Module match_arg_derive 

#[derive(MatchArg)] - Enum ↔ R String with match.arg Support

This module implements the #[derive(MatchArg)] macro which generates the MatchArg trait implementation for C-style enums, enabling automatic conversion between Rust enums and R character strings with partial matching.

Tradeoff

Without #[derive(MatchArg)], an enum-shaped argument lands in the Rust function as a raw String that you match on by hand — invalid values surface as Rust Err deep in the body, with no R-side defaulting and no partial matching. #[derive(MatchArg)] shifts that work to the wrapper boundary: the generated R wrapper picks up match.arg-style partial string matching against the enum’s variant list, fills in the first variant as the R-visible default, and rejects unknown values before the Rust function is even called. The variant list reaches the R side via a write-time placeholder (MX_MATCH_ARG_CHOICES) rather than a string baked into the proc macro, so adding a variant only requires re-running the build.

Usage

#[derive(Copy, Clone, MatchArg)]
enum Mode {
    Fast,
    Safe,
    Debug,
}

// Generates impl MatchArg for Mode, TryFromSexp for Mode, IntoR for Mode.

Attributes

• #[match_arg(rename = "name")] - Rename a variant’s choice string
• #[match_arg(rename_all = "snake_case")] - Rename all variants (snake_case, kebab-case, lower, upper)
• #[match_arg(alias = "name")] - Another spelling of a variant’s choice, matched only when typed in full (MatchArg::ALIASES, #1843). Repeatable, variants only, never renamed by rename_all. An alias that is empty, "NA", a choice, given twice, or a prefix of another variant’s choice is a compile error. Every error that lists the choices names it after them ((or "name" for "choice"), #2148); the formal, the usage and the @param line don’t.
• #[match_arg(alias = "name", listed = false)] - Keep the aliases of this attribute out of every error. listed applies to the aliases of its own #[match_arg(...)] only; one without an alias, a repeated one, or a value other than true / false is a compile error.

Structs

MatchArgAttrs 🔒

Parsed #[match_arg(...)] attributes from an enum or variant.

Functions

check_aliases 🔒

Refuse the aliases #[match_arg(alias = "...")] can’t have, all at once (one error per bad alias, spanned on its literal): an alias that is empty (pmatch() matches "" to nothing, while the R check’s match() would map it), "NA" (it would read as a missing value), a choice, given twice (on any variant), or a prefix of another variant’s choice, since the alias would change what typing it selects. A prefix of its own choice only is harmless. The wrappers writer applies the same rules to a hand-written MatchArg::ALIASES (miniextendr_api::registry::match_arg_aliases_arg).

derive_match_arg

Main entry point for #[derive(MatchArg)].

parse_match_arg_attrs 🔒

Parse #[match_arg(...)] attributes from a list of syn::Attribute.