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, 69 insertions, 0 deletions

diff --git a/src/libutil/deprecated-features.hh b/src/libutil/deprecated-features.hh
new file mode 100644
index 000000000..86a9b8a5a
--- /dev/null
+++ b/src/libutil/deprecated-features.hh
@@ -0,0 +1,69 @@
+#pragma once
+///@file
+
+#include "error.hh"
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * The list of available deprecated features.
+ *
+ * If you update this, don’t forget to also change the map defining
+ * their string representation and documentation in the corresponding
+ * `.cc` file as well.
+ *
+ * Reminder: New deprecated features should start out with a warning without throwing an error.
+ * See the developer documentation for details.
+ */
+enum struct DeprecatedFeature
+{
+};
+
+/**
+ * Just because writing `DeprecatedFeature::UrlLiterals` is way too long
+ */
+using Dep = DeprecatedFeature;
+
+/**
+ * Parse a deprecated feature (enum value) from its name. Deprecated
+ * feature flag names are hyphenated and do not contain spaces.
+ */
+const std::optional<DeprecatedFeature> parseDeprecatedFeature(
+        const std::string_view & name);
+
+/**
+ * Show the name of a deprecated feature. This is the opposite of
+ * parseDeprecatedFeature().
+ */
+std::string_view showDeprecatedFeature(const DeprecatedFeature);
+
+/**
+ * Shorthand for `str << showDeprecatedFeature(feature)`.
+ */
+std::ostream & operator<<(
+        std::ostream & str,
+        const DeprecatedFeature & feature);
+
+/**
+ * Parse a set of strings to the corresponding set of deprecated
+ * features, ignoring (but warning for) any unknown feature.
+ */
+std::set<DeprecatedFeature> parseDeprecatedFeatures(const std::set<std::string> &);
+
+/**
+ * A deprecated feature used for some
+ * operation, but was not enabled.
+ */
+class MissingDeprecatedFeature : public Error
+{
+public:
+    /**
+     * The deprecated feature that was required but not enabled.
+     */
+    DeprecatedFeature missingFeature;
+
+    MissingDeprecatedFeature(DeprecatedFeature missingFeature);
+};
+
+}