Add #[track_caller] support to `?`

[yaahc]

Right now if you construct an error via its Into implementation as provided by the blanket implementation in std it gives the following location.

Location:
   /home/jlusby/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:545

We should add #[track_caller] to this trait and possibily some of the Try trait implementations so that errors that attempt to track their construction point can correctly point to user code.

cc @anp

[BGR360]

For reference this idea was mentioned also in #87401 (comment)

[c410-f3r]

Is there anything preventing this addition? If not, then I can create a PR

[BGR360]

@c410-f3r Since my last comment in August, there has been further discussion about this in various places. IIRC, @yaahc resolved to add #[track_caller] to something, I think it was Into? That change might already be in. I'll go hunting in my notification history to see if I can find what I'm talking about.

[BGR360]

It was to from_residual and it has been merged: #91752

Unless it has been since removed again, I think that means this issue can be closed?

[c410-f3r]

Thanks @BGR360

Unfortunately, such change did not prevent an error I am currently facing that also points stuff to ... library/core/src/convert/mod.rs.

But this can be reported in another issue with an appropriated reproducible snippet

[yaahc]

I was worried it would still be broken, I remember in the past it losing the location when it went through Into and I thought I remembered ? desugaring to use Into but it looks like it uses From, so it seems to work fine for my small snippet using eyre.

use thiserror::Error;

fn main() {
    let err = inner().unwrap_err();
    println!("Error: {:?}", err);
}

#[derive(Debug, Error)]
#[error("my error")]
struct MyError;

fn inner() -> eyre::Result<()> {
    Err(MyError)?
}

It might be that original issue I was referring to was just about explicitly calling into in which case you can still reproduce the issue and I get:

Location:
    /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/convert/mod.rs:543:9

Which might be worth fixing, I'm not really sure. It would be a one line change and we'd need to perf test it but I'm not confident it would pass or be something we'd want to merge.

👍 @c410-f3r for opening a new issue for this specific issue. I'm gonna go ahead and close this one for now.