fmt::Subscriber's output is not captured by cargo test

[e00E]

cargo test captures test Stdout and Stderr output so that it can print only the output of failing tests after all tests have been run. This also prevents tests run in parallel from mixing their output.

fmt::Subscriber can output to Stdout and Stderr but this output is not captured by cargo test. This is annoying because of all the reasons cargo test wants to capture the output.

fn fn_print_and_log() {
    println!("print1");
    tracing::info!("trace1");
    println!("print2");
    tracing::info!("trace2");
}

#[test]
fn test1() {
    tracing_subscriber::fmt::fmt().init();
    fn_print_and_log();
}

$cargo test test1
running 1 test
2022-12-02T13:13:28.521203Z  INFO a: trace
2022-12-02T13:13:28.521235Z  INFO a: trace
test test1 ... ok

$cargo test test1 -- --nocapture
running 1 test
print1
2022-12-02T13:13:55.715215Z  INFO a: trace
print2
2022-12-02T13:13:55.715242Z  INFO a: trace
test test1 ... ok

This happens because output written directly to std::io::stdout() is not captured which is how fmt::Subscriber is implemented.

A stable but ugly solution I came up with is:

struct S;
impl<'a> tracing_subscriber::fmt::MakeWriter<'a> for S {
    type Writer = W;

    fn make_writer(&'a self) -> Self::Writer {
        W
    }
}

struct W;
impl std::io::Write for W {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        // assume fmt::Subscriber only writes utf8 strings
        let string = std::str::from_utf8(buf).unwrap();
        print!("{}", string);
        Ok(buf.len())
    }

    fn flush(&mut self) -> std::io::Result<()> {
        // `print!` already flushes
        Ok(())
    }
}

#[test]
fn test2() {
    tracing_subscriber::fmt::fmt().with_writer(S).init();
    fn_print_and_log();
}

It is ugly because fmt::Subscriber only outputs strings which we convert to bytes for Write, convert back to strings to use print!, convert back to bytes in print!.

I would like this to be fixed in fmt::Subscriber but this this solution feels ugly enough that it shouldn't be used. If someone had an idea for a better solution I would be open to making a PR with it.

[davidbarsky]

Sorry for the delay in responding. This is already handled by https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Layer.html#method.with_test_writer.

[e00E]

The apology is all mine for missing this in the docs. I was searching for issues related to this but didn't find any because you implemented this 2 years ago. You use the same implementation as I came up with so there is no need for any changes.