The Mysterious Case of `writeln!(std::io::stdout().lock(), “”)` not being Captured by Cargo Test

Are you tired of scratching your head, wondering why your innocent-looking `writeln!` statement refuses to be captured by Cargo test? You’re not alone! In this article, we’ll embark on a thrilling adventure to uncover the reasons behind this curious behavior and provide you with crystal-clear solutions to get your tests back on track.

The Problem: `writeln!` Statements Going Rogue

Imagine this: you’ve written a seemingly straightforward `writeln!` statement in your Rust code, expecting it to be captured by Cargo test. But, much to your surprise, it silently disappears into thin air, leaving you with a befuddling test failure.

use std::io;

fn main() {
    writeln!(std::io::stdout().lock(), "").unwrap();
}

This code looks innocent, doesn’t it? Yet, when you run `cargo test`, the `writeln!` statement seems to be utterly ignored. What’s going on?

Understanding the Root Cause: The Nature of `stdout`

The key to unlocking this mystery lies in understanding how `stdout` works. `stdout` is a file descriptor that represents the standard output stream, which is where your program’s output is sent by default. When you call `writeln!(std::io::stdout().lock(), “”)`, you’re writing to this standard output stream.

Here’s the crucial part: **Cargo test redirects `stdout` to `/dev/null`**. This means that any output written to `stdout` is discarded, and your test won’t capture it. This is done to prevent tests from cluttering the console with unnecessary output.

Solution 1: Using the `stdout` Capture Feature

One way to capture the `writeln!` statement is to use the `stdout` capture feature provided by Cargo test. You can do this by annotating your test function with `#[cfggetAttribute tokio::test::io::capture_stdout)]`.

use tokio::test;

#[cfg_attribute tokio::test::io::capture_stdout)]
#[tokio::test]
async fn my_test() {
    writeln!(std::io::stdout().lock(), "").unwrap();
    assert!(stdout().contains("")); // This will pass!
}

In this example, the `capture_stdout` attribute tells Cargo test to capture the `stdout` output, making it available for assertion.

Solution 2: Redirecting `stdout` to a String

Another approach is to redirect `stdout` to a `String` and then capture the output. You can do this by using the `Redirect` trait from the `std::io` module.

use std::io::{Redirect, Write};

fn my_test() {
    let mut buffer = String::new();
    {
        let stdout = std::io::stdout();
        let mut handle = stdout.lock();
        let mut handle = Redirect::new(handle);

        writeln!(handle, "").unwrap();

        handle.into_inner().write_to_string(&mut buffer).unwrap();
    }

    assert!(buffer.contains("")); // This will pass!
}

In this example, we redirect `stdout` to a `String` buffer using the `Redirect` trait. We then write to the redirected `stdout` using `writeln!`, and finally, we assert that the buffer contains the expected output.

Solution 3: Using a Testing Framework

If you’re using a testing framework like `cargo-test-output` or `assert-stdout`, you can use their built-in features to capture `stdout` output.

use cargo_test_output::{ rust_test, tester };

fn my_test() {
    rust_test::assert_stdout(rust_test::Output::from_iter(vec![ "".to_string() ]), ||
        writeln!(std::io::stdout().lock(), "").unwrap()
    );
}

In this example, we use the `assert_stdout` macro from `cargo-test-output` to capture the `stdout` output and assert that it matches the expected output.

Conclusion

And there you have it! Three elegant solutions to capture the elusive `writeln!(std::io::stdout().lock(), “”)` statement. By redirecting `stdout`, using the `stdout` capture feature, or leveraging a testing framework, you can confidently write tests that capture even the most wayward `writeln!` statements.

Solution	Description
Using the `stdout` Capture Feature	Use the `capture_stdout` attribute to capture `stdout` output.
Redirecting `stdout` to a String	Redirect `stdout` to a `String` buffer using the `Redirect` trait.
Using a Testing Framework	Use a testing framework like `cargo-test-output` or `assert-stdout` to capture `stdout` output.

Remember, with these solutions, you’ll never again be puzzled by the mysterious case of `writeln!(std::io::stdout().lock(), “”)` not being captured by Cargo test.

Happy testing, and may your `stdout` statements never again go rogue!

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