
* `#[realia::all(env("FOO"), env("bar"))]`
* Checks if all of the conditions are met.
* `#[realia::attr(env("FOO"), some_attr)]`
* Applies `#[some_attr]` if the condition is met.
You can also specify `const` this way.
## Triggering build on changed conditions
To trigger builds when the conditions change, you'll need to include a
`build.rs` in your project with the environment variables you check.
```rust
fn main() {
// Necessary when using #[realia::env("FOO")]
println!("cargo:rerun-if-env-changed=FOO");
// Necessary when using #[realia::cmd(...)]
println!("cargo:rerun-if-env-changed=PATH");
}
```