Make the Store API more type-safe

Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.

Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.

Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).

11 files changed, 124 insertions, 107 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 4be4cfeea..6539a346a 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -43,6 +43,19 @@ static char * dupString(const char * s)
 }
 
 
+static char * dupStringWithLen(const char * s, size_t size)
+{
+    char * t;
+#if HAVE_BOEHMGC
+    t = GC_STRNDUP(s, size);
+#else
+    t = strndup(s, size);
+#endif
+    if (!t) throw std::bad_alloc();
+    return t;
+}
+
+
 static void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v)
 {
     checkInterrupt();
@@ -330,10 +343,10 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
             auto path = r.second;
 
             if (store->isInStore(r.second)) {
-                PathSet closure;
-                store->computeFSClosure(store->toStorePath(r.second), closure);
+                StorePathSet closure;
+                store->computeFSClosure(store->parseStorePath(store->toStorePath(r.second)), closure);
                 for (auto & path : closure)
-                    allowedPaths->insert(path);
+                    allowedPaths->insert(store->printStorePath(path));
             } else
                 allowedPaths->insert(r.second);
         }
@@ -550,9 +563,11 @@ void mkString(Value & v, const char * s)
 }
 
 
-Value & mkString(Value & v, const string & s, const PathSet & context)
+Value & mkString(Value & v, std::string_view s, const PathSet & context)
 {
-    mkString(v, s.c_str());
+    v.type = tString;
+    v.string.s = dupStringWithLen(s.data(), s.size());
+    v.string.context = 0;
     if (!context.empty()) {
         size_t n = 0;
         v.string.context = (const char * *)
@@ -1639,15 +1654,16 @@ string EvalState::copyPathToStore(PathSet & context, const Path & path)
         throwEvalError("file names are not allowed to end in '%1%'", drvExtension);
 
     Path dstPath;
-    if (srcToStore[path] != "")
-        dstPath = srcToStore[path];
+    auto i = srcToStore.find(path);
+    if (i != srcToStore.end())
+        dstPath = store->printStorePath(i->second);
     else {
-        dstPath = settings.readOnlyMode
-            ? store->computeStorePathForPath(baseNameOf(path), checkSourcePath(path)).first
-            : store->addToStore(baseNameOf(path), checkSourcePath(path), true, htSHA256, defaultPathFilter, repair);
-        srcToStore[path] = dstPath;
-        printMsg(lvlChatty, format("copied source '%1%' -> '%2%'")
-            % path % dstPath);
+        auto p = settings.readOnlyMode
+            ? store->computeStorePathForPath(std::string(baseNameOf(path)), checkSourcePath(path)).first
+            : store->addToStore(std::string(baseNameOf(path)), checkSourcePath(path), true, htSHA256, defaultPathFilter, repair);
+        dstPath = store->printStorePath(p);
+        srcToStore.insert_or_assign(path, std::move(p));
+        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, dstPath);
     }
 
     context.insert(dstPath);
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 85aa1ccfe..cabc92d15 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -17,6 +17,7 @@ namespace nix {
 
 class Store;
 class EvalState;
+struct StorePath;
 enum RepairFlag : bool;
 
 
@@ -42,14 +43,14 @@ struct Env
 };
 
 
-Value & mkString(Value & v, const string & s, const PathSet & context = PathSet());
+Value & mkString(Value & v, std::string_view s, const PathSet & context = PathSet());
 
 void copyContext(const Value & v, PathSet & context);
 
 
 /* Cache for calls to addToStore(); maps source paths to the store
    paths. */
-typedef std::map<Path, Path> SrcToStore;
+typedef std::map<Path, StorePath> SrcToStore;
 
 
 std::ostream & operator << (std::ostream & str, const Value & v);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 4e22a1d77..a3412eab9 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -19,27 +19,27 @@ DrvInfo::DrvInfo(EvalState & state, const string & attrPath, Bindings * attrs)
 DrvInfo::DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPathWithOutputs)
     : state(&state), attrs(nullptr), attrPath("")
 {
-    auto spec = parseDrvPathWithOutputs(drvPathWithOutputs);
+    auto [drvPath, selectedOutputs] = store->parseDrvPathWithOutputs(drvPathWithOutputs);
 
