builtins.throw

This function is implemented in c++ and is part of the native nix runtime.

src/libexpr/primops.cc:1000

static RegisterPrimOp primop_throw(
    {.name = "throw",
     .args = {"s"},
     .doc = R"(
      Throw an error message *s*. This usually aborts Nix expression
      evaluation, but in `nix-env -qa` and other commands that try to
      evaluate a set of derivations to get information about those
      derivations, a derivation that throws an error is silently skipped
      (which is not the case for `abort`).
    )",
     .impl = [](EvalState & state, const PosIdx pos, Value ** args, Value & v) {
         NixStringContext context;
         auto s =
             state.coerceToString(pos, *args[0], context, "while evaluating the error message passed to builtin.throw")
                 .toOwned();
         state.error<ThrownError>(s).setIsFromExpr().debugThrow();
     }})