Rust compilers & the language definition

rustc as the single implementation — with gccrs and mrustc as the outliers and the WASM targets — and the spec situation: the Rust Reference versus the Ferrocene FLS.

Rust ships async/await without an executor — Tokio as the de-facto standard, smol's composable alternative, embassy for embedded, and the unresolved runtime-agnostic library problem.

How editions let Rust make breaking changes without breaking the ecosystem — per-crate opt-in, guaranteed interop, cargo fix migrations, and what each edition changed.

Specs, papers and docs saved for the compilers & language-definition group.