Coverage information is wrong when using tracing

So found an interesting thing when developing cargo-tarpaulin and confirmed it exists also with cargo-llvm-cov and using the raw llvm tools. When #[tracing::instrument] is added to a function the function body is not included in the coverage instrumentation. This doesn't happen with #[tokio::main] and I'm yet to create a more minimal reproduction with a less intense proc macro.

Project setup:

cargo new --lib tracing-instrument
cargo add tracing

In src/lib.rs

use tracing::instrument;

#[instrument]
fn instrumented_function() {
    println!("Hello");
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn is_it_hit() {
        instrumented_function();
    }
}

If you run something like cargo llvm-cov --html on this project you'll see only #[instrument] is marked as covered. If we comment out #[instrument] then the function signature, body and closing brace are marked as covered regions of codes.

This suggests to me there's something a bit wrong with the llvm instrumentation setup and how it interacts with procedural macros.

Start with the minimal reproduction in src/lib.rs and run cargo llvm-cov --html, comparing the instrumented and uninstrumented versions. Then investigate how #[tracing::instrument] interacts with coverage instrumentation, using cargo-tarpaulin, cargo-llvm-cov, and the raw llvm tools as comparison points. Done means the function body receives correct coverage regions without regressing the #[tokio::main] case.