fixup: Re-enable import .drv code

A string is only allowed to create one path component; containing
no slashes.

src/libcmd/common-eval-args.cc

@@ -32,8 +32,7 @@ EvalSettings evalSettings {
                 // FIXME `parseFlakeRef` should take a `std::string_view`.
                 auto flakeRef = parseFlakeRef(fetchSettings, std::string { rest }, {}, true, false);
                 debug("fetching flake search path element '%s''", rest);
-                auto storePath = flakeRef.resolve(store).fetchTree(store).first;
-                return store->toRealPath(storePath);
+                return flakeRef.resolve(store).lazyFetch(store).first;
             },
         },
     },
@@ -174,15 +173,15 @@ SourcePath lookupFileArg(EvalState & state, std::string_view s, const Path * bas
             state.store,
             state.fetchSettings,
             EvalSettings::resolvePseudoUrl(s));
-        auto storePath = fetchToStore(*state.store, SourcePath(accessor), FetchMode::Copy);
-        return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
+        state.registerAccessor(accessor);
+        return SourcePath(accessor);
     }
 
     else if (hasPrefix(s, "flake:")) {
         experimentalFeatureSettings.require(Xp::Flakes);
         auto flakeRef = parseFlakeRef(fetchSettings, std::string(s.substr(6)), {}, true, false);
-        auto storePath = flakeRef.resolve(state.store).fetchTree(state.store).first;
-        return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
+        auto accessor = flakeRef.resolve(state.store).lazyFetch(state.store).first;
+        return SourcePath(accessor);
     }
 
     else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {

src/libexpr/eval-settings.hh

@@ -7,6 +7,7 @@
 namespace nix {
 
 class Store;
+struct SourcePath;
 
 struct EvalSettings : Config
 {
@@ -22,7 +23,7 @@ struct EvalSettings : Config
      * @todo Return (`std::optional` of) `SourceAccssor` or something
      * more structured instead of mere `std::string`?
      */
-    using LookupPathHook = std::optional<std::string>(ref<Store> store, std::string_view);
+    using LookupPathHook = std::optional<SourcePath>(ref<Store> store, std::string_view);
 
     /**
      * Map from "scheme" to a `LookupPathHook`.

src/libexpr/eval.cc

@@ -315,6 +315,7 @@ EvalState::EvalState(
 {
     corepkgsFS->setPathDisplay("<nix", ">");
     internalFS->setPathDisplay("«nix-internal»", "");
+    rootFS->setPathDisplay("/", "");
 
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
@@ -859,6 +860,11 @@ void Value::mkPath(const SourcePath & path)
     mkPath(&*path.accessor, makeImmutableString(path.path.abs()));
 }
 
+void EvalState::registerAccessor(const ref<SourceAccessor> accessor)
+{
+    sourceAccessors.push_back(accessor);
+}
+
 
 inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
 {
@@ -1973,10 +1979,22 @@ void EvalState::concatLists(Value & v, size_t nrLists, Value * const * lists, co
     v.mkList(list);
 }
 
+// FIXME limit recursion
+Value * resolveOutPath(EvalState & state, Value * v, const PosIdx pos)
+{
+    state.forceValue(*v, pos);
+    if (v->type() != nAttrs)
+        return v;
+    auto found = v->attrs()->find(state.sOutPath);
+    if (found != v->attrs()->end())
+        return resolveOutPath(state, found->value, pos);
+    return v;
+}
 
 void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
 {
     NixStringContext context;
+    std::shared_ptr<SourceAccessor> accessor;
     std::vector<BackedStringView> s;
     size_t sSize = 0;
     NixInt n{0};
@@ -2010,8 +2028,9 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     Value * vTmpP = values.data();
 
     for (auto & [i_pos, i] : *es) {
-        Value & vTmp = *vTmpP++;
-        i->eval(state, env, vTmp);
+        Value & vTmp0 = *vTmpP++;
+        i->eval(state, env, vTmp0);
+        Value & vTmp = *resolveOutPath(state, &vTmp0, i_pos);
 
         /* If the first element is a path, then the result will also
            be a path, we don't copy anything (yet - that's done later,
@@ -2019,6 +2038,9 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
            and none of the strings are allowed to have contexts. */
         if (first) {
             firstType = vTmp.type();
+            if (firstType == nPath) {
+                accessor = vTmp.path().accessor;
+            }
         }
 
         if (firstType == nInt) {
@@ -2065,7 +2087,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     else if (firstType == nPath) {
         if (!context.empty())
             state.error<EvalError>("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow();
-        v.mkPath(state.rootPath(CanonPath(canonPath(str()))));
+        v.mkPath({ref(accessor), CanonPath(str())});
     } else
         v.mkStringMove(c_str(), context);
 }
@@ -2315,6 +2337,8 @@ BackedStringView EvalState::coerceToString(
               v.payload.path.path
             : copyToStore
             ? store->printStorePath(copyPathToStore(context, v.path()))
+            : v.path().accessor->toStringReturnsStorePath()
+            ? store->printStorePath(copyPathToStore(context, SourcePath(v.path().accessor, CanonPath::root))) + v.path().path.absOrEmpty()
             : std::string(v.path().path.abs());
     }
 
