🔤 optional human-readable summary/title?

[favonia]

To improve interoperability with Rust-style error messages, I proposed adding an optional title or summary.

Reporter API changes:

• A diagnostic will carry an optional title/summary field.
• For structured reporters, a user has to define a new function default_title/default_summary.
• For both structured and free-form reporters, there will be an optional title/summary argument when appropriate.

Handler changes:

• In TTY, the summary will follow error[code] as error[code]: summary.
• In LSP, the summary will prefix the main message, such as summary: explanation.

I don't know whether summary or title is better. I'm leaning towards summary.

[favonia]

After checking real error messages from the Rust compiler, I feel more negative about this idea. For example, it seems sometimes it's a duplication of the (primary) label:

error[E0277]: `()` is not an iterator
  --> $DIR/conservative_impl_trait.rs:3:33
   |
LL | fn will_ice(something: &u32) -> impl Iterator<Item = &u32> {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not an iterator
   |
   = help: the trait `Iterator` is not implemented for `()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

and very often the text on the primary label is missing:

error: `#[doc(alias("a"))]` expects string literals
  --> $DIR/check-doc-alias-attr.rs:19:13
   |
LL | #[doc(alias(0))]
   |             ^

In either case, they could have just put the full text on the primary label and omit the main message (as we did in asai). I therefore think it's not really useful. I also have not heard any strong support of this feature yet. Closing this as "not planned" until someone complains.