-    drvPath = spec.first;
+    this->drvPath = store->printStorePath(drvPath);
 
     auto drv = store->derivationFromPath(drvPath);
 
-    name = storePathToName(drvPath);
+    name = drvPath.name();
 
-    if (spec.second.size() > 1)
+    if (selectedOutputs.size() > 1)
         throw Error("building more than one derivation output is not supported, in '%s'", drvPathWithOutputs);
 
     outputName =
-        spec.second.empty()
-        ? get(drv.env, "outputName", "out")
-        : *spec.second.begin();
+        selectedOutputs.empty()
+        ? get(drv.env, "outputName").value_or("out")
+        : *selectedOutputs.begin();
 
     auto i = drv.outputs.find(outputName);
     if (i == drv.outputs.end())
-        throw Error("derivation '%s' does not have output '%s'", drvPath, outputName);
+        throw Error("derivation '%s' does not have output '%s'", store->printStorePath(drvPath), outputName);
 
-    outPath = i->second.path;
+    outPath = store->printStorePath(i->second.path);
 }
 
 
diff --git a/src/libexpr/local.mk b/src/libexpr/local.mk
index ccd5293e4..26b9f14ba 100644
--- a/src/libexpr/local.mk
+++ b/src/libexpr/local.mk
@@ -6,7 +6,7 @@ libexpr_DIR := $(d)
 
 libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(d)/primops/*.cc) $(d)/lexer-tab.cc $(d)/parser-tab.cc
 
-libexpr_LIBS = libutil libstore
+libexpr_LIBS = libutil libstore libnixrust
 
 libexpr_LDFLAGS =
 ifneq ($(OS), FreeBSD)
diff --git a/src/libexpr/names.cc b/src/libexpr/names.cc
index 382088c78..d1c8a6101 100644
--- a/src/libexpr/names.cc
+++ b/src/libexpr/names.cc
@@ -16,14 +16,14 @@ DrvName::DrvName()
    a letter.  The `version' part is the rest (excluding the separating
    dash).  E.g., `apache-httpd-2.0.48' is parsed to (`apache-httpd',
    '2.0.48'). */
-DrvName::DrvName(const string & s) : hits(0)
+DrvName::DrvName(std::string_view s) : hits(0)
 {
-    name = fullName = s;
+    name = fullName = std::string(s);
     for (unsigned int i = 0; i < s.size(); ++i) {
         /* !!! isalpha/isdigit are affected by the locale. */
         if (s[i] == '-' && i + 1 < s.size() && !isalpha(s[i + 1])) {
-            name = string(s, 0, i);
-            version = string(s, i + 1);
+            name = s.substr(0, i);
+            version = s.substr(i + 1);
             break;
         }
     }
diff --git a/src/libexpr/names.hh b/src/libexpr/names.hh
index 13c3093e7..00e14b8c7 100644
--- a/src/libexpr/names.hh
+++ b/src/libexpr/names.hh
@@ -15,7 +15,7 @@ struct DrvName
     unsigned int hits;
 
     DrvName();
-    DrvName(const string & s);
+    DrvName(std::string_view s);
     bool matches(DrvName & n);
 
 private:
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index d693a3b20..3ef827ebc 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -44,29 +44,28 @@ std::pair<string, string> decodeContext(const string & s)
 
