diff --git a/.version b/.version
index 7208c2182..f398a2061 100644
--- a/.version
+++ b/.version
@@ -1 +1 @@
-2.4
\ No newline at end of file
+3.0
\ No newline at end of file
diff --git a/doc/manual/command-ref/conf-file.xml b/doc/manual/command-ref/conf-file.xml
index 1fa74a143..9c55526a3 100644
--- a/doc/manual/command-ref/conf-file.xml
+++ b/doc/manual/command-ref/conf-file.xml
@@ -370,34 +370,6 @@ false</literal>.</para>
 
   </varlistentry>
 
-  <varlistentry xml:id="conf-hashed-mirrors"><term><literal>hashed-mirrors</literal></term>
-
-    <listitem><para>A list of web servers used by
-    <function>builtins.fetchurl</function> to obtain files by
-    hash. The default is
-    <literal>http://tarballs.nixos.org/</literal>. Given a hash type
-    <replaceable>ht</replaceable> and a base-16 hash
-    <replaceable>h</replaceable>, Nix will try to download the file
-    from
-    <literal>hashed-mirror/<replaceable>ht</replaceable>/<replaceable>h</replaceable></literal>.
-    This allows files to be downloaded even if they have disappeared
-    from their original URI. For example, given the default mirror
-    <literal>http://tarballs.nixos.org/</literal>, when building the derivation
-
-<programlisting>
-builtins.fetchurl {
-  url = "https://example.org/foo-1.2.3.tar.xz";
-  sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";
-}
-</programlisting>
-
-    Nix will attempt to download this file from
-    <literal>http://tarballs.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae</literal>
-    first. If it is not available there, if will try the original URI.</para></listitem>
-
-  </varlistentry>
-
-
   <varlistentry xml:id="conf-http-connections"><term><literal>http-connections</literal></term>
 
     <listitem><para>The maximum number of parallel TCP connections
diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index e07117496..3579d8fff 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -33,7 +33,7 @@ std::string escapeUri(std::string uri)
 
 static string currentLoad;
 
-static AutoCloseFD openSlotLock(const Machine & m, unsigned long long slot)
+static AutoCloseFD openSlotLock(const Machine & m, uint64_t slot)
 {
     return openLockFile(fmt("%s/%s-%d", currentLoad, escapeUri(m.storeUri), slot), true);
 }
@@ -119,7 +119,7 @@ static int _main(int argc, char * * argv)
                 bool rightType = false;
 
                 Machine * bestMachine = nullptr;
-                unsigned long long bestLoad = 0;
+                uint64_t bestLoad = 0;
                 for (auto & m : machines) {
                     debug("considering building on remote machine '%s'", m.storeUri);
 
@@ -130,8 +130,8 @@ static int _main(int argc, char * * argv)
                         m.mandatoryMet(requiredFeatures)) {
                         rightType = true;
                         AutoCloseFD free;
-                        unsigned long long load = 0;
-                        for (unsigned long long slot = 0; slot < m.maxJobs; ++slot) {
+                        uint64_t load = 0;
+                        for (uint64_t slot = 0; slot < m.maxJobs; ++slot) {
                             auto slotLock = openSlotLock(m, slot);
                             if (lockFile(slotLock.get(), ltWrite, false)) {
                                 if (!free) {
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index 77f3abdeb..c2bb2888b 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -106,6 +106,6 @@ void emitTreeAttrs(
     EvalState & state,
     const fetchers::Tree & tree,
     const fetchers::Input & input,
-    Value & v);
+    Value & v, bool emptyRevFallback = false);
 
 }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 5059286b3..a98cadc80 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -65,7 +65,7 @@ void EvalState::realiseContext(const PathSet & context)
 