@@ -2427,10 +2451,14 @@ SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext
     if (v.type() == nPath)
         return v.path();
 
-    /* Similarly, handle __toString where the result may be a path
+    /* Similarly, handle outPath and __toString where the result may be a path
        value. */
     if (v.type() == nAttrs) {
-        auto i = v.attrs()->find(sToString);
+        auto i = v.attrs()->find(sOutPath);
+        if (i != v.attrs()->end()) {
+            return coerceToPath(pos, *i->value, context, errorCtx);
+        }
+        i = v.attrs()->find(sToString);
         if (i != v.attrs()->end()) {
             Value v1;
             callFunction(*i->value, v, v1, pos);
@@ -3053,12 +3081,14 @@ SourcePath EvalState::findFile(const LookupPath & lookupPath, const std::string_
         if (!suffixOpt) continue;
         auto suffix = *suffixOpt;
 
-        auto rOpt = resolveLookupPathPath(i.path);
+        auto rOpt = resolveLookupPathPath(
+                i.path,
+                true);
         if (!rOpt) continue;
         auto r = *rOpt;
 
-        Path res = suffix == "" ? r : concatStrings(r, "/", suffix);
-        if (pathExists(res)) return rootPath(CanonPath(canonPath(res)));
+        auto res = r / CanonPath(suffix);
+        if (res.pathExists()) return res;
     }
 
     if (hasPrefix(path, "nix/"))
@@ -3073,26 +3103,30 @@ SourcePath EvalState::findFile(const LookupPath & lookupPath, const std::string_
 }
 
 
-std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Path & value0, bool initAccessControl)
+std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Path & value0, bool initAccessControl)
 {
     auto & value = value0.s;
     auto i = lookupPathResolved.find(value);
     if (i != lookupPathResolved.end()) return i->second;
 
-    auto finish = [&](std::string res) {
+    auto finish = [&](SourcePath res) {
         debug("resolved search path element '%s' to '%s'", value, res);
         lookupPathResolved.emplace(value, res);
         return res;
     };
 
+    std::optional<SourcePath> res;
+
     if (EvalSettings::isPseudoUrl(value)) {
         try {
             auto accessor = fetchers::downloadTarball(
                 store,
                 fetchSettings,
                 EvalSettings::resolvePseudoUrl(value));
+            // Traditional search path lookups use the absolute path space for
+            // historical consistency.
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
-            return finish(store->toRealPath(storePath));
+            res.emplace(rootPath(CanonPath(store->toRealPath(storePath))));
         } catch (Error & e) {
             logWarning({
                 .msg = HintFmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)
@@ -3111,22 +3145,22 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
     }
 
     {
-        auto path = absPath(value);
+        auto path = rootPath(value);
 
         /* Allow access to paths in the search path. */
         if (initAccessControl) {
-            allowPath(path);
-            if (store->isInStore(path)) {
+            allowPath(path.path.abs());
+            if (store->isInStore(path.path.abs())) {
                 try {
                     StorePathSet closure;
-                    store->computeFSClosure(store->toStorePath(path).first, closure);
+                    store->computeFSClosure(store->toStorePath(path.path.abs()).first, closure);
                     for (auto & p : closure)
                         allowPath(p);
                 } catch (InvalidPath &) { }
             }
         }
 
-        if (pathExists(path))
+        if (path.pathExists())
             return finish(std::move(path));
         else {
             logWarning({

src/libexpr/eval.hh

@@ -249,6 +249,9 @@ public:
 
     const SourcePath callFlakeInternal;
 
+    /* A collection of InputAccessors, just to keep them alive. */
+    std::list<ref<SourceAccessor>> sourceAccessors;
+
     /**
      * Store used to materialise .drv files.
      */
@@ -339,7 +342,7 @@ private:
 
     LookupPath lookupPath;
 
-    std::map<std::string, std::optional<std::string>> lookupPathResolved;
+    std::map<std::string, std::optional<SourcePath>> lookupPathResolved;
 
     /**
      * Cache used by prim_match().
@@ -381,6 +384,8 @@ public:
      */
     SourcePath rootPath(PathView path);
 
