Stop setting CompletionItem::deprecated

[lnicola]

Closes #2042

We're now using the CompletionItem::tags field to mark CompletionItems as deprecated, and CompletionItem::deprecated is gone from LSP.

[matklad]

Hm, I don't quite understand why :) Reading the linked issue doesn't help. Could you ammend the commit message perhpars to say why, not what? ;)

[lnicola]

Updated (only the description, since bors will copy it into the merge commit anyway).

[matklad]

bors r+

though, I personally still prefer to put info in first-order commit messages -- would work better with blame that way I think

[bors]

Build succeeded:

• Rust (macos-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)

[kjeremy]

This is incorrect. The field isn't gone but deprecated in the protocol. It still needs to be set for older clients.

[lnicola]

But the field still shows up in the response, I assume lsp_types does that?

And we might have other compatibility issues anyway (like in #6779).

[lnicola]

[Trace - 15:47:57] Received response 'completionItem/resolve - (48)' in 1ms.
Result: {
    "label": "foo()",
    "kind": 3,
    "detail": "fn foo()",
    "deprecated": true,
    "filterText": "foo",
    "insertTextFormat": 2,
    "textEdit": {
        "range": {
            "start": {
                "line": 4,
                "character": 4
            },
            "end": {
                "line": 4,
                "character": 6
            }
        },
        "newText": "foo()$0"
    },
    "additionalTextEdits": [],
    "tags": [
        1
    ]
}

[kjeremy]

Both the field (for older) and the tag (newer) should be set. The tag was introduced to the protocol to stop stuffing new info into fields. There are a number of LSP requests that have started to follow this model.

It's deprecated in the sense that newer clients should be looking at the tag set and not the field to determine deprecation status but they should both be filled out. When a new thing is added in the future it will be added to the tag list and not made a field. There shouldn't be any compatibility problems with this approach.