     /* For performance, prefetch all substitute info. */
     StorePathSet willBuild, willSubstitute, unknown;
-    unsigned long long downloadSize, narSize;
+    uint64_t downloadSize, narSize;
     store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
 
     store->buildPaths(drvs);
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
deleted file mode 100644
index 1d64caac3..000000000
--- a/src/libexpr/primops/fetchGit.cc
+++ /dev/null
@@ -1,91 +0,0 @@
-#include "primops.hh"
-#include "eval-inline.hh"
-#include "store-api.hh"
-#include "hash.hh"
-#include "fetchers.hh"
-#include "url.hh"
-
-namespace nix {
-
-static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Value & v)
-{
-    std::string url;
-    std::optional<std::string> ref;
-    std::optional<Hash> rev;
-    std::string name = "source";
-    bool fetchSubmodules = false;
-    PathSet context;
-
-    state.forceValue(*args[0]);
-
-    if (args[0]->type == tAttrs) {
-
-        state.forceAttrs(*args[0], pos);
-
-        for (auto & attr : *args[0]->attrs) {
-            string n(attr.name);
-            if (n == "url")
-                url = state.coerceToString(*attr.pos, *attr.value, context, false, false);
-            else if (n == "ref")
-                ref = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (n == "rev")
-                rev = Hash::parseAny(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA1);
-            else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (n == "submodules")
-                fetchSubmodules = state.forceBool(*attr.value, *attr.pos);
-            else
-                throw EvalError({
-                    .hint = hintfmt("unsupported argument '%s' to 'fetchGit'", attr.name),
-                    .errPos = *attr.pos
-                });
-        }
-
-        if (url.empty())
-            throw EvalError({
-                .hint = hintfmt("'url' argument required"),
-                .errPos = pos
-            });
-
-    } else
-        url = state.coerceToString(pos, *args[0], context, false, false);
-
-    // FIXME: git externals probably can be used to bypass the URI
-    // whitelist. Ah well.
-    state.checkURI(url);
-
-    if (evalSettings.pureEval && !rev)
-        throw Error("in pure evaluation mode, 'fetchGit' requires a Git revision");
-
-    fetchers::Attrs attrs;
-    attrs.insert_or_assign("type", "git");
-    attrs.insert_or_assign("url", url.find("://") != std::string::npos ? url : "file://" + url);
-    if (ref) attrs.insert_or_assign("ref", *ref);
-    if (rev) attrs.insert_or_assign("rev", rev->gitRev());
-    if (fetchSubmodules) attrs.insert_or_assign("submodules", fetchers::Explicit<bool>{true});
-    auto input = fetchers::Input::fromAttrs(std::move(attrs));
-
-    // FIXME: use name?
-    auto [tree, input2] = input.fetch(state.store);
-
-    state.mkAttrs(v, 8);
-    auto storePath = state.store->printStorePath(tree.storePath);
-    mkString(*state.allocAttr(v, state.sOutPath), storePath, PathSet({storePath}));
-    // Backward compatibility: set 'rev' to
-    // 0000000000000000000000000000000000000000 for a dirty tree.
-    auto rev2 = input2.getRev().value_or(Hash(htSHA1));
-    mkString(*state.allocAttr(v, state.symbols.create("rev")), rev2.gitRev());
-    mkString(*state.allocAttr(v, state.symbols.create("shortRev")), rev2.gitShortRev());
-    // Backward compatibility: set 'revCount' to 0 for a dirty tree.
-    mkInt(*state.allocAttr(v, state.symbols.create("revCount")),
-        input2.getRevCount().value_or(0));
-    mkBool(*state.allocAttr(v, state.symbols.create("submodules")), fetchSubmodules);
-    v.attrs->sort();
-
-    if (state.allowedPaths)
-        state.allowedPaths->insert(tree.actualPath);
-}
-
-static RegisterPrimOp r("fetchGit", 1, prim_fetchGit);
-
-}
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 5f480d919..cddcf0e59 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -14,7 +14,8 @@ void emitTreeAttrs(
     EvalState & state,
     const fetchers::Tree & tree,
     const fetchers::Input & input,
-    Value & v)
+    Value & v,
+    bool emptyRevFallback)
 {
     assert(input.isImmutable());
 
