example with default R values fails with `rust_function()`

[sgsokol]

I am trying to reproduce an example from your doc https://extendr.rs/user-guide/default-args.html with rust_function() from the last version on rextendr 0.4.2.900 but got an error:

# R version 4.5.2 (2025-10-31) on Ubuntu 24.04

> rust_function('#[extendr]
fn check_default(#[extendr(default = "NULL")] x: Robj) -> bool {
    x.is_null()
}')
ℹ build directory: /tmp/RtmpsaFvP6/file1aa31653369785
Error in `extract_meta()`:
! Rust code contains invalid attribute macros.
• ✖ No valid `fn` or `impl` block found in the following sample:
• #[extendr]
Run `rlang::last_trace()` to see where the error occurred.
> rlang::last_trace()
<error/rextendr_error>
Error in `extract_meta()`:
! Rust code contains invalid attribute macros.
• ✖ No valid `fn` or `impl` block found in the following sample:
• #[extendr]
---
Backtrace:
     ▆
  1. └─rextendr::rust_function("#[extendr]\nfn check_default(#[extendr(default = \"NULL\")] x: Robj) -> bool {\n    x.is_null()\n}")
  2.   └─rextendr::rust_source(...)
  3.     └─rextendr::make_module_macro(code, opts[["module_name"]])
  4.       └─rextendr:::find_exports(sanitize_rust_code(lines))
  5.         ├─dplyr::select(...)
  6.         ├─dplyr::mutate(...)
  7.         ├─dplyr::bind_rows(...)
  8.         │ └─rlang::list2(...)
  9.         ├─rextendr:::discard(...)
 10.         │ └─rextendr:::.rlang_purrr_probe(.x, .p, ...)
 11.         │   └─rextendr:::map_lgl(.x, .p, ...)
 12.         │     └─rextendr:::.rlang_purrr_map_mold(.x, .f, logical(1), ...)
 13.         │       └─base::vapply(.x, .f, .mold, ..., USE.NAMES = FALSE)
 14.         └─rextendr:::map2(start, end, function(.x, .y) extract_meta(clean_lns[.x:.y]))
 15.           └─base::mapply(.f, .x, .y, MoreArgs = list(...), SIMPLIFY = FALSE)
 16.             └─rextendr (local) `<fn>`(dots[[1L]][[1L]], dots[[2L]][[1L]])
 17.               └─rextendr:::extract_meta(clean_lns[.x:.y])
Run rlang::last_trace(drop = FALSE) to see 1 hidden frame.

Any help?

[albersonmiranda]

@sgsokol Thanks for catching that!

rust_function() is slightly different from running a .rs source code, as the example in the guide. Here's a working version:

rust_function('
fn check_default(#[extendr(default = "NULL")] x: Robj) -> bool {
    x.is_null()
}', use_dev_extendr = TRUE)

We recently changed the way to write default arguments and it's still unreleased, so you'll have to add use_dev_extendr = TRUE. Also, the #[extendr] macro is added automatically in rust_function() so you don't need to add it yourself.

[sgsokol]

Great! It works like a charm now.