serena/rust-analyzer: share one rust-analyzer across sessions via lspmux

[paulnsorensen]

Part 4/4 of the local-llm rightsize effort (research in hallouminate/.cheese/research/{serena-ra-memory,lspmux-serena-status}/).

Problem

Every Claude Code session spawns its own serena MCP server, which spawns its own rust-analyzer at ~4.4 GiB RSS each (3 found concurrently = ~13 GiB; one was 3 days stale). Two upstream causes:

1. serena hardcodes cachePriming.enable=true + cargo.autoreload=true in solidlsp/language_servers/rust_analyzer.py → each instance eagerly indexes the full dep graph (arrow/fastembed) instead of ~400 MB lazy. Fix PR rust-analyzer: lightweight profile (disable cachePriming + cargo.auto… oraios/serena#1557 is maintainer-acknowledged ("makes sense, helps us" — MischaPanch), blocked only on a flaky-test timeout bump (#1559). Serena ships ~1 release/week, so this lands on its own — no local patch.
2. serena never shares LS instances (one per session by design; #693 closed as "client's problem") and doesn't reap children on ungraceful exit (#1490, zero maintainer engagement).

Decision: lspmux — N serena sessions share one rust-analyzer, idle-killed after timeout.

lspmux status (verified 2026-06-11): it IS ra-multiplex renamed at v0.3.0 (GitHub pr2502/ra-multiplex redirects to codeberg.org/p2502/lspmux); last commit 2026-02-25; packaged in Homebrew/Nix/AUR; recommended by the rustaceanvim maintainer; sunshowers/lspmux-rust-analyzer wraps it for exactly the Claude-Code-multi-session case. v0.3.0 fixed processId handling so the LS survives the first client disconnecting.

Tasks

• Install lspmux (cargo install or AUR; pin version) via dotfiles tooling.
• Config: instance_timeout = 300 (idle kill), per-workspace instance keying.
• Shim rust-analyzer so serena launches the lspmux client instead of the real binary. Verification gate first: serena's DependencyProvider may resolve via rustup which rust-analyzer, bypassing a PATH shim — confirm with ps parentage after a fresh session; consider shimming the rustup proxy path or using sunshowers/lspmux-rust-analyzer.
• Gate 2: confirm lspmux passes serena's initializationOptions through unmodified (it rewrites processId in initialize; if it strips cachePriming settings the upstream #1557 fix would be lost through the mux).
• If either gate fails: stop, document findings on this issue — do not silently fall back to patching serena.
• One-time cleanup of current stale rust-analyzers (pkill -f rust-analyzer; serena respawns on demand).
• Watch rust-analyzer: lightweight profile (disable cachePriming + cargo.auto… oraios/serena#1557 — upgrade serena once merged (cuts per-instance RSS independent of lspmux).

Acceptance

• Two concurrent Claude Code sessions in the same Rust workspace → exactly one rust-analyzer process (ps axo pid,ppid,comm | grep rust-analyzer).
• 5+ min after all sessions exit, zero rust-analyzer processes remain.
• serena symbol tools (find_symbol, find_referencing_symbols) still work through the mux in both sessions.

Risks

• Single-author project (no maintained alternative found — absence claim from search exhaustion, not a catalog).
• rustaceanvim flagged lspmux 0.3.x compat as unverified (ra-multiplex -> lspmux mrcjkb/rustaceanvim#870) — relevant if the editor shares the same instance.