@@ -34,10 +35,20 @@ void emitTreeAttrs(
     if (auto rev = input.getRev()) {
         mkString(*state.allocAttr(v, state.symbols.create("rev")), rev->gitRev());
         mkString(*state.allocAttr(v, state.symbols.create("shortRev")), rev->gitShortRev());
+    } else if (emptyRevFallback) {
+        // Backwards compat for `builtins.fetchGit`: dirty repos return an empty sha1 as rev
+        auto emptyHash = Hash(htSHA1);
+        mkString(*state.allocAttr(v, state.symbols.create("rev")), emptyHash.gitRev());
+        mkString(*state.allocAttr(v, state.symbols.create("shortRev")), emptyHash.gitRev());
     }
 
+    if (input.getType() == "git")
+        mkBool(*state.allocAttr(v, state.symbols.create("submodules")), maybeGetBoolAttr(input.attrs, "submodules").value_or(false));
+
     if (auto revCount = input.getRevCount())
         mkInt(*state.allocAttr(v, state.symbols.create("revCount")), *revCount);
+    else if (emptyRevFallback)
+        mkInt(*state.allocAttr(v, state.symbols.create("revCount")), 0);
 
     if (auto lastModified = input.getLastModified()) {
         mkInt(*state.allocAttr(v, state.symbols.create("lastModified")), *lastModified);
@@ -48,10 +59,26 @@ void emitTreeAttrs(
     v.attrs->sort();
 }
 
-static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)
+std::string fixURI(std::string uri, EvalState &state)
 {
-    settings.requireExperimentalFeature("flakes");
+    state.checkURI(uri);
+    return uri.find("://") != std::string::npos ? uri : "file://" + uri;
+}
 
+void addURI(EvalState &state, fetchers::Attrs &attrs, Symbol name, std::string v)
+{
+    string n(name);
+    attrs.emplace(name, n == "url" ? fixURI(v, state) : v);
+}
+
+static void fetchTree(
+    EvalState &state,
+    const Pos &pos,
+    Value **args,
+    Value &v,
+    const std::optional<std::string> type,
+    bool emptyRevFallback = false
+) {
     fetchers::Input input;
     PathSet context;
 
@@ -64,8 +91,15 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
 
         for (auto & attr : *args[0]->attrs) {
             state.forceValue(*attr.value);
-            if (attr.value->type == tString)
-                attrs.emplace(attr.name, attr.value->string.s);
+            if (attr.value->type == tPath || attr.value->type == tString)
+                addURI(
+                    state,
+                    attrs,
+                    attr.name,
+                    state.coerceToString(*attr.pos, *attr.value, context, false, false)
+                );
+            else if (attr.value->type == tString)
+                addURI(state, attrs, attr.name, attr.value->string.s);
             else if (attr.value->type == tBool)
                 attrs.emplace(attr.name, fetchers::Explicit<bool>{attr.value->boolean});
             else if (attr.value->type == tInt)
@@ -75,6 +109,9 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
                     attr.name, showType(*attr.value));
         }
 
+        if (type)
+            attrs.emplace("type", type.value());
+
         if (!attrs.count("type"))
             throw Error({
                 .hint = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
@@ -82,8 +119,18 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
             });
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
-    } else
-        input = fetchers::Input::fromURL(state.coerceToString(pos, *args[0], context, false, false));
+    } else {
+        auto url = fixURI(state.coerceToString(pos, *args[0], context, false, false), state);
+
+        if (type == "git") {
+            fetchers::Attrs attrs;
+            attrs.emplace("type", "git");
+            attrs.emplace("url", url);
+            input = fetchers::Input::fromAttrs(std::move(attrs));
+        } else {
+            input = fetchers::Input::fromURL(url);
+        }
+    }
 
     if (!evalSettings.pureEval && !input.isDirect())
         input = lookupInRegistries(state.store, input).first;
@@ -96,7 +143,13 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
     if (state.allowedPaths)
         state.allowedPaths->insert(tree.actualPath);
 
-    emitTreeAttrs(state, tree, input2, v);
+    emitTreeAttrs(state, tree, input2, v, emptyRevFallback);
+}
+
+static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    settings.requireExperimentalFeature("flakes");
+    fetchTree(state, pos, args, v, std::nullopt);
 }
 
 static RegisterPrimOp r("fetchTree", 1, prim_fetchTree);
@@ -178,7 +231,13 @@ static void prim_fetchTarball(EvalState & state, const Pos & pos, Value * * args
     fetch(state, pos, args, v, "fetchTarball", true, "source");
 }
 
