Introduce EvalStore::storePath

This abstracts over a common case. Good for brevity, and enabling
further experiments.

src/libexpr/eval.cc

@@ -3103,7 +3103,7 @@ std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Pat
                 fetchSettings,
                 EvalSettings::resolvePseudoUrl(value));
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
-            return finish(rootPath(store->printStorePath(storePath)));
+            return finish(this->storePath(storePath));
         } catch (Error & e) {
             logWarning({
                 .msg = HintFmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)

src/libexpr/eval.hh

@@ -389,6 +389,15 @@ public:
      */
     SourcePath rootPath(PathView path);
 
+    /**
+     * Return a `SourcePath` that refers to `path` in the store.
+     *
+     * For now, this has to also be within the root filesystem for
+     * backwards compat, but for Windows and maybe also pure eval, we'll
+     * probably want to do something different.
+     */
+    SourcePath storePath(const StorePath & path);
+
     /**
      * Allow access to a path.
      */

src/libexpr/paths.cc

@@ -1,3 +1,4 @@
+#include "store-api.hh"
 #include "eval.hh"
 
 namespace nix {
@@ -12,4 +13,9 @@ SourcePath EvalState::rootPath(PathView path)
     return {rootFS, CanonPath(absPath(path))};
 }
 
+SourcePath EvalState::storePath(const StorePath & path)
+{
+    return {rootFS, CanonPath{store->printStorePath(path)}};
+}
+
 }