+    void registerAccessor(ref<SourceAccessor> accessor);
+
     /**
      * Allow access to a path.
      */
@@ -446,7 +451,7 @@ public:
      *
      * If it is not found, return `std::nullopt`
      */
-    std::optional<std::string> resolveLookupPathPath(
+    std::optional<SourcePath> resolveLookupPathPath(
         const LookupPath::Path & elem,
         bool initAccessControl = false);
 

src/libexpr/nixexpr.cc

@@ -45,7 +45,7 @@ void ExprString::show(const SymbolTable & symbols, std::ostream & str) const
 
 void ExprPath::show(const SymbolTable & symbols, std::ostream & str) const
 {
-    str << s;
+    str << path;
 }
 
 void ExprVar::show(const SymbolTable & symbols, std::ostream & str) const

src/libexpr/nixexpr.hh

@@ -131,12 +131,15 @@ struct ExprString : Expr
 
 struct ExprPath : Expr
 {
-    ref<SourceAccessor> accessor;
-    std::string s;
+    const SourcePath path;
     Value v;
-    ExprPath(ref<SourceAccessor> accessor, std::string s) : accessor(accessor), s(std::move(s))
+    ExprPath(SourcePath && path)
+        : path(path)
     {
-        v.mkPath(&*accessor, this->s.c_str());
+        v.mkPath(
+            &*path.accessor,
+            // TODO: GC_STRDUP
+            strdup(path.path.abs().c_str()));
     }
     Value * maybeThunk(EvalState & state, Env & env) override;
     COMMON_METHODS

src/libexpr/parser.y

@@ -136,6 +136,10 @@ static Expr * makeCall(PosIdx pos, Expr * fn, Expr * arg) {
   std::vector<std::pair<nix::AttrName, nix::PosIdx>> * inheritAttrs;
   std::vector<std::pair<nix::PosIdx, nix::Expr *>> * string_parts;
   std::vector<std::pair<nix::PosIdx, std::variant<nix::Expr *, nix::StringToken>>> * ind_string_parts;
+  struct {
+      nix::Expr * e;
+      bool appendSlash;
+  } pathStart;
 }
 
 %type <e> start expr expr_function expr_if expr_op
@@ -149,7 +153,8 @@ static Expr * makeCall(PosIdx pos, Expr * fn, Expr * arg) {
 %type <inheritAttrs> attrs
 %type <string_parts> string_parts_interpolated
 %type <ind_string_parts> ind_string_parts
-%type <e> path_start string_parts string_attr
+%type <pathStart> path_start
+%type <e> string_parts string_attr
 %type <id> attr
 %token <id> ID
 %token <str> STR IND_STR
@@ -295,9 +300,11 @@ expr_simple
       $$ = state->stripIndentation(CUR_POS, std::move(*$2));
       delete $2;
   }
-  | path_start PATH_END
+  | path_start PATH_END { $$ = $1.e; }
   | path_start string_parts_interpolated PATH_END {
-      $2->insert($2->begin(), {state->at(@1), $1});
+      if ($1.appendSlash)
+          $2->insert($2->begin(), {noPos, new ExprString("/")});
+      $2->insert($2->begin(), {state->at(@1), $1.e});
       $$ = new ExprConcatStrings(CUR_POS, false, $2);
   }
   | SPATH {
@@ -350,11 +357,17 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, state->basePath.path.abs()));
-    /* add back in the trailing '/' to the first segment */
-    if ($1.p[$1.l-1] == '/' && $1.l > 1)
-      path += "/";
-    $$ = new ExprPath(ref<SourceAccessor>(state->rootFS), std::move(path));
+      std::string_view path({$1.p, $1.l});
+      $$ = {
+          .e = new ExprPath(
+              /* Absolute paths are always interpreted relative to the
+                 root filesystem accessor, rather than the accessor of the
+                 current Nix expression. */
+              hasPrefix(path, "/")
+              ? SourcePath{state->rootFS, CanonPath(path)}
+              : SourcePath{state->basePath.accessor, CanonPath(path, state->basePath.path)}),
+          .appendSlash = hasSuffix(path, "/")
+      };
   }
   | HPATH {
     if (state->settings.pureEval) {
@@ -363,8 +376,8 @@ path_start
             std::string_view($1.p, $1.l)
         );
     }
-    Path path(getHome() + std::string($1.p + 1, $1.l - 1));
-    $$ = new ExprPath(ref<SourceAccessor>(state->rootFS), std::move(path));
+    CanonPath path(getHome() + std::string($1.p + 1, $1.l - 1));
+    $$ = {.e = new ExprPath(SourcePath{state->rootFS, std::move(path)}), .appendSlash = true};
   }
   ;
 

src/libexpr/primops.cc

@@ -178,7 +178,6 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
     auto path = realisePath(state, pos, vPath, std::nullopt);
     auto path2 = path.path.abs();
 
