LSP `CodeActionContext.only`: Rust-Analyzer checks still run (synchronously), not filtered out

rust-analyzer version: rust-analyzer version: 0.4.2604-standalone (fa19df2d0e 2025-09-04) [c:\Users\seri\.vscode-insiders\extensions\rust-lang.rust-analyzer-0.4.2604-win32-x64\server\rust-analyzer.exe]

rustc version: rustc 1.91.0-nightly (af00ff2ce 2025-09-04)

editor or extension:

Version: 1.104.0-insider (user setup)
Commit: 06acd067cb9621b055d9701324477cf75fa0e242
Date: 2025-09-05T09:02:10.455Z
Electron: 37.3.1
ElectronBuildId: 12259562
Chromium: 138.0.7204.235
Node.js: 22.18.0
V8: 13.8.258.31-electron.0
OS: Windows_NT x64 10.0.26200

rust-analyzer v0.4.2604 (pre-release)

relevant settings:

Add the following to your vscode settings:

  "editor.codeActionsOnSave": {
    "source.somethingUnrelatedToRustAnalyzer": true
  },

In my real-life case it was

  "editor.codeActionsOnSave": {
    "source.fixAll.eslint": "explicit"
  },

from the ESLint extension.

code snippet to reproduce:

Create an empty lib project and add this single line (likely you need to have x86_64 as default target, or change to target to it)

use core::arch::x86_64::*;

to a file blocks every manual save action (Ctrl+S), in that file, for 1-3 seconds on my system.

• It does not seem to affect autosave on window change ("files.autoSave": "onWindowChange"). In those cases it immediately saves and runs cargo fmt. I think that's a VSCode setting related to codeActionsOnSave though.
• Using VSCode's entension bisect I confirmed that no other extension causes this.

The LSP logs when any codeActionsOnSave are set start with textDocument/codeAction taking ~2.7s:

[Trace - 9:31:15 PM] Sending request 'textDocument/codeAction - (144)'.
Params: {
    "textDocument": {
        "uri": "file:///x%3A/rustprojs/sdfkkdfgh/src/lib.rs"
    },
    "range": {
        "start": {
            "line": 0,
            "character": 0
        },
        "end": {
            "line": 3,
            "character": 0
        }
    },
    "context": {
        "diagnostics": [],
        "only": [
            "source.somethingUnrelatedToRustAnalyzer"
        ],
        "triggerKind": 2
    }
}


[Trace - 9:31:18 PM] Received response 'textDocument/codeAction - (144)' in 2711ms.
Result: []


[Trace - 9:31:18 PM] Sending request 'textDocument/formatting - (145)'.
Params: {
    "textDocument": {
        "uri": "file:///x%3A/rustprojs/sdfkkdfgh/src/lib.rs"
    },
    "options": {
        "tabSize": 4,
        "insertSpaces": true,
        "trimTrailingWhitespace": true
    }
}


[Trace - 9:31:18 PM] Received response 'textDocument/formatting - (145)' in 49ms.
Result: [
    {
        "range": {
            "start": {
                "line": 2,
                "character": 0
            },
            "end": {
                "line": 3,
                "character": 0
            }
        },
        "newText": ""
    }
]

If I remove the unrelated codeActionsOnSave settings entry the 2.7s call is entirely gone:

[Trace - 9:26:59 PM] Sending request 'textDocument/formatting - (50)'.
Params: {
    "textDocument": {
        "uri": "file:///x%3A/rustprojs/sdfkkdfgh/src/lib.rs"
    },
    "options": {
        "tabSize": 4,
        "insertSpaces": true,
        "trimTrailingWhitespace": true
    }
}


[Trace - 9:26:59 PM] Received response 'textDocument/formatting - (50)' in 55ms.
Result: [
    {
        "range": {
            "start": {
                "line": 2,
                "character": 0
            },
            "end": {
                "line": 3,
                "character": 0
            }
        },
        "newText": ""
    }
]

---

I think what's happening is that r-a does not handle CodeActionContext.only (LSP reference) ideally, and runs its own checks synchronously, before allowing to save, even though only being set should mean that the call should be a no-op if only unrelated settings are set.

use core::arch::x86_64::*; might also be an edge case as it imports so many intrinsics functions?

Start by tracing rust-analyzer's textDocument/codeAction handling for requests whose CodeActionContext.only contains an unrelated source action. Reproduce with the empty lib project and use core::arch::x86_64::*;, then compare the LSP timings; done means unrelated code-action requests avoid the synchronous checks and no longer delay manual saves.