Rust Error Handling

Context	Library
Domain / library crates	thiserror — typed, matchable errors
Infrastructure / adapter crates	thiserror or anyhow with context
CLI entry points / main.rs	anyhow or miette

• Never Box<dyn Error> in domain crates
• Never anyhow in port (trait) signatures

thiserror

#[derive(Debug, thiserror::Error)]
pub enum UserError {
    #[error("User not found: {0}")]
    NotFound(UserId),
    #[error("Repository failure")]
    Repository(#[from] RepositoryError),  // #[from] → impl From<T>, enables ?
}

• #[error(transparent)] — delegates Display and source() to inner error (thin wrappers only)
• One error enum per module boundary — variants named after what went wrong, not where
• No catch-all Other(String) variants — they destroy matchability

anyhow

repository.find(id).await.context("Failed to fetch user")?;
heavy_op().with_context(|| format!("Processing item {id}"))?;
bail!("Item list must not be empty");
ensure!(items.len() <= MAX, "Too many items: {} > {MAX}", items.len());

miette (CLI only)

#[derive(Debug, miette::Diagnostic, thiserror::Error)]
#[error("Parse failed")]
#[diagnostic(code(myapp::parse), help("Check the input format"))]
pub struct ParseError {
    #[source_code] src: NamedSource<String>,
    #[label("here")] span: SourceSpan,
}

unwrap / expect

• No .unwrap() in production code
• .expect("reason") only for invariants that cannot be violated — message must explain why
• Both allowed freely in #[cfg(test)]

Error Propagation

Propagate with ?. Never discard silently with let _ = ... — log before discarding.