CBindgen with (proc-)macros is empty, even when expanding

[Qix-]

The outputted header from cbindgen seems to be consistently false. There are no errors or anything, but it appears the invocation of cargo expand is yielding an empty result. Not quite sure how to debug this one.

I'm running cbindgen from within a build script, like so:

fn main() {
	#[cfg(feature = "header")]
	generate_header();
}

#[cfg(feature = "header")]
fn generate_header() {
	::cbindgen::Builder::new()
		.with_language(::cbindgen::Language::C)
		.with_include_guard("ORO_BOOT_H")
		.with_namespace("oro_boot")
		.with_item_prefix("oro_boot_")
		.with_parse_deps(true)
		.with_parse_expand(&["src/lib.rs"])
		.with_parse_expand_all_features(true)
		.generate()
		.expect("Failed to generate bindings")
		.write_to_file("/tmp/oro_boot.h");
}

However all that gets generated is this:

#ifndef ORO_BOOT_H
#define ORO_BOOT_H

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

#endif  /* ORO_BOOT_H */

The library itself has a bunch of #[repr(C)] pub structs that are defined both using macros as well as standalone, without any macro usage. I'll try to poke around and figure out what's going on but cargo-expand is able to expand these just fine (understanding it's its own binary), so I wonder if it's really an issue with cbindgen or if cargo-expand uses some other mechanism to expand.

---

EDIT: In fact, removing everything but the simple structs (including removing the #[no_std], etc) still results in an empty header.

cargo 1.81.0-nightly (a2b58c3da 2024-07-16)
rustc 1.81.0-nightly (fcc325f1b 2024-07-17)
binary: rustc
commit-hash: fcc325f1bc477975e2ce5ba534fe4c77ff8a8536
commit-date: 2024-07-17
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

EDIT2: Even building with +stable doesn't make any difference.

EDIT3: Running directly via cbindgen CLI works if I just have a single pub const FOO:usize=1234; in there but anything more complex results in an empty output header, too. This is with a config toml being specified along with expand config.

EDIT4: I added a log line that outputs the command line for expanding and it appears to expand just fine. cbindgen isn't using that information though.

---

I originally thought this was a problem with the use of target configs but it wasn't. Original issue here:

Compiling with a custom --target appears to cause cargo expand to fail. Perhaps adding a way to pass additional cargo expand arguments would be helpful here?

  failed to generate header: CargoExpand("test-cbindgen", Compile("error: extra arguments to `rustc` can only be passed to one target, consider filtering\nthe package by passing, e.g., `--lib` or `--bin NAME` to specify a single target\n"))
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/fcc325f1bc477975e2ce5ba534fe4c77ff8a8536/library/std/src/panicking.rs:665:5
     1: core::panicking::panic_fmt
               at /rustc/fcc325f1bc477975e2ce5ba534fe4c77ff8a8536/library/core/src/panicking.rs:74:14
     2: core::result::unwrap_failed
               at /rustc/fcc325f1bc477975e2ce5ba534fe4c77ff8a8536/library/core/src/result.rs:1679:5
     3: core::result::Result<T,E>::expect
               at /rustc/fcc325f1bc477975e2ce5ba534fe4c77ff8a8536/library/core/src/result.rs:1059:23
     4: build_script_build::generate_header
               at ./build.rs:8:17
     5: build_script_build::main
               at ./build.rs:3:2
     6: core::ops::function::FnOnce::call_once
               at /rustc/fcc325f1bc477975e2ce5ba534fe4c77ff8a8536/library/core/src/ops/function.rs:250:5
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Qix-]

Ah. Enum generation isn't even supported :/ Lots of work to do it seems.