Function validate_per_param_attr_conflicts 

pub(crate) fn validate_per_param_attr_conflicts(
    attr: &PerParamMiniextendrAttr,
    param_name: &str,
    is_dots: bool,
    span: Span,
) -> Result<()>

Validate per-parameter attribute conflicts.

Returns Err if:

• coerce + match_arg on the same parameter
• coerce + choices(...) on the same parameter
• choices(...) + explicit default on the same parameter
• match_arg + choices(...) on the same parameter (two sources for one choice list; the match_arg placeholder would shadow the literal list and, with no MatchArg entry to resolve it, dangle in the R formals)
• no_default on a parameter that is neither match_arg nor choices, or together with default (#1828)
• default or formal on a &Dots parameter

The type-dependent checks of a choice parameter live in classify_choice_param, shared with the method paths.