refactor-to-thiserror

name	refactor-to-thiserror
description	Find Rust error types with hand-written Display/Error/From impls and refactor them to derive thiserror::Error, preserving behavior.
allowed-tools	["Bash","Read","Edit","Grep"]

Refactor manual Rust error boilerplate to #[derive(thiserror::Error)].

All paths and commands below are relative to the Rust workspace root — run cd rsworkspace first.

1. Find candidates:

   rg -l 'impl std::error::Error|impl (std::)?fmt::Display for' --type rust crates
   

   Over-matches on purpose — it also catches value-object Display impls, which step 3 filters out.

2. Refactor each error type to the codebase idiom. Read a canonical, test-backed example first (crates/decider/trogon-decider-runtime/src/snapshot/codec/snapshot_decode_error.rs, or find current ones with rg -l 'derive\(.*thiserror::Error' --type rust crates):

   • #[derive(Debug, thiserror::Error)] enum; keep existing derives.
   • One #[error("...")] per variant — copy the old Display text verbatim so to_string() is unchanged.
   • #[source] for wrapped errors; #[from] to replace an impl From; #[error(transparent)] for pass-throughs.
   • Delete the now-redundant impl Display, impl std::error::Error, and replaced impl From blocks.
   • A struct that matches over a field → convert to an enum (one variant per arm).

3. Skip when the derive would change behavior: dynamic Display, custom Error methods beyond source(), non-error Display (value objects), or any public API / exact message text other code depends on. When in doubt, leave it.

4. If missing, add to the crate's Cargo.toml: thiserror = { workspace = true }.

5. Verify: cargo check -p <crate> && cargo test -p <crate>. Tests assert exact messages — fix the derive, never the test.