Clean-code Rust (depth)

On top of the base rules (edition 2021, clippy -D warnings, rustfmt, cargo test).

Errors

• Libraries: define an error enum with thiserror, return Result<_, MyError>, propagate with ?.
• Binaries: anyhow::Result + .context("..."). Never .unwrap()/.expect() outside tests or justified startup.

Ownership & borrows

• Take &T/&mut T in function signatures; own only when you must. Return owned values.
• Reach for Rc<RefCell<_>> only when shared mutability is genuinely required — it's a smell as a default; prefer restructuring ownership or passing &mut.
• Newtypes (struct UserId(u64)) over bare primitives for meaning + type safety.

Style

• Iterators/adapters over manual index loops; ? over match-on-error pyramids.
• Producing many values: return impl Iterator<Item = T> and let the caller decide whether to collect().
• A validated newtype has a constructor returning Result (Normalized::new(v)?), so holding one proves the check ran.
• Derive (Debug, Clone, PartialEq) rather than hand-writing. impl Trait in args for flexibility.
• clippy::pedantic is a good aspiration; silence lints with a reason, never blanket-allow.

Smells

• .clone() sprinkled to dodge the borrow checker → rethink ownership.
• unsafe without // SAFETY: → not acceptable.
• Stringly-typed IDs/states → newtype or enum.