1 files changed, 69 insertions, 13 deletions

diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 9c85fef62..00ba567c6 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -6,26 +6,23 @@
 #include <set>
 #include <string>
 #include <map>
+#include <variant>
 #include <vector>
 
 namespace nix {
 
-using std::list;
-using std::set;
-using std::vector;
-using std::string;
-
-typedef list<string> Strings;
-typedef set<string> StringSet;
-typedef std::map<string, string> StringMap;
+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;
 
 /* Paths are just strings. */
+typedef std::string Path;
+typedef std::string_view PathView;
+typedef std::list<Path> Paths;
+typedef std::set<Path> PathSet;
 
-typedef string Path;
-typedef list<Path> Paths;
-typedef set<Path> PathSet;
-
-typedef vector<std::pair<string, string>> Headers;
+typedef std::vector<std::pair<std::string, std::string>> Headers;
 
 /* Helper class to run code at startup. */
 template<typename T>
@@ -46,4 +43,63 @@ struct Explicit {
     }
 };
 
+
+/* This wants to be a little bit like rust's Cow type.
+   Some parts of the evaluator benefit greatly from being able to reuse
+   existing allocations for strings, but have to be able to also use
+   newly allocated storage for values.
+
+   We do not define implicit conversions, even with ref qualifiers,
+   since those can easily become ambiguous to the reader and can degrade
+   into copying behaviour we want to avoid. */
+class BackedStringView {
+private:
+    std::variant<std::string, std::string_view> data;
+
+    /* Needed to introduce a temporary since operator-> must return
+       a pointer. Without this we'd need to store the view object
+       even when we already own a string. */
+    class Ptr {
+    private:
+        std::string_view view;
+    public:
+        Ptr(std::string_view view): view(view) {}
+        const std::string_view * operator->() const { return &view; }
+    };
+
+public:
+    BackedStringView(std::string && s): data(std::move(s)) {}
+    BackedStringView(std::string_view sv): data(sv) {}
+    template<size_t N>
+    BackedStringView(const char (& lit)[N]): data(std::string_view(lit)) {}
+
+    BackedStringView(const BackedStringView &) = delete;
+    BackedStringView & operator=(const BackedStringView &) = delete;
+
+    /* We only want move operations defined since the sole purpose of
+       this type is to avoid copies. */
+    BackedStringView(BackedStringView && other) = default;
+    BackedStringView & operator=(BackedStringView && other) = default;
+
+    bool isOwned() const
+    {
+        return std::holds_alternative<std::string>(data);
+    }
+
+    std::string toOwned() &&
+    {
+        return isOwned()
+            ? std::move(std::get<std::string>(data))
+            : std::string(std::get<std::string_view>(data));
+    }
+
+    std::string_view operator*() const
+    {
+        return isOwned()
+            ? std::get<std::string>(data)
+            : std::get<std::string_view>(data);
+    }
+    Ptr operator->() const { return Ptr(**this); }
+};
+
 }