Rust Patterns — mmap-guard#

Crate-specific delta only. Authoritative source: AGENTS.md + GOTCHAS.md.

This is a thin, four-file library (lib.rs, file_data.rs, map.rs, load.rs).
Do not introduce repository/service-layer/API-envelope/builder scaffolding from
the generic ECC patterns — none of it applies to a mmap file-I/O boundary.

The FileData enum (the one real domain type)#

• #[non_exhaustive] → every match needs a wildcard arm; adding a variant is
  non-breaking.
• Variants: Mapped(Mmap, File) (map + file handle for advisory locking) and
  Loaded(Vec<u8>). Use .. in matches! (e.g. matches!(d, FileData::Mapped(..))),
  never _.
• Keep the compile-time Send + Sync const assertion in file_data.rs — it guards
  against regressions if variant types change. Do not remove it.
• Implements Debug, Deref<Target=[u8]>, and AsRef<[u8]>. See
  GOTCHAS.md § FileData Enum.

Unsafe boundary#

The single unsafe block lives in src/map.rs. Treat it as the crate's whole
reason to exist — see security.md and GOTCHAS.md § Unsafe Code.