Nix package derivation example

flake.nix

# Fetches the specified version of Odin
odin-fetch = pkgs.stdenv.mkDerivation {
  pname = "odin-source";
  version = odin-version; # references a variable declared earlier in the file
	src = pkgs.fetchurl {
	  url="https://github.com/odin-lang/Odin/releases/download/${odin-version}/odin-linux-amd64-${odin-version}.tar.gz";

		# When you change versions, you should get an error pointing to the correct SHA
		hash = "sha256-zX7CzRqyhAoLfrwY5ctBxnG86HsS8Fb92+8/CAtN3n0=";
};

#nativeBuildInputs = [pkgs.makeWrapper];
#postFixup = ''
	#wrapProgram $out/bin/odin \
	#--set ODIN_ROOT $out/share/odin
#'';
buildInputs = [
  pkgs.llvm
	pkgs.clang
	pkgs.libiconv
];

installPhase = ''
  mkdir -p $out/bin $out/share/odin
	cp odin $out/bin
	cp -r base core vendor $out/share/odin/
'';
};

This shows a basic derivation of how to download a package outside of Nix packages and use it in the rest of your flake.

note that for Odin specifically, you'll need to either add

ODIN_ROOT = "${self.packages.${system}.odin-fetch}/share/odin";

to any derivations that need to utilize Odin or adjust odin-fetch to include

nativeBuildInputs = [pkgs.autoPatchelfHook];

and add a phase

postFixup = ''
  wrapProgram $out/bin/odin \
   --set ODIN_ROOT $out/share/odin
'';