feat: repeated keys

[Malix-Labs]

Is your feature request related to a problem? Please describe.

Currently, there is no report of repeated keys

e.g.

{
  a.b = true;
  a.c = true;
  a = { d = true; };
  a = {
    e.f = true;
    e.g = true;
  };
}

Which, grouped, would be

{
  a = {
    b = true;
    c = true;
    d = true;
    e = { 
      f = true;
      g = true;
    };
  };
}

Describe the solution you'd like

Raising a report for repeated keys

Taking in its scope only the selection, or otherwise the current file / module

The linked code action fix for it would be #831

Describe alternatives you've considered

statix

statix list
"W20 repeated_keys"

Additional context

• Linked : Code Action - Attribute Grouping
  • Code Action: Attributes Grouping #831
• Related : Code Action - Attribute Packing
  • Implementation
    • https://github.com/nix-community/nixd/blob/main/nixd/lib/Controller/CodeActions/PackAttrs.h
    • https://github.com/nix-community/nixd/blob/main/nixd/lib/Controller/CodeActions/PackAttrs.cpp
  • Testing
    • https://github.com/nix-community/nixd/tree/main/nixd/tools/nixd/test/code-action/pack-attrs
  • Feature Request
    • Code Action: Attributes Packing - Context Awareness #830

[inclyc]

Introducing a warning in an LSP definitely requires careful consideration. As I see it, the Nix language already supports attrset merging, and the official writing style—both in configuration.nix and across the wiki—widely uses this syntax. So I’m wondering: what are the actual real‑world scenarios or common pitfalls that would make this warning truly necessary?

[Malix-Labs]

@inclyc I did not suggest a severity, but it might either be a DS_Warning, DS_Info, or DS_Hint

nixd/libnixf/include/nixf/Basic/Diagnostic.h

Lines 145 to 168 in 0e07c08

	/// The super class for all diagnostics.
	/// concret diagnostic types are defined in Diagnostic*.inc
	class Diagnostic : public PartialDiagnostic {
	public:
	/// Each diagnostic contains a severity field,
	/// should be "Fatal", "Error" or "Warning"
	/// this will affect the eval process.
	///
	/// "Warning" -- just a warning.
	enum Severity {
	/// shouldn't eval the code, e.g. parsing error.
	DS_Fatal,
	/// trigger an error in nix, but we can recover & eval the code.
	DS_Error,
	/// A warning.
	DS_Warning,
	/// An information.
	DS_Info,
	/// A hint. Hints are usually not rendered directly in some editor GUI
	/// So this is suitable for liveness analysis results.
	/// For example, "unused xxx"
	DS_Hint,
	};

Repeated keys are valid nix of course, but can be a minor linting issue (suggesting DS_Warning) for readability and localization (allows scattering) concerns

statix has it as :

nix run nixpkgs#statix list
"W20 repeated_keys"

And it seems to raise an equivalent DS_Warning value

[inclyc]

To clarify my point: I'm wondering why we should report this (or encourage users to group attribute sets) at all. As I mentioned, this seems to be a common pattern that appears widely on many official pages. I don't think that just because statix does something, it necessarily means nixd should follow suit. However, I'm open to being convinced if there is strong evidence that this practice is problematic—for example, if it's error-prone, confusing, or deprecated. Otherwise, I'd prefer not to introduce it here.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/linting-repeated-or-grouped-keys-polls/78464/1

[Malix-Labs]

It might be error-prone or confusing in the way that it allows scattered related config instead of grouped ones

It be undesirable, and also a personal preference to avoid

For instance, personally and for the people I work with, we want keys to be repeated as little as possible

I know some other community members also have that preference, but I wouldn't know how to gauge nor if I'm biased, so I created : a community poll in the NixOS discourse

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/linting-repeated-or-grouped-keys-polls/78464/3

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/linting-repeated-or-grouped-keys-polls/78464/7