Use transparent comparators for std::set<std::string> (NFC)

This patch finally applies the transition to std::less<>,
which is a transparent comparator. There's no functional
change and string lookups in sets are now more efficient
and don't produce temporaries (e.g. set.find(std::string_view{"key"})).

src/libstore/include/nix/store/common-protocol-impl.hh

@@ -25,11 +25,11 @@ namespace nix {
         LengthPrefixedProtoHelper<CommonProto, T >::write(store, conn, t); \
     }
 
+#define COMMA_ ,
 COMMON_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::vector<T>)
-COMMON_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::set<T>)
+COMMON_USE_LENGTH_PREFIX_SERIALISER(template<typename T COMMA_ typename Compare>, std::set<T COMMA_ Compare>)
 COMMON_USE_LENGTH_PREFIX_SERIALISER(template<typename... Ts>, std::tuple<Ts...>)
 
-#define COMMA_ ,
 COMMON_USE_LENGTH_PREFIX_SERIALISER(
     template<typename K COMMA_ typename V>,
     std::map<K COMMA_ V>)

src/libstore/include/nix/store/common-protocol.hh

@@ -72,14 +72,14 @@ DECLARE_COMMON_SERIALISER(DrvOutput);
 template<>
 DECLARE_COMMON_SERIALISER(Realisation);
 
+#define COMMA_ ,
 template<typename T>
 DECLARE_COMMON_SERIALISER(std::vector<T>);
-template<typename T>
-DECLARE_COMMON_SERIALISER(std::set<T>);
+template<typename T, typename Compare>
+DECLARE_COMMON_SERIALISER(std::set<T COMMA_ Compare>);
 template<typename... Ts>
 DECLARE_COMMON_SERIALISER(std::tuple<Ts...>);
 
-#define COMMA_ ,
 template<typename K, typename V>
 DECLARE_COMMON_SERIALISER(std::map<K COMMA_ V>);
 #undef COMMA_

src/libstore/include/nix/store/derivations.hh

@@ -343,7 +343,7 @@ struct Derivation : BasicDerivation
     /**
      * inputs that are sub-derivations
      */
-    DerivedPathMap<std::set<OutputName>> inputDrvs;
+    DerivedPathMap<std::set<OutputName, std::less<>>> inputDrvs;
 
     /**
      * Print a derivation.

src/libstore/include/nix/store/length-prefixed-protocol-helper.hh

@@ -52,8 +52,10 @@ struct LengthPrefixedProtoHelper;
 template<class Inner, typename T>
 LENGTH_PREFIXED_PROTO_HELPER(Inner, std::vector<T>);
 
-template<class Inner, typename T>
-LENGTH_PREFIXED_PROTO_HELPER(Inner, std::set<T>);
+#define COMMA_ ,
+template<class Inner, typename T, typename Compare>
+LENGTH_PREFIXED_PROTO_HELPER(Inner, std::set<T COMMA_ Compare>);
+#undef COMMA_
 
 template<class Inner, typename... Ts>
 LENGTH_PREFIXED_PROTO_HELPER(Inner, std::tuple<Ts...>);
@@ -86,12 +88,11 @@ LengthPrefixedProtoHelper<Inner, std::vector<T>>::write(
     }
 }
 
-template<class Inner, typename T>
-std::set<T>
-LengthPrefixedProtoHelper<Inner, std::set<T>>::read(
+template<class Inner, typename T, typename Compare>
+std::set<T, Compare> LengthPrefixedProtoHelper<Inner, std::set<T, Compare>>::read(
     const StoreDirConfig & store, typename Inner::ReadConn conn)
 {
-    std::set<T> resSet;
+    std::set<T, Compare> resSet;
     auto size = readNum<size_t>(conn.from);
     while (size--) {
         resSet.insert(S<T>::read(store, conn));
@@ -99,10 +100,9 @@ LengthPrefixedProtoHelper<Inner, std::set<T>>::read(
     return resSet;
 }
 
-template<class Inner, typename T>
-void
-LengthPrefixedProtoHelper<Inner, std::set<T>>::write(
-    const StoreDirConfig & store, typename Inner::WriteConn conn, const std::set<T> & resSet)
+template<class Inner, typename T, typename Compare>
+void LengthPrefixedProtoHelper<Inner, std::set<T, Compare>>::write(
+    const StoreDirConfig & store, typename Inner::WriteConn conn, const std::set<T, Compare> & resSet)
 {
     conn.to << resSet.size();
     for (auto & key : resSet) {

src/libstore/include/nix/store/outputs-spec.hh

@@ -27,20 +27,24 @@ struct OutputsSpec {
     /**
      * A non-empty set of outputs, specified by name
      */
-    struct Names : std::set<OutputName> {
-        using std::set<OutputName>::set;
+    struct Names : std::set<OutputName, std::less<>> {
+    private:
+        using BaseType = std::set<OutputName, std::less<>>;
+
+    public:
+        using BaseType::BaseType;
 
         /* These need to be "inherited manually" */
 
-        Names(const std::set<OutputName> & s)
-            : std::set<OutputName>(s)
+        Names(const BaseType & s)
+            : BaseType(s)
         { assert(!empty()); }
 
         /**
          * Needs to be "inherited manually"
          */
-        Names(std::set<OutputName> && s)
-            : std::set<OutputName>(s)
+        Names(BaseType && s)
+            : BaseType(std::move(s))
         { assert(!empty()); }
 
         /* This set should always be non-empty, so we delete this

src/libstore/include/nix/store/serve-protocol-impl.hh

@@ -26,7 +26,9 @@ namespace nix {
     }
 
 SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::vector<T>)
-SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::set<T>)
+#define COMMA_ ,
+SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename T COMMA_ typename Compare>, std::set<T COMMA_ Compare>)
+#undef COMMA_
 SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename... Ts>, std::tuple<Ts...>)
 
