libexpr: Introduce Deprecated features

They are like experimental features, but opt-in instead of opt-out. They
will allow us to gracefully remove language features. See #437

Change-Id: I9ca04cc48e6926750c4d622c2b229b25cc142c42

1 files changed, 29 insertions, 0 deletions

diff --git a/src/libutil/deprecated-features-json.hh b/src/libutil/deprecated-features-json.hh
new file mode 100644
index 000000000..787be40dc
--- /dev/null
+++ b/src/libutil/deprecated-features-json.hh
@@ -0,0 +1,29 @@
+#pragma once
+///@file
+
+#include "deprecated-features.hh"
+#include "json-utils.hh"
+
+namespace nix {
+
+/**
+ * Compute the documentation of all deprecated features.
+ *
+ * See `doc/manual` for how this information is used.
+ */
+nlohmann::json documentDeprecatedFeatures();
+
+/**
+ * Semi-magic conversion to and from json.
+ * See the nlohmann/json readme for more details.
+ */
+void to_json(nlohmann::json &, const DeprecatedFeature &);
+void from_json(const nlohmann::json &, DeprecatedFeature &);
+
+/**
+ * It is always rendered as a string
+ */
+template<>
+struct json_avoids_null<DeprecatedFeature> : std::true_type {};
+
+};