//! Macro expansion, conditionals, include resolution, and the header cache.
//!
//! Design: `spec/05-preprocessor.md`. Layer rank 5, see `spec/18-package-layout.md`.
//!
//! # Status
//!
//! Not implemented. This crate exists from the first commit so that the layer rank it holds
//! is real and `cargo xtask layers` has something to check. The work lands in M1.
//!
//! Every crate in the workspace is published, and publishing implies a promise. This one is
//! tier 3: its Rust API is explicitly unstable and will change without a major version bump.
//! Depend on the `rucc` binary's behaviour, not on this.
/// The milestone in `spec/17-milestones.md` that fills this crate in.
pub const MILESTONE: &str = "M1";