Improve explicit fields completion in constructor

[MangelMaxime]

Details

type Attributes2() =
    member val FieldId: string = "" with get, set
    member val Label: string = "" with get, set

let x =
    Attributes2(FieldId = "")

Trying to fill the explicit fields between the parentheses is difficult unless you already know what you are looking for.

If you don't fill anything, nothing is proposed but once you start typing La and request a new list then Label is suggested. It would be nice, if explicit fields where always suggested and if possible at the top of the list in this situation because abs, acos functions is probably not what the user is looking for.

CleanShot.2024-10-19.at.17.55.45.mp4

Checklist

• I have looked through existing issues to make sure that this feature has not been requested before
• I have provided a descriptive title for this issue
• I am aware that even valid feature requests may be rejected if they do not align with the project's goals
• I or my company would be willing to contribute this feature

[github-actions]

🤖 This is an automated response from Repo Assist.

Thanks for the detailed report and video! This is a known gap in the completion experience.

Root cause analysis:

The issue is in how FCS's GetDeclarationListInfo handles the case where no partial identifier has been typed yet. When the cursor is at an empty position inside a constructor call (e.g. Attributes2(FieldId = "", ┃)), QuickParse.GetPartialLongNameEx returns an empty partial name. FCS currently doesn't return named-argument completions for an empty partial name in this context — it only returns them once a character is typed and the partial name is non-empty.

Where to fix:

There are two potential approaches:

1. FSAC side (ParseAndCheckResults.TryGetCompletions): When the completion context is inside a constructor/method call with named args and the partial name is empty, FSAC could manually enumerate the remaining unprovided named arguments by:

   • Looking at the current call site via the parse tree
   • Identifying which named parameters are already supplied
   • Injecting synthetic completion items for the remaining ones

2. FCS side (GetDeclarationListInfo): The FCS completion engine could be taught to return named-argument suggestions when the partial name is empty in a named-argument position. This would be a cleaner fix but requires a change upstream in dotnet/fsharp.

The FSAC-side approach is more tractable and has been done for other completion scenarios (e.g. CustomOperation CE keywords). The ParsedInput.TryGetCompletionContext result and the parse tree at the call site could be used to identify remaining named params.

If anyone wants to take this on, the starting point would be ParseAndCheckResults.TryGetCompletions in src/FsAutoComplete.Core/ParseAndCheckResults.fs and the Completion.atPos context detection.

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