Rust Errors

Rust Errors

An error type is a contract about what can go wrong and what the caller does about it: which failures panic, which become a Result, which shape the error takes, how much context survives the call stack. Making the failure impossible is /type-driven-design; whether shape changes break callers, /rust-api-design.

The panic policy

unwrap and expect are assertions about invariants, not error handling. Acceptable: in tests, in main for a startup precondition, and after a check the compiler cannot see — where expect names the invariant and carries the values. Unacceptable: in library code on any input-derived value, or anywhere the justification is "this can't fail" without saying why.

Not every failure is a Result. A broken internal invariant with no caller-recoverable path should panic — the program is already in a state the type system promised it would not reach. Input validation is the opposite: whatever the input, the caller can respond, so it is always a Result.

Result all the way to a boundary

Errors propagate with ? to the layer that can actually decide — retry, report, exit. ? converts through From, which is why a #[from] on an enum variant removes a map_err closure at every call site.

Which shape: the boundary decides

One question settles it: does this error cross a public API boundary you will maintain across releases?

No — an internal crate, a closed set of failures, an error the caller only ever prints — it is an enum, and the enum is the cheaper, better answer:

#[derive(Debug, thiserror::Error)]
enum LoadError {
    #[error("cannot read {path}")]
    Read { path: PathBuf, #[source] source: std::io::Error },
    #[error("malformed config in {path}")]
    Parse { path: PathBuf, #[source] source: toml::de::Error },
}

Yes — a published library whose callers upgrade without editing their code — it is a struct wrapping a private kind, exposing the questions callers actually ask:

pub struct ConfigError { kind: ErrorKind }

impl ConfigError {
    pub fn is_not_found(&self) -> bool { /* match on the private kind */ }
    pub fn path(&self) -> &Path { /* ... */ }
}

The full struct pattern, compiling, is in ERROR-TYPES.md.

The public enum fails at that boundary in four independent ways: a variant like Parse(toml::de::Error) promises for ever which TOML crate you use; adding a failure mode breaks caller matches; #[non_exhaustive] does not save it, because it forces every caller into a _ => arm from day one and so removes the exhaustive matching that was the reason to expose an enum; and it pushes work onto callers, who re-derive "the config file is missing" from ConfigError::Io(e) if e.kind() == NotFound. The precedent is std::io::Error, which is exactly this pattern.

thiserror still does the mechanical work either way — #[source] chaining and Display forwarding. Binaries use anyhow: a binary error is a message a human reads once, so a boxed dynamic error with context is enough. A binary large enough to have internal library crates uses thiserror inside them and anyhow only at the top, and anyhow in a published library forces every downstream caller to give up matching on concrete types — that is the actual reason for the split, not taste.

Error taxonomy

Split the enum by what the caller does about it, not by where it was raised. If two variants always get identical handling, they are one variant. If one variant is retryable and another fatal, they must be distinguishable without string-matching a message — an is_retryable() or kind() accessor on a coarse enum. On a public struct error the growth question does not arise, because the kind is private and adding to it is not a breaking change; #[non_exhaustive] is for the enum that is public despite the guidance above, buying non-breaking growth at the cost of exhaustive matching. The semver rules live in /rust-api-design.

Panics that are the right answer

Panic means stop the program, not "return an error loudly": a detected bug — a violated internal invariant — panics rather than returning an Error, because there is no caller who can do anything correct with it. catch_unwind is a last resort, followed by a controlled restart of the unit that panicked, never by carrying on as though nothing happened. For invariants too costly to check in release builds, debug_assert! keeps the check in dev and test builds.

Deferrals

Making the illegal state unrepresentable is /type-driven-design; whether a shape change is a semver break is /rust-api-design; cancellation and timeouts are /async-rust; expression-level cleanup is /idiomatic-rust.

Verification

cargo clippy --all-targets   # add -- -D warnings only if the repo has no lint config
cargo test

Run clippy at the lint level configured in the repo — a clippy.toml or its own lint attributes stay, and a stricter level never goes on top. Then the targeted audit the agent runs itself and reads:

rg '\.unwrap\(\)|\.expect\(' --glob '!**/tests/**' --glob '!**/benches/**' src/

Every surviving hit needs a one-line justification. clippy::unwrap_used and clippy::expect_used are restriction lints: propose them for the repo lint config, do not switch them on unilaterally, or override a level already set.