core/pkgs/by-name/li/libiconv/packages.nix

{ ... }:
res: pkgs: super:

with pkgs; {
  # GNU libc provides libiconv so systems with glibc don't need to
  # build libiconv separately. Additionally, Apple forked/repackaged
  # libiconv so we use that instead of the vanilla version on that OS,
  # and BSDs include libiconv in libc.
  #
  # We also provide `libiconvReal`, which will always be a standalone libiconv,
  # just in case you want it regardless of platform.
  libiconv = if lib.elem stdenv.hostPlatform.libc [
    "glibc"
    "musl"
    "nblibc"
    "wasilibc"
  ] then
    libcIconv (if stdenv.hostPlatform != stdenv.buildPlatform then
      libcCross
    else
      stdenv.cc.libc)
    # else if stdenv.hostPlatform.isDarwin
    #   then darwin.libiconv
  else
    libiconvReal;

  libcIconv = libc:
    let
      inherit (libc) pname version;
      libcDev = lib.getDev libc;
    in runCommand "${pname}-iconv-${version}" { strictDeps = true; } ''
      mkdir -p $out/include
      ln -sv ${libcDev}/include/iconv.h $out/include
    '';

  libiconvReal = callPackage ./. { };

  iconv = if lib.elem stdenv.hostPlatform.libc [ "glibc" "musl" ] then
    lib.getBin stdenv.cc.libc
  else if stdenv.hostPlatform.isDarwin then
    lib.getBin darwin.libiconv
  else
    lib.getBin libiconvReal;
}