anyhow versus thiserror in Rust error handling

WHAT THIS TESTS It checks whether you understand the community convention for structuring errors based on whether your code is a reusable library or a top-level application.

A GOOD ANSWER COVERS anyhow offers a single, type-erased error type, anyhow::Error, that can wrap any error implementing the standard Error trait. It makes propagation trivial (functions return anyhow::Result<T>), supports attaching human-readable context with .context(...), and captures backtraces. It is aimed at applications and binaries where you generally do not need callers to programmatically distinguish error kinds; you propagate up and report at the top. thiserror is a derive macro that helps you define your own concrete error enum: you annotate variants with display messages and source/from attributes, and it generates the boilerplate Error and Display impls and From conversions. This exposes a precise, matchable set of variants to callers, which is what a library should do so consumers can handle specific failures.

DESIGN TRADE-OFFS anyhow optimizes for convenience and speed of development but erases type information, so callers cannot match on specific causes; that is fine inside an app, harmful in a public API. thiserror keeps errors concrete and stable for callers at the cost of more upfront definition. They compose: a library defines thiserror enums, and an application consuming several libraries can collapse them into anyhow.

COMMON WRONG ANSWERS Saying anyhow and thiserror are interchangeable. Recommending anyhow for a library's public API. Thinking thiserror provides a runtime error type rather than generating impls for yours.

LIKELY FOLLOW-UPS How does the From conversion generated by thiserror cooperate with the ? operator? Can you use both in one workspace? What is the cost of boxing in anyhow?

ONE CONCRETE EXAMPLE A parsing library defines enum ParseError with thiserror, giving variants like UnexpectedEof and InvalidToken that downstream code can match on to recover differently. The command-line application that uses that library returns anyhow::Result<()> from main, propagates ParseError and IO errors alike with ?, attaches context such as .context("reading config"), and prints the resulting chain, never needing to match individual variants itself.