Macro defer_warning 

macro_rules! defer_warning {
    ($($t:tt)*) => { ... };
}

Queue an R warning with rust_warning class layering and return normally; the surrounding guarded call or callback signals it after its value is computed.

Same grammar as crate::warning!: optional call = none (no call, like R’s warning(..., call. = FALSE)), optional class = … (one class or a vector, most specific first), optional data = … (a pair, a bracketed list of pairs, or { name = value } sugar), then the format! message. For a typed payload use crate::defer_warning() with a #[derive(RCondition)] type (#[condition(call = none)] on the type or a variant drops the call there).

A later Err does not cancel this warning. To avoid warnings on failure, queue after the last fallible step; see warning placement.

use miniextendr_api::defer_warning;

#[miniextendr]
fn parse_all(xs: Vec<String>) -> Vec<i32> {
    let (ok, bad): (Vec<_>, Vec<_>) = xs.iter().map(|s| s.parse::<i32>()).partition(Result::is_ok);
    if !bad.is_empty() {
        defer_warning!(class = "pkg_unparsed", data = ("n", bad.len() as i32), "{} values could not be parsed", bad.len());
    }
    ok.into_iter().map(Result::unwrap).collect()
}