 
 InvalidPathError::InvalidPathError(const Path & path) :
-    EvalError(format("path '%1%' is not valid") % path), path(path) {}
+    EvalError("path '%s' is not valid", path), path(path) {}
 
 void EvalState::realiseContext(const PathSet & context)
 {
-    PathSet drvs;
+    std::vector<StorePathWithOutputs> drvs;
 
     for (auto & i : context) {
         std::pair<string, string> decoded = decodeContext(i);
-        Path ctx = decoded.first;
-        assert(store->isStorePath(ctx));
+        auto ctx = store->parseStorePath(decoded.first);
         if (!store->isValidPath(ctx))
-            throw InvalidPathError(ctx);
-        if (!decoded.second.empty() && nix::isDerivation(ctx)) {
-            drvs.insert(decoded.first + "!" + decoded.second);
+            throw InvalidPathError(store->printStorePath(ctx));
+        if (!decoded.second.empty() && ctx.isDerivation()) {
+            drvs.push_back(StorePathWithOutputs{ctx.clone(), {decoded.second}});
 
             /* Add the output of this derivation to the allowed
                paths. */
             if (allowedPaths) {
-                auto drv = store->derivationFromPath(decoded.first);
+                auto drv = store->derivationFromPath(store->parseStorePath(decoded.first));
                 DerivationOutputs::iterator i = drv.outputs.find(decoded.second);
                 if (i == drv.outputs.end())
                     throw Error("derivation '%s' does not have an output named '%s'", decoded.first, decoded.second);
-                allowedPaths->insert(i->second.path);
+                allowedPaths->insert(store->printStorePath(i->second.path));
             }
         }
     }
@@ -74,10 +73,11 @@ void EvalState::realiseContext(const PathSet & context)
     if (drvs.empty()) return;
 
     if (!evalSettings.enableImportFromDerivation)
-        throw EvalError(format("attempted to realize '%1%' during evaluation but 'allow-import-from-derivation' is false") % *(drvs.begin()));
+        throw EvalError("attempted to realize '%1%' during evaluation but 'allow-import-from-derivation' is false",
+            store->printStorePath(drvs.begin()->path));
 
     /* For performance, prefetch all substitute info. */
-    PathSet willBuild, willSubstitute, unknown;
+    StorePathSet willBuild, willSubstitute, unknown;
     unsigned long long downloadSize, narSize;
     store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
     store->buildPaths(drvs);
@@ -100,8 +100,9 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
 
     Path realPath = state.checkSourcePath(state.toRealPath(path, context));
 
-    if (state.store->isStorePath(path) && state.store->isValidPath(path) && isDerivation(path)) {
-        Derivation drv = readDerivation(realPath);
+    // FIXME
+    if (state.store->isStorePath(path) && state.store->isValidPath(state.store->parseStorePath(path)) && isDerivation(path)) {
+        Derivation drv = readDerivation(*state.store, realPath);
         Value & w = *state.allocValue();
         state.mkAttrs(w, 3 + drv.outputs.size());
         Value * v2 = state.allocAttr(w, state.sDrvPath);
@@ -115,7 +116,7 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
 
         for (const auto & o : drv.outputs) {
             v2 = state.allocAttr(w, state.symbols.create(o.first));
-            mkString(*v2, o.second.path, {"!" + o.first + "!" + path});
+            mkString(*v2, state.store->printStorePath(o.second.path), {"!" + o.first + "!" + path});
             outputsVal->listElems()[outputs_index] = state.allocValue();
             mkString(*(outputsVal->listElems()[outputs_index++]), o.first);
         }
@@ -676,24 +677,24 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
            runs. */
         if (path.at(0) == '=') {
             /* !!! This doesn't work if readOnlyMode is set. */
-            PathSet refs;
-            state.store->computeFSClosure(string(path, 1), refs);
+            StorePathSet refs;
+            state.store->computeFSClosure(state.store->parseStorePath(std::string_view(path).substr(1)), refs);
             for (auto & j : refs) {
-                drv.inputSrcs.insert(j);
-                if (isDerivation(j))
-                    drv.inputDrvs[j] = state.store->queryDerivationOutputNames(j);
+                drv.inputSrcs.insert(j.clone());
+                if (j.isDerivation())
+                    drv.inputDrvs[j.clone()] = state.store->queryDerivationOutputNames(j);
             }
         }
 
