Function build_match_arg_prelude 

pub(crate) fn build_match_arg_prelude(
    inputs: &Punctuated<FnArg, Comma>,
    per_param: &HashMap<String, ParamAttrs>,
    c_ident: &str,
    attribution: CallAttribution,
) -> Vec<String>

Build R prelude lines that validate match_arg / choices / several_ok parameters before the .Call().

Returns an empty vector when the method declares none. Every form is one statement from CallAttribution::match_arg_statement with the method’s own attribution: the strict scalar helper (NULL and the formal default select the first choice, a factor is read as its labels), the Option<T> form that skips it for NULL (#1473), and the strict several_ok helper (every element must match, NULL selects all; #1472). An Either layer, over a scalar or a several_ok list (#1612), guards either helper with is.character(x) || is.factor(x), so everything else reaches the R arm; Missing adds !missing(x) (#1551), and for a single choice !identical(x, <choices>) (#2152). For match_arg the choice list is the write-time placeholder (c_ident keys it, the same one effective_r_defaults puts in the formal); for choices(...) it is the literal.

The match_arg statements come first, then the choices(...) ones, each group in signature order (inputs), as in a standalone function’s wrapper.

Shared by MethodContext::match_arg_prelude (inherent impls) and TraitMethodContext::match_arg_prelude (trait impls) — see audit/2026-07-03-dogfooding-macros-codegen.md finding #1 (trait methods previously had no match_arg support at all).