-    // FIXME
     auto isValidDerivationInStore = [&]() -> std::optional<StorePath> {
         if (!state.store->isStorePath(path2))
             return std::nullopt;
@@ -216,9 +215,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         state.forceFunction(**state.vImportedDrvToDerivation, pos, "while evaluating imported-drv-to-derivation.nix.gen.hh");
         v.mkApp(*state.vImportedDrvToDerivation, w);
         state.forceAttrs(v, pos, "while calling imported-drv-to-derivation.nix.gen.hh");
-    }
-
-    else {
+    } else {
         if (!vScope)
             state.evalFile(path, v);
         else {
@@ -1700,7 +1697,10 @@ static std::string_view legacyBaseNameOf(std::string_view path)
 static void prim_baseNameOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     NixStringContext context;
-    v.mkString(legacyBaseNameOf(*state.coerceToString(pos, *args[0], context,
+    if (v.type() == nPath)
+        v.mkString(baseNameOf(v.path().path.abs()), context);
+    else
+        v.mkString(legacyBaseNameOf(*state.coerceToString(pos, *args[0], context,
             "while evaluating the first argument passed to builtins.baseNameOf",
             false, false)), context);
 }

src/libexpr/primops/fetchTree.cc

@@ -19,21 +19,20 @@ namespace nix {
 
 void emitTreeAttrs(
     EvalState & state,
-    const StorePath & storePath,
     const fetchers::Input & input,
     Value & v,
+    std::function<void(Value &)> setOutPath,
     bool emptyRevFallback,
     bool forceDirty)
 {
     auto attrs = state.buildBindings(100);
 
-    state.mkStorePathString(storePath, attrs.alloc(state.sOutPath));
+    setOutPath(attrs.alloc(state.sOutPath));
 
     // FIXME: support arbitrary input attributes.
 
-    auto narHash = input.getNarHash();
-    assert(narHash);
-    attrs.alloc("narHash").mkString(narHash->to_string(HashFormat::SRI, true));
+    if (auto narHash = input.getNarHash())
+        attrs.alloc("narHash").mkString(narHash->to_string(HashFormat::SRI, true));
 
     if (input.getType() == "git")
         attrs.alloc("submodules").mkBool(
@@ -72,10 +71,28 @@ void emitTreeAttrs(
     v.mkAttrs(attrs);
 }
 
+void emitTreeAttrs(
+    EvalState & state,
+    const SourcePath & storePath,
+    const fetchers::Input & input,
+    Value & v,
+    bool emptyRevFallback,
+    bool forceDirty)
+{
+    emitTreeAttrs(state, input, v,
+        [&](Value & vOutPath) {
+            state.registerAccessor(storePath.accessor);
+            vOutPath.mkPath(storePath);
+        },
+        emptyRevFallback,
+        forceDirty);
+}
+
 struct FetchTreeParams {
     bool emptyRevFallback = false;
     bool allowNameArgument = false;
     bool isFetchGit = false;
+    bool returnPath = true; // whether to return a SourcePath instead of a StorePath
 };
 
 static void fetchTree(
@@ -112,7 +129,9 @@ static void fetchTree(
 
         for (auto & attr : *args[0]->attrs()) {
             if (attr.name == state.sType) continue;
+
             state.forceValue(*attr.value, attr.pos);
+
             if (attr.value->type() == nPath || attr.value->type() == nString) {
                 auto s = state.coerceToString(attr.pos, *attr.value, context, "", false, false).toOwned();
                 attrs.emplace(state.symbols[attr.name],
@@ -193,11 +212,31 @@ static void fetchTree(
 
     state.checkURI(input.toURLString());
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    if (params.returnPath) {
+        // Clang16+: change to `auto [accessor, input2] =`
+        auto pair = input.getAccessor(state.store);
+        auto & accessor = pair.first;
+        auto & input2 = pair.second;
 
-    state.allowPath(storePath);
+        emitTreeAttrs(state, input2, v,
+            [&](Value & vOutPath) {
+                state.registerAccessor(accessor);
+                vOutPath.mkPath(SourcePath { accessor, CanonPath::root });
+            },
+            params.emptyRevFallback, false);
+    } else {
+        auto pair = input.fetchToStore(state.store);
+        auto & storePath = pair.first;
+        auto & input2 = pair.second;
 
