language-rustlisted

# Rust ## Ownership in one paragraph Every value has one owner. Passing by value moves; borrow with `&` (shared, many) or `&mut` (exclusive, one). Shared and mutable references cannot coexist. Most "fights with the borrow checker" are a design smell — the data model mixed ownership and sharing. ## Borrowing rules of thumb - `&T` when you only read. - `&mut T` when you mutate in place. - `T` (by value) when you consume — builders, transforming constructors. - Return owned types unless the caller clearly benefits from a borrow tied to a parameter's lifetime. ## Lifetimes - Start without annotations; add them when the compiler asks. - `'static` does not mean "lives forever" — it means "no non-static references inside". Prefer owned types over `'static` juggling. - If a struct needs many lifetimes, consider owned data or `Arc` instead. ## Error handling - Return `Result<T, E>`; use `?` to propagate. - **Library code:** custom enum with `thiserror`. - **Application code:** `anyhow::Error` for ergonomics; add context with `.with_context(|| "doing X")`. - Reserve `panic!` for invariant violations the caller cannot recover from. ```rust #[derive(thiserror::Error, Debug)] pub enum LoadError { #[error("io: {0}")] Io(#[from] std::io::Error), #[error("bad format: {0}")] Format(String), } ``` ## Traits - Design around capability, not identity (`Read`, not `IsFile`). - Default methods let traits evolve without breaking implementors. - `impl Trait` in arguments → static dispat