update to new Rust error format and/or move to json solely

[oli-obk]

We have 17 days until the old error format won't even be in stable anymore.

I'd be happy with moving to json entirely, it still has some quirks, but we could circumvent them for now by showing the json field that contains the full message in the popups.

[alygin]

I agree that it's time to switch to JSON as the default option.

At the same time, I personally would prefer to keep the possibility of switching to the standard format even though the new version is not parser-friendly anymore. I like the human readable output. Maybe it's just a matter of habit, but there might be people who feels the same.

I would break the issue into the following subtasks (from the highest priority to the lowest):

1. Make JSON the default output format.
2. Fix the known problems in the JSON parser.
3. Implement a parser for the new format.
4. Keep the old format (the one that we have now) parser too for those who doesn't switch to the new Rust versions quickly. Switching between the new and the old formats must be automatic.

What do you think?

I could certainly do 1 and 2 and then give a try to 3 and 4. But I know nothing about the quirks of the JSON parser because I haven't been using it much. @oli-obk, could you, please, amplify on this? Maybe give an example?

[oli-obk]

I don't have any examples off hand, but I often need to read the json in the build window to grok what the error popup is telling me. We don't display all information.

[alygin]

Last time when I tweaked things in the JSON parser and tested it on various examples, I saw a lot of duplication in JSON messages. Some nodes convey the same information but use different wording. For instance, here's the JSON for the simple "mismatched types" error:

    {
        "message": "mismatched types",
        "code": {
            "code": "E0308",
            "explanation": "\nThis error occurs when the compiler was unable ..."
        },
        "level": "error",
        "spans": [{
                "file_name": "src/main.rs",
                "byte_start": 110,
                "byte_end": 115,
                "line_start": 11,
                "line_end": 11,
                "column_start": 9,
                "column_end": 14,
                "is_primary": true,
                "text": [{
                        "text": "    b = 43i32;",
                        "highlight_start": 9,
                        "highlight_end": 14
                }],
                "label": "expected u32, found i32",
                "suggested_replacement": null,
                "expansion": null
        }],
        "children": [{
                "message": "expected type `u32`",
                "code": null,
                "level": "note",
                "spans": [],
                "children": [],
                "rendered": null
            }, {
                "message": "   found type `i32`",
                "code": null,
                    "level": "note",
                "spans": [],
                "children": [],
                "rendered": null
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        "rendered": null
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Such error is now displayed this way:

It will be better if we add the message from the span to the main message (it will become "mismatched types: expected u32, found i32") so sometimes there will be no need to expand the message. But we already have this information in the Notes. So, actually, no useful information is lost, it just is not duplicated as it is in the JSON.

All other examples I encountered were pretty similar to this one. If you come across some error message with a useful information completely removed in the lint, please, post it here.

[alygin]

Errors in macros are tricky ones. They repeat the definition of the macro several times. I'm not sure if all this stuff is really needed to be displayed.

Below is an example produced by println!("{}");. The only suggestion I have is to add the text of the span to the main error message as a separate line. So the main error lint in this case will be "invalid reference to argument 0 (no arguments given)" and contain an additional line

print ! ( concat ! ( $ fmt , \"\\n\" ) , $ ( $ arg ) * ) ) ;

It will help when the message itself doesn't say enough to figure out the error cause.

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    "code": null,
    "level": "error",
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            "byte_end": 3981,
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            "line_end": 3,
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