         /* Handle derivation outputs of the form ‘!<name>!<path>’. */
         else if (path.at(0) == '!') {
             std::pair<string, string> ctx = decodeContext(path);
-            drv.inputDrvs[ctx.first].insert(ctx.second);
+            drv.inputDrvs[state.store->parseStorePath(ctx.first)].insert(ctx.second);
         }
 
         /* Otherwise it's a source file. */
         else
-            drv.inputSrcs.insert(path);
+            drv.inputSrcs.insert(state.store->parseStorePath(path));
     }
 
     /* Do we have all required attributes? */
@@ -703,10 +704,8 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         throw EvalError(format("required attribute 'system' missing, at %1%") % posDrvName);
 
     /* Check whether the derivation name is valid. */
-    checkStoreName(drvName);
     if (isDerivation(drvName))
-        throw EvalError(format("derivation names are not allowed to end in '%1%', at %2%")
-            % drvExtension % posDrvName);
+        throw EvalError("derivation names are not allowed to end in '%s', at %s", drvExtension, posDrvName);
 
     if (outputHash) {
         /* Handle fixed-output derivations. */
@@ -716,52 +715,51 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         HashType ht = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
         Hash h(*outputHash, ht);
 
-        Path outPath = state.store->makeFixedOutputPath(outputHashRecursive, h, drvName);
-        if (!jsonObject) drv.env["out"] = outPath;
-        drv.outputs["out"] = DerivationOutput(outPath,
-            (outputHashRecursive ? "r:" : "") + printHashType(h.type),
-            h.to_string(Base16, false));
+        auto outPath = state.store->makeFixedOutputPath(outputHashRecursive, h, drvName);
+        if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);
+        drv.outputs.insert_or_assign("out", DerivationOutput(std::move(outPath),
+                (outputHashRecursive ? "r:" : "") + printHashType(h.type),
+                h.to_string(Base16, false)));
     }
 
     else {
-        /* Construct the "masked" store derivation, which is the final
-           one except that in the list of outputs, the output paths
-           are empty, and the corresponding environment variables have
-           an empty value.  This ensures that changes in the set of
-           output names do get reflected in the hash. */
+        /* Compute a hash over the "masked" store derivation, which is
+           the final one except that in the list of outputs, the
+           output paths are empty strings, and the corresponding
+           environment variables have an empty value.  This ensures
+           that changes in the set of output names do get reflected in
+           the hash. */
         for (auto & i : outputs) {
             if (!jsonObject) drv.env[i] = "";
-            drv.outputs[i] = DerivationOutput("", "", "");
         }
 
-        /* Use the masked derivation expression to compute the output
-           path. */
-        Hash h = hashDerivationModulo(*state.store, drv);
+        Hash h = hashDerivationModulo(*state.store, Derivation(drv), true);
 
-        for (auto & i : drv.outputs)
-            if (i.second.path == "") {
-                Path outPath = state.store->makeOutputPath(i.first, h, drvName);
-                if (!jsonObject) drv.env[i.first] = outPath;
-                i.second.path = outPath;
-            }
+        for (auto & i : outputs) {
+            auto outPath = state.store->makeOutputPath(i, h, drvName);
+            if (!jsonObject) drv.env[i] = state.store->printStorePath(outPath);
+            drv.outputs.insert_or_assign(i,
+                DerivationOutput(std::move(outPath), "", ""));
+        }
     }
 
     /* Write the resulting term into the Nix store directory. */
-    Path drvPath = writeDerivation(state.store, drv, drvName, state.repair);
+    auto drvPath = writeDerivation(state.store, drv, drvName, state.repair);
+    auto drvPathS = state.store->printStorePath(drvPath);
 
