Legacy vs non-legacy `to_string`/`parse` for `DerivedPath`

As requested by @roberth, it is good to call out the specific instances
we care about, which is `!` for the RPC protocols, and `^` for humans.

This doesn't take advantage of parametricity as much, but since the
human and computer interfaces are good to decouple anyways (we don't
care if they drift further apart over time in the slightest) some
separation and slight duplication is fine.

Also, unit test both round trips.

1 files changed, 27 insertions, 3 deletions

diff --git a/src/libstore/derived-path.hh b/src/libstore/derived-path.hh
index 9fc64e2f2..5f7acbebc 100644
--- a/src/libstore/derived-path.hh
+++ b/src/libstore/derived-path.hh
@@ -48,8 +48,18 @@ struct DerivedPathBuilt {
     StorePath drvPath;
     OutputsSpec outputs;
 
-    std::string to_string(const Store & store, char separator = '!') const;
-    static DerivedPathBuilt parse(const Store & store, std::string_view, std::string_view);
+    /**
+     * Uses `^` as the separator
+     */
+    std::string to_string(const Store & store) const;
+    /**
+     * Uses `!` as the separator
+     */
+    std::string to_string_legacy(const Store & store) const;
+    /**
+     * The caller splits on the separator, so it works for both variants.
+     */
+    static DerivedPathBuilt parse(const Store & store, std::string_view drvPath, std::string_view outputs);
     nlohmann::json toJSON(ref<Store> store) const;
 
     GENERATE_CMP(DerivedPathBuilt, me->drvPath, me->outputs);
@@ -81,8 +91,22 @@ struct DerivedPath : _DerivedPathRaw {
         return static_cast<const Raw &>(*this);
     }
 
-    std::string to_string(const Store & store, char separator = '!') const;
+    /**
+     * Uses `^` as the separator
+     */
+    std::string to_string(const Store & store) const;
+    /**
+     * Uses `!` as the separator
+     */
+    std::string to_string_legacy(const Store & store) const;
+    /**
+     * Uses `^` as the separator
+     */
     static DerivedPath parse(const Store & store, std::string_view);
+    /**
+     * Uses `!` as the separator
+     */
+    static DerivedPath parseLegacy(const Store & store, std::string_view);
 };
 
 /**