 #define SERVE_USE_LENGTH_PREFIX_SERIALISER_COMMA ,

src/libstore/include/nix/store/serve-protocol.hh

@@ -180,12 +180,12 @@ DECLARE_SERVE_SERIALISER(ServeProto::BuildOptions);
 
 template<typename T>
 DECLARE_SERVE_SERIALISER(std::vector<T>);
-template<typename T>
-DECLARE_SERVE_SERIALISER(std::set<T>);
+#define COMMA_ ,
+template<typename T, typename Compare>
+DECLARE_SERVE_SERIALISER(std::set<T COMMA_ Compare>);
 template<typename... Ts>
 DECLARE_SERVE_SERIALISER(std::tuple<Ts...>);
 
-#define COMMA_ ,
 template<typename K, typename V>
 DECLARE_SERVE_SERIALISER(std::map<K COMMA_ V>);
 #undef COMMA_

src/libstore/include/nix/store/worker-protocol-impl.hh

@@ -26,7 +26,9 @@ namespace nix {
     }
 
 WORKER_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::vector<T>)
-WORKER_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::set<T>)
+#define COMMA_ ,
+WORKER_USE_LENGTH_PREFIX_SERIALISER(template<typename T COMMA_ typename Compare>, std::set<T COMMA_ Compare>)
+#undef COMMA_
 WORKER_USE_LENGTH_PREFIX_SERIALISER(template<typename... Ts>, std::tuple<Ts...>)
 
 #define WORKER_USE_LENGTH_PREFIX_SERIALISER_COMMA ,

src/libstore/include/nix/store/worker-protocol.hh

@@ -135,7 +135,7 @@ struct WorkerProto
     }
 
     using Feature = std::string;
-    using FeatureSet = std::set<Feature>;
+    using FeatureSet = std::set<Feature, std::less<>>;
 
     static const FeatureSet allFeatures;
 };
@@ -273,12 +273,12 @@ DECLARE_WORKER_SERIALISER(WorkerProto::ClientHandshakeInfo);
 
 template<typename T>
 DECLARE_WORKER_SERIALISER(std::vector<T>);
-template<typename T>
-DECLARE_WORKER_SERIALISER(std::set<T>);
+#define COMMA_ ,
+template<typename T, typename Compare>
+DECLARE_WORKER_SERIALISER(std::set<T COMMA_ Compare>);
 template<typename... Ts>
 DECLARE_WORKER_SERIALISER(std::tuple<Ts...>);
 
-#define COMMA_ ,
 template<typename K, typename V>
 DECLARE_WORKER_SERIALISER(std::map<K COMMA_ V>);
 #undef COMMA_

src/libutil/include/nix/util/json-utils.hh

@@ -90,8 +90,8 @@ struct json_avoids_null<std::vector<T>> : std::true_type {};
 template<typename T>
 struct json_avoids_null<std::list<T>> : std::true_type {};
 
-template<typename T>
-struct json_avoids_null<std::set<T>> : std::true_type {};
+template<typename T, typename Compare>
+struct json_avoids_null<std::set<T, Compare>> : std::true_type {};
 
 template<typename K, typename V>
 struct json_avoids_null<std::map<K, V>> : std::true_type {};

src/libutil/include/nix/util/topo-sort.hh

@@ -5,13 +5,13 @@
 
 namespace nix {
 
-template<typename T>
-std::vector<T> topoSort(std::set<T> items,
-        std::function<std::set<T>(const T &)> getChildren,
+template<typename T, typename Compare>
+std::vector<T> topoSort(std::set<T, Compare> items,
+        std::function<std::set<T, Compare>(const T &)> getChildren,
         std::function<Error(const T &, const T &)> makeCycleError)
 {
     std::vector<T> sorted;
-    std::set<T> visited, parents;
+    decltype(items) visited, parents;
 
     std::function<void(const T & path, const T * parent)> dfsVisit;
 
@@ -21,7 +21,7 @@ std::vector<T> topoSort(std::set<T> items,
         if (!visited.insert(path).second) return;
         parents.insert(path);
 
-        std::set<T> references = getChildren(path);
+        auto references = getChildren(path);
 
         for (auto & i : references)
             /* Don't traverse into items that don't exist in our starting set. */

src/libutil/include/nix/util/types.hh

@@ -12,17 +12,35 @@
 namespace nix {
 
 typedef std::list<std::string> Strings;
-typedef std::set<std::string> StringSet;
 typedef std::map<std::string, std::string> StringMap;
 typedef std::map<std::string, std::string> StringPairs;
 
+/**
+ * Alias to ordered set container with transparent comparator.
+ *
+ * Used instead of std::set<std::string> to use C++14 N3657 [1]
+ * heterogenous lookup consistently across the whole codebase.
+ * Transparent comparators get rid of creation of unnecessary
+ * temporary variables when looking up keys by `std::string_view`
+ * or C-style `const char *` strings.
+ *
+ * [1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3657.htm
+ */
+using StringSet = std::set<std::string, std::less<>>;
+
 /**
  * Paths are just strings.
  */
 typedef std::string Path;
 typedef std::string_view PathView;
 typedef std::list<Path> Paths;
-typedef std::set<Path> PathSet;
+
+/**
+ * Alias to an ordered set of `Path`s. Uses transparent comparator.
+ *
+ * @see StringSet
+ */
+using PathSet = std::set<Path, std::less<>>;
 
 typedef std::vector<std::pair<std::string, std::string>> Headers;