Recurring defect class: post-MSRV syntax caught only by the MSRV CI job #455

Readiness: needs-decision — owner picks a prevention home.

Class

Code was written with syntax newer than rust-version = "1.85.0" (let-chains). stable clippy/test stay green; only the Rust (1.85.0) CI job catches it mid-PR.

Evidence

• Refs fix: adversarial audit — parser, emitter, settings, CLI defects incl. localized round-trip #433 (2026-10-13) — commits "fix: drop let-chains in settings parser for Rust 1.85 MSRV" and "test: use any() in regression assertions for MSRV clippy" after Rust (1.85.0) / Quality failed while Rust (stable) passed on the preceding head.
• Refs fix(parser): parse every declared localized catalog spelling in its locale #449 (2026-10-14) — commit "fix: keep MSRV 1.85 compatibility (no let-chains)" after the same job failed identically on the preceding head.

Two independent merged PRs within a rolling three-day window — the same author-side mistake, caught only at CI.

Ask

Owner chooses the smallest effective prevention:

1. enable clippy::incompatible_msrv in workspace lints so local cargo clippy reads rust-version and flags post-MSRV constructs; or
2. a rule in AGENTS.md documenting the +1.85.0 check before pushing.

Recorded in docs/LESSONS.md via the merged-work retrospective.