Move the GENERATE_CMP macro to its own file

Despite being an ugly hack, it can probably be useful in a couple extra
places

src/libstore/realisation.hh

@@ -2,34 +2,7 @@
 
 #include "path.hh"
 #include <nlohmann/json_fwd.hpp>
-
-
-/* Awfull hacky generation of the comparison operators by doing a lexicographic
- * comparison between the choosen fields
- * ```
- * GENERATE_CMP(ClassName, my->field1, my->field2, ...)
- * ```
- *
- * will generate comparison operators semantically equivalent to:
- * ```
- * bool operator<(const ClassName& other) {
- *   return field1 < other.field1 && field2 < other.field2 && ...;
- * }
- * ```
- */
-#define GENERATE_ONE_CMP(COMPARATOR, MY_TYPE, FIELDS...) \
-    bool operator COMPARATOR(const MY_TYPE& other) const { \
-      const MY_TYPE* me = this; \
-      auto fields1 = std::make_tuple( FIELDS ); \
-      me = &other; \
-      auto fields2 = std::make_tuple( FIELDS ); \
-      return fields1 COMPARATOR fields2; \
-    }
-#define GENERATE_EQUAL(args...) GENERATE_ONE_CMP(==, args)
-#define GENERATE_LEQ(args...) GENERATE_ONE_CMP(<, args)
-#define GENERATE_CMP(args...) \
-    GENERATE_EQUAL(args) \
-    GENERATE_LEQ(args)
+#include "comparator.hh"
 
 namespace nix {
 

src/libutil/comparator.hh

@@ -0,0 +1,30 @@
+#pragma once
+
+/* Awfull hacky generation of the comparison operators by doing a lexicographic
+ * comparison between the choosen fields.
+ *
+ * ```
+ * GENERATE_CMP(ClassName, me->field1, me->field2, ...)
+ * ```
+ *
+ * will generate comparison operators semantically equivalent to:
+ *
+ * ```
+ * bool operator<(const ClassName& other) {
+ *   return field1 < other.field1 && field2 < other.field2 && ...;
+ * }
+ * ```
+ */
+#define GENERATE_ONE_CMP(COMPARATOR, MY_TYPE, FIELDS...) \
+    bool operator COMPARATOR(const MY_TYPE& other) const { \
+      const MY_TYPE* me = this; \
+      auto fields1 = std::make_tuple( FIELDS ); \
+      me = &other; \
+      auto fields2 = std::make_tuple( FIELDS ); \
+      return fields1 COMPARATOR fields2; \
+    }
+#define GENERATE_EQUAL(args...) GENERATE_ONE_CMP(==, args)
+#define GENERATE_LEQ(args...) GENERATE_ONE_CMP(<, args)
+#define GENERATE_CMP(args...) \
+    GENERATE_EQUAL(args) \
+    GENERATE_LEQ(args)