Make rootFS's showPath() render the paths from the original accessors

This makes paths in error messages behave similar to lazy-trees,
e.g. instead of store paths like

       error: attribute 'foobar' missing
       at /nix/store/ddzfiipzqlrh3gnprmqbadnsnrxsmc9i-source/machine/configuration.nix:209:7:
          208|
          209|       pkgs.foobar
             |       ^
          210|     ];

you now get

       error: attribute 'foobar' missing
       at /home/eelco/Misc/eelco-configurations/machine/configuration.nix:209:7:
          208|
          209|       pkgs.foobar
             |       ^
          210|     ];

src/libexpr/eval.cc

@@ -14,6 +14,7 @@
 #include "profiles.hh"
 #include "print.hh"
 #include "filtering-source-accessor.hh"
+#include "forwarding-source-accessor.hh"
 #include "memory-source-accessor.hh"
 #include "gc-small-vector.hh"
 #include "url.hh"
@@ -180,6 +181,34 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
     }
 }
 
+struct PathDisplaySourceAccessor : ForwardingSourceAccessor
+{
+    ref<EvalState::StorePathAccessors> storePathAccessors;
+
+    PathDisplaySourceAccessor(
+        ref<SourceAccessor> next,
+        ref<EvalState::StorePathAccessors> storePathAccessors)
+        : ForwardingSourceAccessor(next)
+        , storePathAccessors(storePathAccessors)
+    {
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        /* Find the accessor that produced `path`, if any, and use it
+           to render a more informative path
+           (e.g. `«github:foo/bar»/flake.nix` rather than
+           `/nix/store/hash.../flake.nix`). */
+        auto ub = storePathAccessors->upper_bound(path);
+        if (ub != storePathAccessors->begin())
+            ub--;
+        if (ub != storePathAccessors->end() && path.isWithin(ub->first))
+            return ub->second->showPath(path.removePrefix(ub->first));
+        else
+            return next->showPath(path);
+    }
+};
+
 static constexpr size_t BASE_ENV_SIZE = 128;
 
 EvalState::EvalState(
@@ -245,6 +274,7 @@ EvalState::EvalState(
     }
     , repair(NoRepair)
     , emptyBindings(0)
+    , storePathAccessors(make_ref<StorePathAccessors>())
     , rootFS(
         ({
             /* In pure eval mode, we provide a filesystem that only
@@ -270,6 +300,8 @@ EvalState::EvalState(
                     : makeUnionSourceAccessor({accessor, storeFS});
             }
 
+            accessor = make_ref<PathDisplaySourceAccessor>(accessor, storePathAccessors);
+
             /* Apply access control if needed. */
             if (settings.restrictEval || settings.pureEval)
                 accessor = AllowListSourceAccessor::create(accessor, {},

src/libexpr/eval.hh

@@ -262,6 +262,16 @@ public:
     /** `"unknown"` */
     Value vStringUnknown;
 
+    using StorePathAccessors = std::map<CanonPath, ref<SourceAccessor>>;
+
+    /**
+     * A map back to the original `SourceAccessor`s used to produce
+     * store paths. We keep track of this to produce error messages
+     * that refer to the original flakerefs.
+     * FIXME: use Sync.
+     */
+    ref<StorePathAccessors> storePathAccessors;
+
     /**
      * The accessor for the root filesystem.
      */

src/libexpr/primops/fetchMercurial.cc

@@ -64,7 +64,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
     if (rev) attrs.insert_or_assign("rev", rev->gitRev());
     auto input = fetchers::Input::fromAttrs(state.fetchSettings, std::move(attrs));
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    auto [storePath, accessor, input2] = input.fetchToStore(state.store);
 
     auto attrs2 = state.buildBindings(8);
     state.mkStorePathString(storePath, attrs2.alloc(state.sOutPath));

src/libexpr/primops/fetchTree.cc

@@ -200,10 +200,12 @@ static void fetchTree(
             throw Error("input '%s' is not allowed to use the '__final' attribute", input.to_string());
     }
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    auto [storePath, accessor, input2] = input.fetchToStore(state.store);
 
     state.allowPath(storePath);
 
+    state.storePathAccessors->insert_or_assign(CanonPath(state.store->printStorePath(storePath)), accessor);
+
     emitTreeAttrs(state, storePath, input2, v, params.emptyRevFallback, false);
 }