-    emitTreeAttrs(state, storePath, input2, v, params.emptyRevFallback, false);
+        state.allowPath(storePath);
+
+        emitTreeAttrs(state, input2, v,
+            [&](Value & vOutPath) {
+                state.mkStorePathString(storePath, vOutPath);
+            },
+            params.emptyRevFallback, false);
+    }
 }
 
 static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)
@@ -614,7 +653,8 @@ static void prim_fetchGit(EvalState & state, const PosIdx pos, Value * * args, V
         FetchTreeParams {
             .emptyRevFallback = true,
             .allowNameArgument = true,
-            .isFetchGit = true
+            .isFetchGit = true,
+            .returnPath = false,
         });
 }
 

src/libfetchers/fetchers.cc

@@ -167,6 +167,8 @@ bool Input::contains(const Input & other) const
 
 std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
 {
+// TODO: lazy-trees gets rid of this. Why?
+#if 0
     if (!scheme)
         throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
@@ -187,6 +189,7 @@ std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
             debug("substitution of input '%s' failed: %s", to_string(), e.what());
         }
     }
+#endif
 
     auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
         try {
@@ -194,8 +197,9 @@ std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
 
             auto storePath = nix::fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, final.getName());
 
-            auto narHash = store->queryPathInfo(storePath)->narHash;
-            final.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
+            // TODO: do we really want to throw this out?
+            // auto narHash = store->queryPathInfo(storePath)->narHash;
+            // final.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
 
             scheme->checkLocks(*this, final);
 

src/libflake/flake/flake.cc

@@ -19,62 +19,17 @@ using namespace flake;
 
 namespace flake {
 
-typedef std::pair<StorePath, FlakeRef> FetchedFlake;
-typedef std::vector<std::pair<FlakeRef, FetchedFlake>> FlakeCache;
-
-static std::optional<FetchedFlake> lookupInFlakeCache(
-    const FlakeCache & flakeCache,
-    const FlakeRef & flakeRef)
-{
-    // FIXME: inefficient.
-    for (auto & i : flakeCache) {
-        if (flakeRef == i.first) {
-            debug("mapping '%s' to previously seen input '%s' -> '%s",
-                flakeRef, i.first, i.second.second);
-            return i.second;
-        }
-    }
-
-    return std::nullopt;
-}
-
-static std::tuple<StorePath, FlakeRef, FlakeRef> fetchOrSubstituteTree(
+static FlakeRef maybeResolve(
     EvalState & state,
     const FlakeRef & originalRef,
-    bool allowLookup,
-    FlakeCache & flakeCache)
+    bool useRegistries)
 {
-    auto fetched = lookupInFlakeCache(flakeCache, originalRef);
-    FlakeRef resolvedRef = originalRef;
-
-    if (!fetched) {
         if (originalRef.input.isDirect()) {
-            fetched.emplace(originalRef.fetchTree(state.store));
-        } else {
-            if (allowLookup) {
-                resolvedRef = originalRef.resolve(state.store);
-                auto fetchedResolved = lookupInFlakeCache(flakeCache, originalRef);
-                if (!fetchedResolved) fetchedResolved.emplace(resolvedRef.fetchTree(state.store));
-                flakeCache.push_back({resolvedRef, *fetchedResolved});
-                fetched.emplace(*fetchedResolved);
-            }
-            else {
+            if (!useRegistries)
                 throw Error("'%s' is an indirect flake reference, but registry lookups are not allowed", originalRef);
-            }
-        }
-        flakeCache.push_back({originalRef, *fetched});
-    }
-
-    auto [storePath, lockedRef] = *fetched;
-
-    debug("got tree '%s' from '%s'",
-        state.store->printStorePath(storePath), lockedRef);
-
-    state.allowPath(storePath);
-
-    assert(!originalRef.input.getNarHash() || storePath == originalRef.input.computeStorePath(*state.store));
-
-    return {std::move(storePath), resolvedRef, lockedRef};
+            return originalRef.resolve(state.store);
+        } else
+            return originalRef;
 }
 
 static void forceTrivialValue(EvalState & state, Value & value, const PosIdx pos)
@@ -316,24 +271,19 @@ static Flake getFlake(
     EvalState & state,
     const FlakeRef & originalRef,
     bool allowLookup,
-    FlakeCache & flakeCache,
     InputPath lockRootPath)
 {
-    auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
-        state, originalRef, allowLookup, flakeCache);
+    auto resolvedRef = maybeResolve(state, originalRef, allowLookup);
+    auto [accessor, lockedRef] = resolvedRef.lazyFetch(state.store);
 
