core/pkgs/build-support/fetchhg/default.nix

{ lib, stdenvNoCC, mercurial }:
{ name ? null, url, rev ? null, sha256 ? null, hash ? null
, fetchSubrepos ? false, preferLocalBuild ? true }:

if hash != null && sha256 != null then
  throw "Only one of sha256 or hash can be set"
else
# TODO: statically check if mercurial as the https support if the url starts woth https.
  stdenvNoCC.mkDerivation {
    name = "hg-archive" + (lib.optionalString (name != null) "-${name}");
    builder = ./builder.sh;
    nativeBuildInputs = [ mercurial ];

    impureEnvVars = lib.fetchers.proxyImpureEnvVars;

    subrepoClause = lib.optionalString fetchSubrepos "S";

    outputHashAlgo = if hash != null then null else "sha256";
    outputHashMode = "recursive";
    outputHash = if hash != null then
      hash
    else if sha256 != null then
      sha256
    else
      lib.fakeSha256;

    inherit url rev;
    inherit preferLocalBuild;
  }