-    printMsg(lvlChatty, format("instantiated '%1%' -> '%2%'")
-        % drvName % drvPath);
+    printMsg(lvlChatty, "instantiated '%1%' -> '%2%'", drvName, drvPathS);
 
     /* Optimisation, but required in read-only mode! because in that
        case we don't actually write store derivations, so we can't
        read them later. */
-    drvHashes[drvPath] = hashDerivationModulo(*state.store, drv);
+    drvHashes.insert_or_assign(drvPath.clone(),
+        hashDerivationModulo(*state.store, Derivation(drv), false));
 
     state.mkAttrs(v, 1 + drv.outputs.size());
-    mkString(*state.allocAttr(v, state.sDrvPath), drvPath, {"=" + drvPath});
+    mkString(*state.allocAttr(v, state.sDrvPath), drvPathS, {"=" + drvPathS});
     for (auto & i : drv.outputs) {
         mkString(*state.allocAttr(v, state.symbols.create(i.first)),
-            i.second.path, {"!" + i.first + "!" + drvPath});
+            state.store->printStorePath(i.second.path), {"!" + i.first + "!" + drvPathS});
     }
     v.attrs->sort();
 }
@@ -814,7 +812,7 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
         throw EvalError(format("path '%1%' is not in the Nix store, at %2%") % path % pos);
     Path path2 = state.store->toStorePath(path);
     if (!settings.readOnlyMode)
-        state.store->ensurePath(path2);
+        state.store->ensurePath(state.store->parseStorePath(path2));
     context.insert(path2);
     mkString(v, path, context);
 }
@@ -1010,17 +1008,17 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
     string name = state.forceStringNoCtx(*args[0], pos);
     string contents = state.forceString(*args[1], context, pos);
 
-    PathSet refs;
+    StorePathSet refs;
 
     for (auto path : context) {
         if (path.at(0) != '/')
             throw EvalError(format("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%") % name % pos);
-        refs.insert(path);
+        refs.insert(state.store->parseStorePath(path));
     }
 
-    Path storePath = settings.readOnlyMode
+    auto storePath = state.store->printStorePath(settings.readOnlyMode
         ? state.store->computeStorePathForText(name, contents, refs)
-        : state.store->addTextToStore(name, contents, refs, state.repair);
+        : state.store->addTextToStore(name, contents, refs, state.repair));
 
     /* Note: we don't need to add `context' to the context of the
        result, since `storePath' itself has references to the paths
@@ -1060,21 +1058,18 @@ static void addPath(EvalState & state, const Pos & pos, const string & name, con
         return state.forceBool(res, pos);
     }) : defaultPathFilter;
 
-    Path expectedStorePath;
-    if (expectedHash) {
-        expectedStorePath =
-            state.store->makeFixedOutputPath(recursive, expectedHash, name);
-    }
+    std::optional<StorePath> expectedStorePath;
+    if (expectedHash)
+        expectedStorePath = state.store->makeFixedOutputPath(recursive, expectedHash, name);
     Path dstPath;
-    if (!expectedHash || !state.store->isValidPath(expectedStorePath)) {
-        dstPath = settings.readOnlyMode
+    if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
+        dstPath = state.store->printStorePath(settings.readOnlyMode
             ? state.store->computeStorePathForPath(name, path, recursive, htSHA256, filter).first
-            : state.store->addToStore(name, path, recursive, htSHA256, filter, state.repair);
-        if (expectedHash && expectedStorePath != dstPath) {
-            throw Error(format("store path mismatch in (possibly filtered) path added from '%1%'") % path);
-        }
+            : state.store->addToStore(name, path, recursive, htSHA256, filter, state.repair));
+        if (expectedHash && expectedStorePath != state.store->parseStorePath(dstPath))
+            throw Error("store path mismatch in (possibly filtered) path added from '%s'", path);
     } else
-        dstPath = expectedStorePath;
+        dstPath = state.store->printStorePath(*expectedStorePath);
 
     mkString(v, dstPath, {dstPath});
 }
@@ -1091,7 +1086,7 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
     if (args[0]->type != tLambda)
         throw TypeError(format("first argument in call to 'filterSource' is not a function but %1%, at %2%") % showType(*args[0]) % pos);
 
