error-stack: Why does error_stack::Report::frames() produce a breadth-first iterator?

[mkatychev]

Given the rust code below:

use error_stack::Report;

#[derive(Debug, thiserror::Error)]
#[error("A")]
pub struct AError;

#[derive(Debug, thiserror::Error)]
#[error("C")]
pub struct CAttachment;

#[derive(Debug, thiserror::Error)]
#[error("D")]
pub struct DAttachment;

fn main() -> Result<(), Report<AError>> {
    let report = Report::new(CAttachment)
        .attach_printable(Report::new(DAttachment).attach_printable("E"))
        .change_context(AError)
        .attach_printable("B");

    for frame in report.frames() {
        println!("{frame:?}");
    }
    println!();
    Err(report)
}

The output below is given:

Frame { attachment: "B" }
Frame { .. }
Frame { context: AError }
Frame { attachment: D
├╴at src/lib.rs:17:27
╰╴E }
Frame { .. }
Frame { context: CAttachment }

Error: A
├╴at src/lib.rs:18:10
├╴B
│
╰─▶ C
    ├╴at src/lib.rs:16:18
    ╰╴D

Why does the Display on Report<AError> produce the right Depth-first ordering inline with the error_stack::iter::Frames documentation:

A
├╴...
├╴B
│
╰─▶ C
    ├╴...
    ├╴D
    ╰ E (hidden)

While calling Report::frames()::iter produces a breadth first iteration:

B
A
D
E
C

[TimDiekmann]

This looks like a bug to me, cc @indietyp. Thank you for reporting this!

[indietyp]

That's interesting, especially considering that we're testing exactly that case: https://github.com/hashintel/hash/blob/main/libs/error-stack/tests/test_iter.rs#L23-L65

Will try to figure out what is causing the divergence.

[indietyp]

Okay, I think I have an answer, this is quite peculiar, but basically what's happening is that you're constructing:

    let report = Report::new(CAttachment)
        .attach_printable(Report::new(DAttachment).attach_printable("E"))
        .change_context(AError)
        .attach_printable("B");

leads to the following tree:

B
|
A
|
D
|
C

So the output will be: B A D C, you are not using a context, so when attaching a printable you are basically attaching D\nE, which leads to:

B
A
D
E
C

[indietyp]

To attach a report "correctly", you would need to use extend() or extend_one()

[TimDiekmann]

Thanks, @indietyp, I also just figured out why the behavior is as it is 😅 🚀

[mkatychev]

@indietyp do you mind showing the let report above using extend()? That will allow me to mark a comment as accepted answer.

[indietyp]

Sure! Sorry for the long wait. I kind of forgot to answer 😅

use error_stack::Report;

#[derive(Debug, thiserror::Error)]
#[error("A")]
pub struct AError;

#[derive(Debug, thiserror::Error)]
#[error("C")]
pub struct CAttachment;

#[derive(Debug, thiserror::Error)]
#[error("D")]
pub struct DAttachment;

fn main() -> Result<(), Report<AError>> {
    let mut report = Report::new(CAttachment);
	report.extend_one(Report::new(DAttachment).attach_printable("E"));
	
    let report = report
        .change_context(AError)
        .attach_printable("B");

    for frame in report.frames() {
        println!("{frame:?}");
    }
    println!();
    Err(report)
}

This should fix the problem (notice the call to extend_one)

[mkatychev]

Appreciate the follow-up