+static void prim_fetchGit(EvalState &state, const Pos &pos, Value **args, Value &v)
+{
+    fetchTree(state, pos, args, v, "git", true);
+}
+
 static RegisterPrimOp r2("__fetchurl", 1, prim_fetchurl);
 static RegisterPrimOp r3("fetchTarball", 1, prim_fetchTarball);
+static RegisterPrimOp r4("fetchGit", 1, prim_fetchGit);
 
 }
diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 47af59bc4..08b83b0db 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -134,7 +134,7 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
 
     if (auto prevNarHash = getNarHash()) {
         if (narHash != *prevNarHash)
-            throw Error("NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
+            throw Error((unsigned int) 102, "NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
                 to_string(), tree.actualPath, prevNarHash->to_string(SRI, true), narHash->to_string(SRI, true));
     }
 
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 52718c231..2b1f25ca3 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -36,7 +36,7 @@ void printGCWarning()
 
 void printMissing(ref<Store> store, const std::vector<StorePathWithOutputs> & paths, Verbosity lvl)
 {
-    unsigned long long downloadSize, narSize;
+    uint64_t downloadSize, narSize;
     StorePathSet willBuild, willSubstitute, unknown;
     store->queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
     printMissing(store, willBuild, willSubstitute, unknown, downloadSize, narSize, lvl);
@@ -45,7 +45,7 @@ void printMissing(ref<Store> store, const std::vector<StorePathWithOutputs> & pa
 
 void printMissing(ref<Store> store, const StorePathSet & willBuild,
     const StorePathSet & willSubstitute, const StorePathSet & unknown,
-    unsigned long long downloadSize, unsigned long long narSize, Verbosity lvl)
+    uint64_t downloadSize, uint64_t narSize, Verbosity lvl)
 {
     if (!willBuild.empty()) {
         if (willBuild.size() == 1)
@@ -384,7 +384,7 @@ RunPager::~RunPager()
 }
 
 
-string showBytes(unsigned long long bytes)
+string showBytes(uint64_t bytes)
 {
     return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
 }
diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index f558247c0..ffae5d796 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -47,7 +47,7 @@ void printMissing(
 
 void printMissing(ref<Store> store, const StorePathSet & willBuild,
     const StorePathSet & willSubstitute, const StorePathSet & unknown,
-    unsigned long long downloadSize, unsigned long long narSize, Verbosity lvl = lvlInfo);
+    uint64_t downloadSize, uint64_t narSize, Verbosity lvl = lvlInfo);
 
 string getArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end);
@@ -110,7 +110,7 @@ extern volatile ::sig_atomic_t blockInt;
 
 /* GC helpers. */
 
-string showBytes(unsigned long long bytes);
+string showBytes(uint64_t bytes);
 
 struct GCResults;
 
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index d26f96b74..4156cd678 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -297,7 +297,7 @@ public:
     GoalPtr makeDerivationGoal(const StorePath & drvPath, const StringSet & wantedOutputs, BuildMode buildMode = bmNormal);
     std::shared_ptr<DerivationGoal> makeBasicDerivationGoal(const StorePath & drvPath,
         const BasicDerivation & drv, BuildMode buildMode = bmNormal);
-    GoalPtr makeSubstitutionGoal(const StorePath & storePath, RepairFlag repair = NoRepair);
+    GoalPtr makeSubstitutionGoal(const StorePath & storePath, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
 
     /* Remove a dead goal. */
     void removeGoal(GoalPtr goal);
@@ -1206,7 +1206,7 @@ void DerivationGoal::haveDerivation()
        them. */
     if (settings.useSubstitutes && parsedDrv->substitutesAllowed())
         for (auto & i : invalidOutputs)
-            addWaitee(worker.makeSubstitutionGoal(i, buildMode == bmRepair ? Repair : NoRepair));
+            addWaitee(worker.makeSubstitutionGoal(i, buildMode == bmRepair ? Repair : NoRepair, getDerivationCA(*drv)));
 
