Add VSCode command to show the `rustc --explain EXXXX` output

[ivangabriele]

Here is a user story example:

1. I use VSCode.
2. I have a compilation (analysis) error.
3. This error has an explanation code (For more information about this error, try `rustc --explain EXXXX`.).
4. I open the command palette and run the command rust-analyzer: Explain.
5. It opens a new tab containing the rustc --explain EXXXX output.

There are other considerations to take into account:

• multiple errors
• (rather?) add it in code actions
• how would that reflect in other IDEs/text editors?

If this feature is acceptable (and once the specs would be agreed upon), I could try to contribute by working on it but I must warn that I'm a beginner in Rust, so that would take some time.

[Veykril]

So we kind of have this already in a sense. In editors other than VSCode if the editor exposes the error link they can already get redirected to the explanation webpage of the error code. In VSCode we overwrite it for the diagnostics view, but I think it would make sense to make the error code in that view clickable and either have that be a link to the explanation page on the rust lang website or maybe show a hover with it.

[bjorn3]

but I think it would make sense to make the error code in that view clickable

Since rust-lang/rust#107838 rustc itself actually does this in terminals supporting it when using -Zterminal-urls.

[ivangabriele]

@Veykril Oh that seem easier to integrate it like that then. From what I understand it would "only" require to touch editors/code TS code to achieve that?

For the code action and/or tooltip link, I wonder if this error code is available as a specific prop in TS reflecting rustc code or if it must be "regexed" out? (I'm looking at the codebase right now to check that).

As for the command palette command, it has the advantage to allow custom keybindings for the end-user. But I have no idea if most other users would find that useful or not.

[Veykril]

We can also have both, that is a command as you described as well as the link in the diagnostics view.

Both of these things can be fully done in the client only, so no need to touch the server for this. For the command you'd need to fetch the current diagnostics and filter for the ones that apply to the cursor position, then you can probably extract the error code (link) from there.

[ivangabriele]

@Veykril That's what I guessed looking at the code, I'll try to make a first poke locally to see if I'm able to make that work.

[ivangabriele]

@Veykril I have an issue:

When I debug this code :

fn main() {
    let a = "a";

    a = "b";
}

the cli debugger output this error with an explain link:

error[E0384]: cannot assign twice to immutable variable `a`
 --> src/main.rs:4:5
  |
2 |     let a = "a";
  |         -
  |         |
  |         first assignment to `a`
  |         help: consider making this binding mutable: `mut a`
3 |
4 |     a = "b";
  |     ^^^^^^^ cannot assign twice to immutable variable

For more information about this error, try `rustc --explain E0384`.

but when I log the diagnostic in handleDiagnostics() (client.ts file), I receive this object:

ProtocolDiagnostic {
  range: n { c: p { c: 3, e: 4 }, e: p { c: 3, e: 11 } },
  message: 'cannot mutate immutable variable `a`',
  severity: 0,
  data: undefined,
  hasDiagnosticCode: false,
  code: {
    value: 'need-mut',
    target: f {
      scheme: 'https',
      authority: 'rust-analyzer.github.io',
      path: '/manual.html',
      query: '',
      fragment: 'need-mut',
      _formatted: null,
      _fsPath: null
    }
  },
  source: 'rust-analyzer'
}

I don't have any error link there. I thought errorCode was something but it's false (I may suggest to rename that to hasErrorCore in my PR).

I also wonder if the server uses the user rustc version or an custom version since even the message is different. Or maybe original errors are transformed somewhere?

[bjorn3]

That diagnostic looks like it comes from rust-analyzer itself (source is rust-analyzer, not rustc). There should be a second diagnostic coming from rustc if you saved.

[lnicola]

You can see it here:

        {
            "range": {
                "start": {
                    "line": 2,
                    "character": 4
                },
                "end": {
                    "line": 2,
                    "character": 11
                }
            },
            "severity": 1,
            "code": "E0384",
            "codeDescription": {
                "href": "https://doc.rust-lang.org/error-index.html#E0384"
            },
            "source": "rustc",
            "message": "cannot assign twice to immutable variable `a`\ncannot assign twice to immutable variable",
            "relatedInformation": [
                {
                    "location": {
                        "uri": "file:///home/grayshade/Projects/hello/src/main.rs",
                        "range": {
                            "start": {
                                "line": 1,
                                "character": 8
                            },
                            "end": {
                                "line": 1,
                                "character": 9
                            }
                        }
                    },
                    "message": "first assignment to `a`"
                },
                {
                    "location": {
                        "uri": "file:///home/grayshade/Projects/hello/src/main.rs",
                        "range": {
                            "start": {
                                "line": 1,
                                "character": 8
                            },
                            "end": {
                                "line": 1,
                                "character": 9
                            }
                        }
                    },
                    "message": "consider making this binding mutable: `mut a`"
                }
            ],
            "data": {
                "rendered": "\u001b[0m\u001b[1m\u001b[38;5;9merror[E0384]\u001b[0m\u001b[0m\u001b[1m: cannot assign twice to immutable variable `a`\u001b[0m\n\u001b[0m \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m--> \u001b[0m\u001b[0msrc/main.rs:3:5\u001b[0m\n\u001b[0m  \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m|\u001b[0m\n\u001b[0m\u001b[1m\u001b[38;5;12m2\u001b[0m\u001b[0m \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m|\u001b[0m\u001b[0m \u001b[0m\u001b[0m    let a = \"a\";\u001b[0m\n\u001b[0m  \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m| \u001b[0m\u001b[0m        \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m-\u001b[0m\n\u001b[0m  \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m| \u001b[0m\u001b[0m        \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m|\u001b[0m\n\u001b[0m  \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m| \u001b[0m\u001b[0m        \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12mfirst assignment to `a`\u001b[0m\n\u001b[0m  \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m| \u001b[0m\u001b[0m        \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12mhelp: consider making this binding mutable: `mut a`\u001b[0m\n\u001b[0m\u001b[1m\u001b[38;5;12m3\u001b[0m\u001b[0m \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m|\u001b[0m\u001b[0m \u001b[0m\u001b[0m    a = \"b\";\u001b[0m\n\u001b[0m  \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;12m| \u001b[0m\u001b[0m    \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;9m^^^^^^^\u001b[0m\u001b[0m \u001b[0m\u001b[0m\u001b[1m\u001b[38;5;9mcannot assign twice to immutable variable\u001b[0m\n\n"
            }
        },

The compiler even gives us the explanation message, but there's no place for us to put it (data isn't really designed for this).

[ivangabriele]

@lnicola How did you log this (I saw the data.rendered is used when it exists in handleDiagnostics() iterator).

[lnicola]

"rust-analyzer.trace.server": "verbose" in the Code settings.

[ivangabriele]

I think I will need some help to know how to pass/access this data in the client.