Idiomatic Rust

Contract

Field	Bound contract
Trigger	Rust code is being written, reviewed, or refactored, or a Rust design choice about borrowing, error handling, dispatch, or testing needs the idiomatic answer.
Authority	Reversible local: writes only the Rust source and project files the invoking task already touches; rollback is version-control restore. No remote mutation.
Side effect	Rust source files may change under the invoking task; chapter guidance and verification output go to chat.
Done	The code or review follows the conventions below, the routed chapters were consulted, and the toolchain verification ran clean or its failure is reported.

The Rust task (required): code to write, a diff to review, or a design question.
The crate or workspace root (required when it is not the current directory): supplies Cargo.toml, rust-toolchain.toml, and the edition pin.
The toolchain pin (optional): defaults to the grounded pin in step 1.