Macros not included in back-traces when used in a crate

When a macro causes a panic (directly or indirectly) between crates, the line of code from which the macro is used isn't included in the back-trace.

This makes it impossible to know which use of the macro caused the panic.

I ran into this problem because my tests are in a separate directory, and use the code they tests as a crate.

Possibly this is just unsupported feature / known limit in macro system, reporting because I wasn't sure.

Git repository for the example below.

tests/tests.rs

extern crate test_macro;
#[test]
fn main() {
    let file: Vec<u8> = Vec::new();
    test_macro::decode_main(&*file).unwrap();
}

src/lib.rs

use ::std::io;

macro_rules! read_exact {
    ($f:expr, $r:expr) => {
        io::Read::read_exact($f, $r).unwrap()
    }
}

fn decode_blocks<R: io::Read>(mut file: R) -> Result<(), io::Error> {
    let mut bhead: [u8; 320] = [0; 320];
    read_exact!(&mut file, &mut bhead);  // <-- this line isn't in the backtrace
    Ok(())
}

pub fn decode_main<R: io::Read>(mut file: R) -> Result<(), io::Error> {
    decode_blocks(file)?;
    Ok(())
}

Calling:

RUST_BACKTRACE=1 cargo test

Gives the backtrace:

                        at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/result.rs:737
  11:     0x562061ffd7da - test_macro::decode_blocks::h391253710f990761
                        at /src/test_macro/src/lib.rs:5
  12:     0x562061ffd5ca - test_macro::decode_main::h43b3b33b47640ea4
                        at /src/test_macro/src/lib.rs:16
  13:     0x562061fff1ad - tests::main::hc17f25ef21544d0b
                        at /src/test_macro/tests/tests.rs:5
  14:     0x56206200d54e - <F as test::FnBox<T>>::call_box::h04563d623b7ebdf9

Notice line 11 isn't included.

Using Rust stable 1.15.

1. I'm not exactly sure when this was fixed (there's been a lot of backtrace work over the years), but using the latest stable (1.48.0), the backtrace does now include line 11:

      3: core::result::Result<T,E>::unwrap
                at /home/achin/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/result.rs:973
      4: test_macro::decode_blocks
                at ./src/lib.rs:11
      5: test_macro::decode_main
                at ./src/lib.rs:16
      6: tests::main
                at ./tests/tests.rs:5
      7: tests::main::{{closure}}
                at ./tests/tests.rs:3
   

   This issue can be closed

   Andrew Chin at 2020-11-27 19:34:57