Publish diagnostics sends partial or empty information causing markup blinking

After a didSave request is issued, Rust Analyzer often sends multiple publishDiagnostics notifications in a row, with the first one containing no diagnostics, then maybe some with a few diagnostics and finally one with all the available diagnostics. The first notification with empty diagnostics causes editors such as VS Code to remove the markup and then add it again causing some blinking behaviour if you have autoSave enabled.

Here is an example in VS Code:

Looking at the server trace, it's possible to see the notifications with empty diagnostics. For example:

Trace - 11:46:11 AM] Sending notification 'textDocument/didSave'.
Params: {
    "textDocument": {
        "uri": "file:///Users/manuel.ceron/code/sandbox/rtest/src/main.rs"
    }
}


[Trace - 11:46:11 AM] Received request 'window/workDoneProgress/create - (25)'.
Params: {
    "token": "rustAnalyzer/cargo check"
}


[Trace - 11:46:11 AM] Sending response 'window/workDoneProgress/create - (25)'. Processing request took 0ms
No result returned.


[Trace - 11:46:11 AM] Received notification '$/progress'.
Params: {
    "token": "rustAnalyzer/cargo check",
    "value": {
        "kind": "begin",
        "title": "cargo check"
    }
}


[Trace - 11:46:11 AM] Received notification 'textDocument/publishDiagnostics'.
Params: {
    "uri": "file:///Users/manuel.ceron/code/sandbox/rtest/src/main.rs",
    "diagnostics": [],
    "version": 1
}


[Trace - 11:46:12 AM] Received notification 'textDocument/publishDiagnostics'.
Params: {
    "uri": "file:///Users/manuel.ceron/code/sandbox/rtest/src/main.rs",
    "diagnostics": [
        {
            "range": {
                "start": {
                    "line": 5,
                    "character": 3
                },
                "end": {
                    "line": 5,
                    "character": 6
                }
            },
            "severity": 2,
            "code": "dead_code",
            "source": "rustc",
            "message": "function is never used: `foo`\n`#[warn(dead_code)]` on by default",
            "tags": [
                1
            ]
        },
        {
            "range": {
                "start": {
                    "line": 8,
                    "character": 3
                },
                "end": {
                    "line": 8,
                    "character": 6
                }
            },
            "severity": 2,
            "code": "dead_code",
            "source": "rustc",
            "message": "function is never used: `bar`",
            "tags": [
                1
            ]
        }
    ],
    "version": 1
}

This was tested on rust-analyzer release from 2021-05-17

Reproduce the didSave sequence in VS Code and inspect how rust-analyzer emits textDocument/publishDiagnostics notifications. Compare the empty, partial, and final notifications in the server trace; done means editors no longer receive an unnecessary empty or incomplete update that causes markup to blink.