lib.fixedPoints.toExtension: init

[ShamrockLee]

Description of changes

This function is taken from stdenv.mkDerivation's implementation of overrideAttrs, and acts as a toFunction analogy for overlays.

Some packages/package sets have an argument in their override interface to take override functions, which might be anything from a plain attribute set to an overlay. To use the override interface while keeping the previous overriding, one would extend it with lib.composeExtensions, which only accepts overlays.

#225051 locally uses a function like this to convert the overrideModAttrs argument to passthru.overrideModAttrs. The former could be anything to pass to .overrideAttrs, but the latter must be an attribute overlay so that people could override with lib.composeExtension.

Other potential use cases include enabling custom overriders such as overridePythonAttrs support overlay-style overriding (drv.overridePythonAttrs (finalAttrs: previousPythonAttrs: { ... })), but that is out of the scope of this PR.

Things done

• Built on platform(s)
  • x86_64-linux (no rebuild)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[ShamrockLee]

Result of nixpkgs-review run on x86_64-linux 1

1 package blacklisted:

• nixos-install-tools

2 packages built:

• disko
• nixpkgs-manual

[roberth]

Great idea!

[roberth]

Have a few suggestions for the docs, but otherwise LGTM!

[doronbehar]

Thanks for this PR, it is a little bit above my league and above my current level of available focus to review it properly, but it looks good and useful. Do you think it'd be possible to use these changes to help with #44426 ?

[roberth]

Thank you @ShamrockLee!

@doronbehar Yes, it seems like a useful function for that, especially if it needs to support both the arity-1 and arity-2 style overriding like overrideAttrs does. After calling this, you always have overlays, and you can work with the functions for overlays to handle them.