Add support for children in Diagnostic object for complex errors

[trixnz]

While working with languages like Rust with rich information in its errors, it's become apparent that not all of this information can be conveyed cleanly with the current Problems window. Take, for example, this error:

warning: this `if` has the same condition as a previous if, #[warn(ifs_same_cond)] on by default
 --> src\main.rs:6:15
  |
6 |     } else if a == b {
  |               ^^^^^^
  |
note: same as this
 --> src\main.rs:4:8
  |
4 |     if a == b {
  |        ^^^^^^
  = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#ifs_same_cond

The note referencing where the first condition was used would be helpful to the user, but we must omit it because the error list is sorted on the Range of the error, resulting in the note being sorted before the actual error:

While the above example doesn't look too ambiguous, when there are several errors spread out through the window, it quickly becomes extremely difficult to figure out what relates to each other.

If Diagnostic had a children: Diagnostic[] field where we could add additional context around the error, we would be able to convey more information to the user about the events that led to the error/warning.

[dbaeumer]

The same as this is IMO not a separate error / problem. The solution to this should be a more complex message instead of a diagnostic tree. @sandy081 what happens with multi line messages today in the problems view.

[sophiajt]

When @nikomatsakis and I were working on the new errors, one of the ways we envisioned them possibly working may be close to what @dbaeumer (also, hi Dirk! long time no see!) is suggesting.

As a first step, I think a diagnostic tree is an okay fix. But I think ideally (and apologies I don't have a mockup so you'll have to visualize), is something like this:

• Errors/Warnings are squiggled, but notes aren't shown
• When you click on an Error/Warn squiggle, you get a popup that is like peek. It shows you the rest of the notes that are important, but otherwise these are hidden

@dbaeumer - does vscode support lightbulbs or additional info notifications? Maybe this is something where the lightbulb could pop up if there are additional notes.

[dbaeumer]

@jonathandturner (Hi Jonathon, indeed long time no see. See you have fun with rust now) Yes, there is support for a lightbulb in VS Code. Have a look at the code action API. This allows you to provide code actions for every position in the document. We then show the light bulb with the corresponding actions to run.

The peak UI for errors is already there. You can pressed F8 in the editor to activate this.

[sandy081]

Long messages are not shown completely in the problems view. A partial message with a ... is shown at the end. Complete message is shown as title (on hover). For more information refer to this feature request - #1927

I would make this a dup of #1927 if this is only about showing multi line messages in the problems view.

[trixnz]

@sandy081 I don't think this issue is necessarily about multi line messages. Consider the following error:

error[E0502]: cannot borrow `x` as immutable because it is also borrowed as mutable
  --> src\main.rs:29:20
   |
27 |     let y = &mut x;
   |                  - mutable borrow occurs here
28 |     *y += 1;
29 |     println!("{}", x);
   |                    ^ immutable borrow occurs here
30 | }
   | - mutable borrow ends here

Presently there doesn't appear (unless I am mistaken) to be a way to add squiggles to lines 27 and 29 without adding a new diagnostic entry for each, which would be ambiguous when there are other errors to look through. As a result, we can only add the cannot borrowxas immutable because it is also borrowed as mutable line to the diagnostic list.

Allowing for children in the tree (or an alternative means) would allow us to highlight the two borrows which would lead back to the original error. As I understand, the lightbulbs are for code actions and wouldn't really fit the context here as there is no action to be taken.

[sandy081]

@trixnz In the above example, is not it enough to add a single diagnostic entry to line 29 providing a detailed message and mentioning where mutable borrow occurred. I do not think we need to add a squiggle or a child entry for line 27. IMO line 27 is a reference that has to be made in diagnostic entry of line 29.