-    addPath(state, pos, baseNameOf(path), path, args[0], true, Hash(), v);
+    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], true, Hash(), v);
 }
 
 static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value & v)
@@ -2151,7 +2146,7 @@ void EvalState::createBaseEnv()
     }
 
     if (!evalSettings.pureEval) {
-        mkString(v, settings.thisSystem);
+        mkString(v, settings.thisSystem.get());
         addConstant("__currentSystem", v);
     }
 
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 2d79739ea..94fa0158c 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -148,7 +148,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
         if (!state.store->isStorePath(i.name))
             throw EvalError("Context key '%s' is not a store path, at %s", i.name, i.pos);
         if (!settings.readOnlyMode)
-            state.store->ensurePath(i.name);
+            state.store->ensurePath(state.store->parseStorePath(i.name));
         state.forceAttrs(*i.value, *i.pos);
         auto iter = i.value->attrs->find(sPath);
         if (iter != i.value->attrs->end()) {
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 9d0c64291..4aee1073e 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -69,7 +69,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
                 return files.count(file);
             };
 
-            gitInfo.storePath = store->addToStore("source", uri, true, htSHA256, filter);
+            gitInfo.storePath = store->printStorePath(store->addToStore("source", uri, true, htSHA256, filter));
 
             return gitInfo;
         }
@@ -156,7 +156,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
         gitInfo.storePath = json["storePath"];
 
-        if (store->isValidPath(gitInfo.storePath)) {
+        if (store->isValidPath(store->parseStorePath(gitInfo.storePath))) {
             gitInfo.revCount = json["revCount"];
             return gitInfo;
         }
@@ -176,7 +176,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
     unpackTarfile(*source, tmpDir);
 
-    gitInfo.storePath = store->addToStore(name, tmpDir);
+    gitInfo.storePath = store->printStorePath(store->addToStore(name, tmpDir));
 
     gitInfo.revCount = std::stoull(runProgram("git", true, { "-C", cacheDir, "rev-list", "--count", gitInfo.rev }));
 
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index a907d0e1c..db274fa4f 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -63,7 +63,7 @@ HgInfo exportMercurial(ref<Store> store, const std::string & uri,
                 return files.count(file);
             };
 
-            hgInfo.storePath = store->addToStore("source", uri, true, htSHA256, filter);
+            hgInfo.storePath = store->printStorePath(store->addToStore("source", uri, true, htSHA256, filter));
 
             return hgInfo;
         }
@@ -134,7 +134,7 @@ HgInfo exportMercurial(ref<Store> store, const std::string & uri,
 
         hgInfo.storePath = json["storePath"];
 
-        if (store->isValidPath(hgInfo.storePath)) {
+        if (store->isValidPath(store->parseStorePath(hgInfo.storePath))) {
             printTalkative("using cached Mercurial store path '%s'", hgInfo.storePath);
             return hgInfo;
         }
@@ -150,7 +150,7 @@ HgInfo exportMercurial(ref<Store> store, const std::string & uri,
 
     deletePath(tmpDir + "/.hg_archival.txt");
 
-    hgInfo.storePath = store->addToStore(name, tmpDir);
+    hgInfo.storePath = store->printStorePath(store->addToStore(name, tmpDir));
 
     nlohmann::json json;
     json["storePath"] = hgInfo.storePath;
diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index 91faea122..7ba5e1c14 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -38,7 +38,12 @@ public:
         return s < s2.s;
     }
 
-    operator const string & () const
+    operator const std::string & () const
+    {
+        return *s;
+    }
+
+    operator const std::string_view () const
     {
         return *s;
     }