modern Rust API adoption gated behind MSRV bump (1.88 -> 1.9x) #9

Description

Per the new mandatory per-cycle "Modern Rust API / MSRV audit" check in .claude/rules/continuous-improvement.md, this cycle scanned crates/rmcp/src and crates/rmcp-macros/src against the rust-agents:rust-modern-apis trigger table (stable APIs added 1.89-1.97). Current workspace MSRV is 1.88 (Cargo.toml rust-version).

Findings

• Duration::from_secs(60) appears as a literal one-minute duration in 9 call sites (task_manager.rs, transport/streamable_http_server/session/local.rs, model/request_state.rs). Duration::from_mins(1) (stable 1.91) would read marginally clearer but the win is purely cosmetic for a self-evidently-60-second constant.
• assert!(matches!(...)) appears 90 times, all inside #[cfg(test)] modules or crates/rmcp/tests/* integration tests (23 files) — assert_matches! (stable 1.96) would give better failure output (prints the actual value), but per the audit skill's own guidance this is test-only styling, not a correctness issue.
• No occurrences of the other trigger patterns (with_extension add-suffix misuse, try_into().unwrap() for fixed arrays, compare_exchange CAS loops, cfg_if crate usage, fs2/fd-lock external locking, bit-manipulation via leading_zeros/wrapping_neg, unbounded_channel, manual file_stem composition).

Assessment

None of the above rises to a correctness or safety liability — all are readability-only. Bumping MSRV to unlock them (nearest useful target: 1.91 for Duration::from_mins, 1.96 for assert_matches!) is itself a breaking change under this project's versioning discipline (cargo-semver-checks/cargo public-api diff gates, downstream toolchain compatibility for every consumer pinned between 1.88 and the target). That cost is not justified by a handful of cosmetic call sites, most of which are test-only.

Recommendation: do not bump MSRV for this alone. Revisit if/when an MSRV bump is already warranted for an unrelated reason (e.g. a dependency requiring a newer toolchain) — at that point, adopt Duration::from_mins/from_hours and assert_matches! opportunistically as part of the same bump rather than as a standalone change.

Environment

• Version: 3.1.2 (02c62ae)
• MSRV: 1.88 (workspace Cargo.toml)

Upstream

None found — searched modelcontextprotocol/rust-sdk issues/PRs (state: all) for "MSRV modern API" / "assert_matches" / "Duration::from_mins", no matches.

Logs / Evidence

Grep commands used: Duration::from_secs(60, assert!(matches!(, with_extension(, try_into().unwrap(), compare_exchange, cfg_if, fs2|fd-lock, leading_zeros|wrapping_neg, file_stem, unbounded_channel across crates/rmcp/src, crates/rmcp/tests, crates/rmcp-macros/src.