Function aliases_placeholder 

pub(crate) fn aliases_placeholder(c_ident: &str, r_param: &str) -> String

R-side placeholder for the trailing aliases = argument of a match_arg parameter’s check (#1843), written as , <placeholder> after the other arguments. The wrapper writer replaces , <placeholder> with , aliases = base::c("grey" = "gray"), or with nothing when the type has no aliases (registry::match_arg_aliases_arg), from the same MX_MATCH_ARG_CHOICES entry as choices_placeholder. The macro can’t tell which: MatchArg::ALIASES resolves at link time, as CHOICES does.