     if (waitees.empty()) /* to prevent hang (no wake-up event) */
         outputsSubstituted();
@@ -1646,13 +1646,13 @@ void DerivationGoal::buildDone()
                So instead, check if the disk is (nearly) full now.  If
                so, we don't mark this build as a permanent failure. */
 #if HAVE_STATVFS
-            unsigned long long required = 8ULL * 1024 * 1024; // FIXME: make configurable
+            uint64_t required = 8ULL * 1024 * 1024; // FIXME: make configurable
             struct statvfs st;
             if (statvfs(worker.store.realStoreDir.c_str(), &st) == 0 &&
-                (unsigned long long) st.f_bavail * st.f_bsize < required)
+                (uint64_t) st.f_bavail * st.f_bsize < required)
                 diskFull = true;
             if (statvfs(tmpDir.c_str(), &st) == 0 &&
-                (unsigned long long) st.f_bavail * st.f_bsize < required)
+                (uint64_t) st.f_bavail * st.f_bsize < required)
                 diskFull = true;
 #endif
 
@@ -2851,7 +2851,7 @@ struct RestrictedStore : public LocalFSStore
 
     void queryMissing(const std::vector<StorePathWithOutputs> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-        unsigned long long & downloadSize, unsigned long long & narSize) override
+        uint64_t & downloadSize, uint64_t & narSize) override
     {
         /* This is slightly impure since it leaks information to the
            client about what paths will be built/substituted or are
@@ -4276,6 +4276,10 @@ private:
     /* The store path that should be realised through a substitute. */
     StorePath storePath;
 
+    /* The path the substituter refers to the path as. This will be
+     * different when the stores have different names. */
+    std::optional<StorePath> subPath;
+
     /* The remaining substituters. */
     std::list<ref<Store>> subs;
 
@@ -4309,8 +4313,11 @@ private:
     typedef void (SubstitutionGoal::*GoalState)();
     GoalState state;
 
+    /* Content address for recomputing store path */
+    std::optional<ContentAddress> ca;
+
 public:
-    SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair = NoRepair);
+    SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
     ~SubstitutionGoal();
 
     void timedOut(Error && ex) override { abort(); };
@@ -4340,10 +4347,11 @@ public:
 };
 
 
-SubstitutionGoal::SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair)
+SubstitutionGoal::SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair, std::optional<ContentAddress> ca)
     : Goal(worker)
     , storePath(storePath)
     , repair(repair)
+    , ca(ca)
 {
     state = &SubstitutionGoal::init;
     name = fmt("substitution of '%s'", worker.store.printStorePath(this->storePath));
@@ -4418,14 +4426,18 @@ void SubstitutionGoal::tryNext()
     sub = subs.front();
     subs.pop_front();
 
-    if (sub->storeDir != worker.store.storeDir) {
+    if (ca) {
+        subPath = sub->makeFixedOutputPathFromCA(storePath.name(), *ca);
+        if (sub->storeDir == worker.store.storeDir)
+            assert(subPath == storePath);
+    } else if (sub->storeDir != worker.store.storeDir) {
         tryNext();
         return;
     }
 
     try {
         // FIXME: make async
-        info = sub->queryPathInfo(storePath);
+        info = sub->queryPathInfo(subPath ? *subPath : storePath);
     } catch (InvalidPath &) {
         tryNext();
         return;
@@ -4444,6 +4456,19 @@ void SubstitutionGoal::tryNext()
         throw;
     }
 
+    if (info->path != storePath) {
+        if (info->isContentAddressed(*sub) && info->references.empty()) {
+            auto info2 = std::make_shared<ValidPathInfo>(*info);
+            info2->path = storePath;
+            info = info2;
+        } else {
+            printError("asked '%s' for '%s' but got '%s'",
+                sub->getUri(), worker.store.printStorePath(storePath), sub->printStorePath(info->path));
+            tryNext();
+            return;
+        }
+    }
+
     /* Update the total expected download size. */
     auto narInfo = std::dynamic_pointer_cast<const NarInfo>(info);
 
