libutil: add a result type using boost outcome

we're using boost::outcome rather than leaf or stl types because stl
types are not available everywhere and leaf does not provide its own
storage for error values, relying on thread-locals and the stack. if
we want to use promises we won't have a stack and would have to wrap
everything into leaf-specific allocating wrappers, so outcome it is.

Change-Id: I35111a1f9ed517e7f12a839e2162b1ba6a993f8f

src/libutil/meson.build

@@ -105,6 +105,7 @@ libutil_headers = files(
   'regex-combinators.hh',
   'regex.hh',
   'repair-flag.hh',
+  'result.hh',
   'serialise.hh',
   'shlex.hh',
   'signals.hh',

src/libutil/result.hh

@@ -0,0 +1,24 @@
+#pragma once
+/// @file
+
+#include <boost/outcome/std_outcome.hpp>
+#include <boost/outcome/std_result.hpp>
+#include <boost/outcome/success_failure.hpp>
+#include <exception>
+
+namespace nix {
+
+template<typename T, typename E = std::exception_ptr>
+using Result = boost::outcome_v2::std_result<T, E>;
+
+template<typename T, typename D, typename E = std::exception_ptr>
+using Outcome = boost::outcome_v2::std_outcome<T, D, E>;
+
+namespace result {
+
+using boost::outcome_v2::success;
+using boost::outcome_v2::failure;
+
+}
+
+}