mcpls rust-config.instructions.md

Type safety and idiomatic config

Config option enums must derive Default with #[default] on the intended default variant rather than implementing Default manually. This keeps the default co-located with the type definition and is less error-prone.

Boolean config fields that represent a choice between more than two states should be enum, not bool. A bool cannot be extended without a breaking change; an enum can gain variants under #[non_exhaustive].

New #[serde(default)] fields must have a test that deserialises a config with the field omitted and asserts the default value. Cover both JSON and TOML — parsers can behave differently on edge cases like empty strings and absent vs. null keys.

Enum config options must have a round-trip test for every variant. A typo in #[serde(rename_all = "...")] silently changes the on-disk format and breaks existing user configs without a compile error.

MSRV and dependencies

When introducing a new dependency, check whether the functionality is already available in std for the current MSRV. Prefer std over external crates to reduce compile time and supply-chain surface.

New crates require a license check in the same PR. Add the license to deny.toml before merging — cargo deny check licenses fails fast and blocks the pipeline.

If a new stable Rust API (e.g. OnceLock at 1.70 replacing once_cell::sync::OnceCell) makes an existing dependency redundant, suggest removing the dependency and bumping rust-version instead. Document the bump in CHANGELOG.