-    return readFlake(state, originalRef, resolvedRef, lockedRef, state.rootPath(state.store->toRealPath(storePath)), lockRootPath);
-}
+    state.registerAccessor(accessor);
 
-Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup, FlakeCache & flakeCache)
-{
-    return getFlake(state, originalRef, allowLookup, flakeCache, {});
+    return readFlake(state, originalRef, resolvedRef, lockedRef, SourcePath {accessor, CanonPath::root}, lockRootPath);
 }
 
 Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup)
 {
-    FlakeCache flakeCache;
-    return getFlake(state, originalRef, allowLookup, flakeCache);
+    return getFlake(state, originalRef, allowLookup, {});
 }
 
 static LockFile readLockFile(
@@ -355,11 +305,9 @@ LockedFlake lockFlake(
 {
     experimentalFeatureSettings.require(Xp::Flakes);
 
-    FlakeCache flakeCache;
-
     auto useRegistries = lockFlags.useRegistries.value_or(settings.useRegistries);
 
-    auto flake = getFlake(state, topRef, useRegistries, flakeCache);
+    auto flake = getFlake(state, topRef, useRegistries);
 
     if (lockFlags.applyNixConfig) {
         flake.config.apply(settings);
@@ -553,7 +501,7 @@ LockedFlake lockFlake(
                         }
 
                         if (mustRefetch) {
-                            auto inputFlake = getFlake(state, oldLock->lockedRef, false, flakeCache, inputPath);
+                            auto inputFlake = getFlake(state, oldLock->lockedRef, false, inputPath);
                             nodePaths.emplace(childNode, inputFlake.path.parent());
                             computeLocks(inputFlake.inputs, childNode, inputPath, oldLock, lockRootPath, parentPath, false);
                         } else {
@@ -586,7 +534,7 @@ LockedFlake lockFlake(
                             if (localRef.input.getType() == "path")
                                 localPath = absPath(*input.ref->input.getSourcePath(), parentPath);
 
-                            auto inputFlake = getFlake(state, localRef, useRegistries, flakeCache, inputPath);
+                            auto inputFlake = getFlake(state, localRef, useRegistries, inputPath);
 
                             auto childNode = make_ref<LockedNode>(inputFlake.lockedRef, ref);
 
@@ -615,12 +563,17 @@ LockedFlake lockFlake(
                         }
 
                         else {
-                            auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
-                                state, *input.ref, useRegistries, flakeCache);
+                            auto [path, lockedRef] = [&]() -> std::tuple<SourcePath, FlakeRef>
+                            {
+                                auto resolvedRef = maybeResolve(state, *input.ref, useRegistries);
+                                auto [accessor, lockedRef] = resolvedRef.lazyFetch(state.store);
+                                state.registerAccessor(accessor);
+                                return {SourcePath(accessor), lockedRef};
+                            }();
 
                             auto childNode = make_ref<LockedNode>(lockedRef, ref, false);
 
-                            nodePaths.emplace(childNode, state.rootPath(state.store->toRealPath(storePath)));
+                            nodePaths.emplace(childNode, path);
 
                             node->inputs.insert_or_assign(id, childNode);
                         }
@@ -723,8 +676,7 @@ LockedFlake lockFlake(
                            repo, so we should re-read it. FIXME: we could
                            also just clear the 'rev' field... */
                         auto prevLockedRef = flake.lockedRef;
-                        FlakeCache dummyCache;
-                        flake = getFlake(state, topRef, useRegistries, dummyCache);
+                        flake = getFlake(state, topRef, useRegistries);
 
                         if (lockFlags.commitLockFile &&
                             flake.lockedRef.input.getRev() &&
@@ -768,21 +720,23 @@ void callFlake(EvalState & state,
 
         auto lockedNode = node.dynamic_pointer_cast<const LockedNode>();
 
+        /*
         // FIXME: This is a hack to support chroot stores. Remove this
         // once we can pass a sourcePath rather than a storePath to
         // call-flake.nix.
-        auto path = sourcePath.path.abs();
-        if (auto store = state.store.dynamic_pointer_cast<LocalFSStore>()) {
-            auto realStoreDir = store->getRealStoreDir();
-            if (isInDir(path, realStoreDir))
-                path = store->storeDir + path.substr(realStoreDir.size());
-        }
-
-        auto [storePath, subdir] = state.store->toStorePath(path);
+        // auto path = sourcePath.path.abs();
+        // if (auto store = state.store.dynamic_pointer_cast<LocalFSStore>()) {
+        //    auto realStoreDir = store->getRealStoreDir();
+        //    if (isInDir(path, realStoreDir))
+        //        path = store->storeDir + path.substr(realStoreDir.size());
+        // }
+        //
+        // auto [storePath, subdir] = state.store->toStorePath(path);
+        */
 
