rust-errorslisted

# 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 —