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ADR-0001: Implement the P04 compositor in pure Rust

  • Status: Accepted
  • Date: 2026-09-13
  • Authors: @lusoris (decision), roadmap decision D08

Context

The P04 Mercurius blueprint rejects libweston and Smithay for the fast runtime loop and mandates a C wlroots implementation that owns the event loop (requirement REQ-P04-01). The only code candidate for P04 is a Rust scaffold with no wlroots binding, and no wlroots version is pinned anywhere in the sources. Keeping the mandate would add a C toolchain, an FFI boundary, and a second memory-safety regime to the component that sits at the centre of the IPC mesh (P05, P07, P08, P10, P11 and P15 all connect to it).

The roadmap had left the choice open until M07 with a backend-agnostic registry and IPC state machine. The maintainer decided the question directly.

Decision

P04 aegis-compositor is implemented as a pure Rust compositor. The C wlroots mandate in the P04 blueprint is superseded. The Rust compositor library is selected at M07 through the template matrix after checking current upstream support; no C compositor toolchain is admitted.

Consequences

  • Positive: one language and one memory-safety model across the host daemons; the HISS-07 and HISS-09 rules apply to the compositor without an FFI exception; no C toolchain admission on the M07 path.
  • Negative: the blueprint's latency argument for wlroots (control of the event loop for the fast runtime loop) is no longer backed by its own rationale. The frame-pacing and IPC latency budgets must be demonstrated with Rust implementation evidence in M07 and on real GPU hardware in M12 before they are claimed.
  • REQ-P04-01 stays in the requirement set as source evidence and is marked superseded by this ADR.

References

  • Roadmap decision D08, docs/roadmap/README.md (Risks and decisions).
  • Requirement REQ-P04-01, docs/roadmap/requirements.md.
  • Maintainer answer, 2026-09-13, to "The P04 blueprint mandates a C wlroots compositor core, but the only code candidate is pure Rust with no wlroots binding. Which way?": "Pure Rust compositor".