         emitTreeAttrs(
             state,
-            storePath,
+            SourcePath(sourcePath.accessor),
             lockedNode ? lockedNode->lockedRef.input : lockedFlake.flake.lockedRef.input,
             vSourceInfo,
             false,
@@ -793,7 +747,7 @@ void callFlake(EvalState & state,
 
         override
             .alloc(state.symbols.create("dir"))
-            .mkString(CanonPath(subdir).rel());
+            .mkString(sourcePath.path.rel());
 
         overrides.alloc(state.symbols.create(key->second)).mkAttrs(override);
     }

src/libflake/flake/flake.hh

@@ -218,7 +218,7 @@ void callFlake(
 
 void emitTreeAttrs(
     EvalState & state,
-    const StorePath & storePath,
+    const SourcePath & storePath,
     const fetchers::Input & input,
     Value & v,
     bool emptyRevFallback = false,

src/libflake/flake/flakeref.cc

@@ -287,10 +287,10 @@ FlakeRef FlakeRef::fromAttrs(
         fetchers::maybeGetStrAttr(attrs, "dir").value_or(""));
 }
 
-std::pair<StorePath, FlakeRef> FlakeRef::fetchTree(ref<Store> store) const
+std::pair<ref<SourceAccessor>, FlakeRef> FlakeRef::lazyFetch(ref<Store> store) const
 {
-    auto [storePath, lockedInput] = input.fetchToStore(store);
-    return {std::move(storePath), FlakeRef(std::move(lockedInput), subdir)};
+    auto [accessor, lockedInput] = input.getAccessor(store);
+    return {accessor, FlakeRef(std::move(lockedInput), subdir)};
 }
 
 std::tuple<FlakeRef, std::string, ExtendedOutputsSpec> parseFlakeRefWithFragmentAndExtendedOutputsSpec(

src/libflake/flake/flakeref.hh

@@ -63,7 +63,7 @@ struct FlakeRef
         const fetchers::Settings & fetchSettings,
         const fetchers::Attrs & attrs);
 
-    std::pair<StorePath, FlakeRef> fetchTree(ref<Store> store) const;
+    std::pair<ref<SourceAccessor>, FlakeRef> lazyFetch(ref<Store> store) const;
 };
 
 std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);

src/libutil/posix-source-accessor.cc

@@ -195,4 +195,9 @@ ref<SourceAccessor> makeFSSourceAccessor(std::filesystem::path root)
 {
     return make_ref<PosixSourceAccessor>(std::move(root));
 }
+
+bool PosixSourceAccessor::toStringReturnsStorePath() const {
+    return false;
+}
+
 }

src/libutil/posix-source-accessor.hh

@@ -57,6 +57,8 @@ struct PosixSourceAccessor : virtual SourceAccessor
      */
     static SourcePath createAtRoot(const std::filesystem::path & path);
 
+    virtual bool toStringReturnsStorePath() const override;
+
 private:
 
     /**

src/libutil/source-accessor.cc

@@ -64,7 +64,9 @@ void SourceAccessor::setPathDisplay(std::string displayPrefix, std::string displ
 
 std::string SourceAccessor::showPath(const CanonPath & path)
 {
-    return displayPrefix + path.abs() + displaySuffix;
+    return displayPrefix
+        + ((displayPrefix.empty() || displayPrefix.ends_with("/")) ? "" : ":") + path.rel()
+        + displaySuffix;
 }
 
 CanonPath SourceAccessor::resolveSymlinks(
@@ -105,4 +107,8 @@ CanonPath SourceAccessor::resolveSymlinks(
     return res;
 }
 
+bool SourceAccessor::toStringReturnsStorePath() const {
+    return true;
+}
+
 }

src/libutil/source-accessor.hh

@@ -161,6 +161,15 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
 
     virtual std::string showPath(const CanonPath & path);
 
+    /**
+     * System paths: `toString /foo/bar = "/foo/bar"`
+     * Virtual paths: fetched to the store
+     *
+     * In both cases, the returned string functionally identifies the path,
+     * and can still be read.
+     */
+    virtual bool toStringReturnsStorePath() const;
+
     /**
      * Resolve any symlinks in `path` according to the given
      * resolution mode.

src/nix/flake.cc

@@ -18,6 +18,7 @@
 #include "markdown.hh"
 #include "users.hh"
 #include "terminal.hh"
+#include "value/context.hh"
 
 #include <filesystem>
 #include <nlohmann/json.hpp>
@@ -213,9 +214,6 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
         auto lockedFlake = lockFlake();
         auto & flake = lockedFlake.flake;
 
