Expose the fact that storeFS is a MountedSourceAccessor

This will become useful.

Author: edolstra

src/libexpr/eval.cc

@@ -240,7 +240,7 @@ EvalState::EvalState(
 
         auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
         if (settings.pureEval || store->storeDir != realStoreDir) {
-            accessor = settings.pureEval ? storeFS : makeUnionSourceAccessor({accessor, storeFS});
+            accessor = settings.pureEval ? storeFS.cast<SourceAccessor>() : makeUnionSourceAccessor({accessor, storeFS});
         }
 
         /* Apply access control if needed. */

src/libexpr/include/nix/expr/eval.hh

@@ -48,6 +48,7 @@ class StorePath;
 struct SingleDerivedPath;
 enum RepairFlag : bool;
 struct MemorySourceAccessor;
+struct MountedSourceAccessor;
 
 namespace eval_cache {
 class EvalCache;
@@ -319,7 +320,7 @@ public:
     /**
      * The accessor corresponding to `store`.
      */
-    const ref<SourceAccessor> storeFS;
+    const ref<MountedSourceAccessor> storeFS;
 
     /**
      * The accessor for the root filesystem.