## Meta attributes

The `meta` attribute set should always be placed last in the derivativion and any other "meta"-like attribute sets like `passthru` should be written before it.

* `meta.description` must:
  * Be short, just one sentence.
  * Be capitalized.
  * Not start with the definite or an indefinite article.
  * Not start with the package name.
    * More generally, it should not refer to the package name.
  * Not end with a period (or any punctuation for that matter).
  * Provide factual information.
    * Avoid subjective language.
* `meta.license` must be set and match the upstream license.
  * If there is no upstream license, `meta.license` should default to `lib.licenses.unfree`.
  * If in doubt, try to contact the upstream developers for clarification.
* `meta.mainProgram` must be set to the name of the executable which facilitates the primary function or purpose of the package, if there is such an executable in `$bin/bin/` (or `$out/bin/`, if there is no `"bin"` output).
  * Packages that only have a single executable in the applicable directory above should set `meta.mainProgram`. For example, the package `ripgrep` only has a single executable `rg` under `$out/bin/`, so `ripgrep.meta.mainProgram` is set to `"rg"`.
  * Packages like `polkit_gnome` that have no executables in the applicable directory should not set `meta.mainProgram`.
  * Packages like `e2fsprogs` that have multiple executables, none of which can be considered the main program, should not set `meta.mainProgram`.
  * Packages which are not primarily used for a single executable do not need to set `meta.mainProgram`.
  * Always prefer using a hardcoded string (don't use `pname`, for example).
  * When in doubt, ask for reviewer input.
* `meta.maintainers` must be set for new packages.

See the Nixpkgs manual for more details on [standard meta-attributes](https://nixos.org/nixpkgs/manual/#sec-standard-meta-attributes).

## Import From Derivation

[Import From Derivation](https://nixos.org/manual/nix/unstable/language/import-from-derivation) (IFD) is disallowed in Nixpkgs for performance reasons:
[Hydra](https://github.com/NixOS/hydra) evaluates the entire package set, and sequential builds during evaluation would increase evaluation times to become impractical.

Import From Derivation can be worked around in some cases by committing generated intermediate files to version control and reading those instead.

## Sources

Always fetch source files using [Nixpkgs fetchers](https://nixos.org/manual/nixpkgs/unstable/#chap-pkgs-fetchers).
Use reproducible sources with a high degree of availability.
Prefer protocols that support proxies.

A list of schemes for `mirror://` URLs can be found in [`pkgs/build-support/fetchurl/mirrors.nix`](build-support/fetchurl/mirrors.nix), and is supported by [`fetchurl`](https://nixos.org/manual/nixpkgs/unstable/#fetchurl).
Other fetchers which end up relying on `fetchurl` may also support mirroring.

The preferred source hash type is `sha256`.

Examples going from bad to best practices:

- Bad: Uses `git://` which won't be proxied.

  ```nix
  {
    src = fetchgit {
      url = "git://github.com/NixOS/nix.git";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
    };
  }
  ```

- Better: This is ok, but an archive fetch will still be faster.

  ```nix
  {
    src = fetchgit {
      url = "https://github.com/NixOS/nix.git";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
    };
  }
  ```

- Best: Fetches a snapshot archive for the given revision.

  ```nix
  {
    src = fetchFromGitHub {
      owner = "NixOS";
      repo = "nix";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
    };
  }
  ```

> [!Note]
> When fetching from GitHub, always reference revisions by their full commit hash.
> GitHub shares commit hashes among all forks and returns `404 Not Found` when a short commit hash is ambiguous.