-        // Currently, all flakes are in the Nix store via the rootFS accessor.
-        auto storePath = store->printStorePath(store->toStorePath(flake.path.path.abs()).first);
-
         if (json) {
             nlohmann::json j;
             if (flake.description)
@@ -236,7 +234,6 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 j["revCount"] = *revCount;
             if (auto lastModified = flake.lockedRef.input.getLastModified())
                 j["lastModified"] = *lastModified;
-            j["path"] = storePath;
             j["locks"] = lockedFlake.lockFile.toJSON().first;
             if (auto fingerprint = lockedFlake.getFingerprint(store))
                 j["fingerprint"] = fingerprint->to_string(HashFormat::Base16, false);
@@ -253,9 +250,6 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 logger->cout(
                     ANSI_BOLD "Description:" ANSI_NORMAL "   %s",
                     *flake.description);
-            logger->cout(
-                ANSI_BOLD "Path:" ANSI_NORMAL "          %s",
-                storePath);
             if (auto rev = flake.lockedRef.input.getRev())
                 logger->cout(
                     ANSI_BOLD "Revision:" ANSI_NORMAL "      %s",
@@ -891,7 +885,9 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
         auto cursor = installable.getCursor(*evalState);
 
         auto templateDirAttr = cursor->getAttr("path");
-        auto templateDir = templateDirAttr->getString();
+        auto & v = templateDirAttr->forceValue();
+        NixStringContext ctx;
+        auto templateDir = evalState->coerceToString(noPos, v, ctx, "while casting the template value to a path", false, true).toOwned();
 
         if (!store->isInStore(templateDir))
             evalState->error<TypeError>(
@@ -1523,21 +1519,15 @@ struct CmdFlakePrefetch : FlakeCommand, MixJSON
     {
         auto originalRef = getFlakeRef();
         auto resolvedRef = originalRef.resolve(store);
-        auto [storePath, lockedRef] = resolvedRef.fetchTree(store);
-        auto hash = store->queryPathInfo(storePath)->narHash;
+        auto [accessor, lockedRef] = resolvedRef.lazyFetch(store);
 
         if (json) {
             auto res = nlohmann::json::object();
-            res["storePath"] = store->printStorePath(storePath);
-            res["hash"] = hash.to_string(HashFormat::SRI, true);
             res["original"] = fetchers::attrsToJSON(resolvedRef.toAttrs());
             res["locked"] = fetchers::attrsToJSON(lockedRef.toAttrs());
             logger->cout(res.dump());
         } else {
-            notice("Downloaded '%s' to '%s' (hash '%s').",
-                lockedRef.to_string(),
-                store->printStorePath(storePath),
-                hash.to_string(HashFormat::SRI, true));
+            notice("Fetched '%s'.", lockedRef.to_string());
         }
     }
 };

tests/functional/flakes/flakes.sh

@@ -192,7 +192,6 @@ nix flake metadata "$flake1Dir" | grepQuiet 'URL:.*flake1.*'
 # Test 'nix flake metadata --json'.
 json=$(nix flake metadata flake1 --json | jq .)
 [[ $(echo "$json" | jq -r .description) = 'Bla bla' ]]
-[[ -d $(echo "$json" | jq -r .path) ]]
 [[ $(echo "$json" | jq -r .lastModified) = $(git -C "$flake1Dir" log -n1 --format=%ct) ]]
 hash1=$(echo "$json" | jq -r .revision)
 [[ -n $(echo "$json" | jq -r .fingerprint) ]]

tests/functional/lang/eval-fail-derivation-name.err.exp

@@ -1,20 +1,20 @@
 error:
        … while evaluating the attribute 'outPath'
-         at <nix/derivation-internal.nix>:<number>:<number>:
+         at <nix:derivation-internal.nix>:<number>:<number>:
      <number>|       value = commonAttrs // {
      <number>|         outPath = builtins.getAttr outputName strict;
              |         ^
      <number>|         drvPath = strict.drvPath;
 
        … while calling the 'getAttr' builtin
-         at <nix/derivation-internal.nix>:<number>:<number>:
+         at <nix:derivation-internal.nix>:<number>:<number>:
      <number>|       value = commonAttrs // {
      <number>|         outPath = builtins.getAttr outputName strict;
              |                   ^
      <number>|         drvPath = strict.drvPath;
 
        … while calling the 'derivationStrict' builtin
-         at <nix/derivation-internal.nix>:<number>:<number>:
+         at <nix:derivation-internal.nix>:<number>:<number>:
      <number>|
      <number>|   strict = derivationStrict drvAttrs;
              |            ^