libutil: allow marking settings as deprecated

this is a bit of a hack, but it's apparently the cleanest way of doing
this in the absence of any kind of priority/provenance information for
values of some given setting. we'll need this to deprecate build-hook.

Change-Id: I03644a9c3f17681c052ecdc610b4f1301266ab9e

src/libutil/config-impl.hh

@@ -66,9 +66,13 @@ void BaseSetting<T>::appendOrSet(T newValue, bool append)
 template<typename T>
 void BaseSetting<T>::set(const std::string & str, bool append)
 {
-    if (experimentalFeatureSettings.isEnabled(experimentalFeature))
+    if (experimentalFeatureSettings.isEnabled(experimentalFeature)) {
-        appendOrSet(parse(str), append);
+        auto parsed = parse(str);
-    else {
+        if (deprecated && (append || parsed != value)) {
+            warn("deprecated setting '%s' found (set to '%s')", name, str);
+        }
+        appendOrSet(std::move(parsed), append);
+    } else {
         assert(experimentalFeature);
         warn("Ignoring setting '%s' because experimental feature '%s' is not enabled",
             name,

src/libutil/config.hh

@@ -175,6 +175,10 @@ class AbstractSetting
 
 public:
 
+    struct deprecated_t {
+        explicit deprecated_t() = default;
+    };
+
     const std::string name;
     const std::string description;
     const std::set<std::string> aliases;
@@ -225,6 +229,7 @@ protected:
     T value;
     const T defaultValue;
     const bool documentDefault;
+    const bool deprecated;
 
     /**
      * Parse the string into a `T`.
@@ -250,11 +255,13 @@ public:
         const std::string & name,
         const std::string & description,
         const std::set<std::string> & aliases = {},
-        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt)
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt,
+        bool deprecated = false)
         : AbstractSetting(name, description, aliases, experimentalFeature)
         , value(def)
         , defaultValue(def)
         , documentDefault(documentDefault)
+        , deprecated(deprecated)
     { }
 
     operator const T &() const { return value; }
@@ -322,12 +329,25 @@ public:
         const std::string & description,
         const std::set<std::string> & aliases = {},
         const bool documentDefault = true,
-        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt)
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt,
-        : BaseSetting<T>(def, documentDefault, name, description, aliases, std::move(experimentalFeature))
+        bool deprecated = false)
+        : BaseSetting<T>(def, documentDefault, name, description, aliases, std::move(experimentalFeature), deprecated)
     {
         options->addSetting(this);
     }
 
+    Setting(AbstractSetting::deprecated_t,
+        Config * options,
+        const T & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {},
+        const bool documentDefault = true,
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt)
+        : Setting(options, def, name, description, aliases, documentDefault, std::move(experimentalFeature), true)
+    {
+    }
+
     void operator =(const T & v) { this->assign(v); }
 };