Having problems expanding macros with build.rs

[iquerejeta]

I encountered a problem with the expansion of macros using build.rs. Here is a minimal example to point out the problems I'm having. I have the following lib.rs.

// lib.rs file

macro_rules! number {
    ($name:ident, $number:expr) => {
        #[no_mangle]
        pub extern "C" fn $name () -> u64 {
            $number
        }
    }
}

number!(two, 2);

with the following cbindgen.toml

language = "C"

[parse.expand]
crates = ["testing"]

If I run rustup run nightly cbindgen ./ --config cbindgen.toml --crate testing --output bindings.h, the macros are expanded, and I get the expected file bindings.h.

However, if I try to expand macros and build the binding using a build.rs file, the compilation takes a lot and returns an error at the end of it. The build files I've tried are the following:

// With the builder struct

extern crate cbindgen;
use std::env;

fn main() {
    cbindgen::Builder::new()
        .with_crate(env::var("CARGO_MANIFEST_DIR").unwrap())
        .with_parse_expand(&["testing"])
        .generate()
        .expect("Unable to generate bindings")
        .write_to_file("bindings.h");
}

and

// With the generate function using the same cbindgen.toml as above
extern crate cbindgen;

fn main() {
    cbindgen::generate(std::env::var("CARGO_MANIFEST_DIR").unwrap())
        .expect("Unable to generate bindings")
        .write_to_file("bindings.h");
}

When running RUST_BACKTRACE=1 cargo build I get a ton of lines like this (not copying all because would be huge):

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\n               at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/ops/function.rs:227:5\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

and the final lines of the error are

  stack backtrace:
     0: rust_begin_unwind
               at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/std/src/panicking.rs:498:5
     1: core::panicking::panic_fmt
               at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/panicking.rs:116:14
     2: core::result::unwrap_failed
               at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/result.rs:1690:5
     3: core::result::Result<T,E>::expect
               at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/result.rs:975:23
     4: build_script_build::main
               at ./build.rs:4:5
     5: core::ops::function::FnOnce::call_once
               at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/ops/function.rs:227:5
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Is this a known issue? Or am I maybe doing something wrong?

Thank you!

[emilio]

Which version of cbindgen are you running as a dependency? I think https://github.com/eqrion/cbindgen/pull/747 should've fixed this.

[iquerejeta]

Seems that this error is different:

> cbindgen --version
cbindgen 0.23.0

and in the cargo file

[build-dependencies]
cbindgen = "0.23.0"

Also

> cargo --version
cargo 1.60.0 (d1fd9fe2c 2022-03-01)
> rustc --version
rustc 1.60.0 (7737e0b5c 2022-04-04)

[emilio]

If you can put together a test-case I can look into it.

[iquerejeta]

Test case as in a little repo to reproduce the error? I've put the minimal failing example in this repo https://github.com/iquerejeta/cbindgen-test

Thanks for looking into it!

[dariusc93]

I'm running into the same thing recently. While I have not had an error yet, it does take a long time when using Builder::with_parse_expand. Have not tried with cbindgen and a configuration just yet and will be trying that sometime today.

EDIT: Forgot to update that it works fine with cbindgen cli but just stalls when using it as a build dependency. Havent had time to dive into it to debug it (since i have a workaround by just calling cbindgen directly but is an annoying process) but curious as to why its an issue now unless its something that was introduced into 0.23.

[kczimm]

Anyone have updates on this? I'm running into the same issue on cbindgen = "0.24.3".

[dariusc93]

Anyone have updates on this? I'm running into the same issue on cbindgen = "0.24.3".

Its still an issue the last I checked. I think the solution is to either build with nightly (believe i have tested this but cannot confirm right now) or just run cbindgen directly or in your build.rs

[kczimm]

I've resorted to running cbindgen in the build.rs as so:

Command::new("rustup")
        .arg("run")
        .arg("nightly")
        .arg("cbindgen")
        .arg("./")
        .arg("--config")
        .arg("cbindgen.toml")
        .arg("--crate")
        .arg("testing")
        .arg("--output")
        .arg(bindings)
        .output()
        .expect("failed to execute cbindgen cli");

but even so, the build times are very long (almost 10 minutes) because of this invocation of cbindgen.