Rendering: Provide some way to show multiple nested errors (or display related as nested)

[elkowar]

Currently, the related errors of a given report will show just one after another, without any clear indication of them belonging tightly to the outer error.

I.e., in my project, right now I have errors like these:

Which are supposed to express that the two related errors are actually part of the outer error -- clearly not quite the intended use for related.

I can get closer to a more intuitive visualization by storing the related errors in a simple struct ErrList(#[related] Vec<Report>):

However, this now requires me to have another error message be displayed, which often doesn't add any semantic value, and takes up a lot of space.

For cases like these, where I really want to show several siblings as nested within one outer error, I'd optimally want to have them be displayed roughly like this:

My proposal here would be to add a variant of "related" errors that explicitly declare them as nested, i.e.:

#[derive(Diagnostic, Error, Debug)]
#[error("{message}")]
#[diagnostic]
struct ManyErrors {
  message: String
  #[related(nested)]
  errors: Vec<Report>
}

[zkat]

This is a reasonable proposal! I’d be happy to take this change :)

[elkowar]

Nice! I'd love to take a stab at this, but I'm a bit unsure on how to approach an implementation here:

If the idea is to actually have this as a property of the related attribute, then I don't know how to express that within the current structure of the Diagnostic trait.
Alternatively, I could just add a separate nested field to the Diagnostic trait, which would potentially feel cleaner, but is also a much larger change to the API.
A third, minimally invasive, approach would be to just make the way related errors are shown a global setting in the GraphicalReportHandler.

What do you think, @zkat? Or is there something else I'm missing?

[zkat]

Probably a configuration in the GraphicalReportHandler is probably best, esp since this is all about controlling how things look, rather than some fundamental property about the relationship between diagnostics