@@ -4533,7 +4558,7 @@ void SubstitutionGoal::tryToRun()
             PushActivity pact(act.id);
 
             copyStorePath(ref<Store>(sub), ref<Store>(worker.store.shared_from_this()),
-                storePath, repair, sub->isTrusted ? NoCheckSigs : CheckSigs);
+                subPath ? *subPath : storePath, repair, sub->isTrusted ? NoCheckSigs : CheckSigs);
 
             promise.set_value();
         } catch (...) {
@@ -4666,11 +4691,11 @@ std::shared_ptr<DerivationGoal> Worker::makeBasicDerivationGoal(const StorePath
 }
 
 
-GoalPtr Worker::makeSubstitutionGoal(const StorePath & path, RepairFlag repair)
+GoalPtr Worker::makeSubstitutionGoal(const StorePath & path, RepairFlag repair, std::optional<ContentAddress> ca)
 {
     GoalPtr goal = substitutionGoals[path].lock(); // FIXME
     if (!goal) {
-        goal = std::make_shared<SubstitutionGoal>(path, *this, repair);
+        goal = std::make_shared<SubstitutionGoal>(path, *this, repair, ca);
         substitutionGoals.insert_or_assign(path, goal);
         wakeUp(goal);
     }
@@ -5038,7 +5063,7 @@ void Worker::markContentsGood(const StorePath & path)
 static void primeCache(Store & store, const std::vector<StorePathWithOutputs> & paths)
 {
     StorePathSet willBuild, willSubstitute, unknown;
-    unsigned long long downloadSize, narSize;
+    uint64_t downloadSize, narSize;
     store.queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
 
     if (!willBuild.empty() && 0 == settings.maxBuildJobs && getMachines().empty())
diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index 03bb77488..6585a480d 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -58,26 +58,6 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         }
     };
 
-    /* We always have one output, and if it's a fixed-output derivation (as
-       checked below) it must be the only output */
-    auto & output = drv.outputs.begin()->second;
-
-    /* Try the hashed mirrors first. */
-    if (auto hash = std::get_if<DerivationOutputFixed>(&output.output)) {
-        if (hash->hash.method == FileIngestionMethod::Flat) {
-            for (auto hashedMirror : settings.hashedMirrors.get()) {
-                try {
-                    if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
-                    fetch(hashedMirror + printHashType(hash->hash.hash.type) + "/" + hash->hash.hash.to_string(Base16, false));
-                    return;
-                } catch (Error & e) {
-                    debug(e.what());
-                }
-            }
-        }
-    }
-
-    /* Otherwise try the specified URL. */
     fetch(mainUrl);
 }
 
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index b760d74fa..5e568fc94 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -86,7 +86,7 @@ struct TunnelLogger : public Logger
     }
 
     /* startWork() means that we're starting an operation for which we
-      want to send out stderr to the client. */
+       want to send out stderr to the client. */
     void startWork()
     {
         auto state(state_.lock());
@@ -579,7 +579,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         auto path = store->parseStorePath(readString(from));
         logger->startWork();
         SubstitutablePathInfos infos;
-        store->querySubstitutablePathInfos({path}, infos);
+        store->querySubstitutablePathInfos({{path, std::nullopt}}, infos);
         logger->stopWork();
         auto i = infos.find(path);
         if (i == infos.end())
@@ -595,10 +595,16 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopQuerySubstitutablePathInfos: {
-        auto paths = readStorePaths<StorePathSet>(*store, from);
-        logger->startWork();
         SubstitutablePathInfos infos;
-        store->querySubstitutablePathInfos(paths, infos);
+        StorePathCAMap pathsMap = {};
+        if (GET_PROTOCOL_MINOR(clientVersion) < 22) {
+            auto paths = readStorePaths<StorePathSet>(*store, from);
+            for (auto & path : paths)
+                pathsMap.emplace(path, std::nullopt);
+        } else
+            pathsMap = readStorePathCAMap(*store, from);
+        logger->startWork();
+        store->querySubstitutablePathInfos(pathsMap, infos);
         logger->stopWork();
         to << infos.size();
         for (auto & i : infos) {
@@ -703,24 +709,84 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (!trusted)
             info.ultimate = false;
 
-        std::unique_ptr<Source> source;
-        if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
-            source = std::make_unique<TunnelSource>(from, to);
-        else {
-            StringSink saved;
-            TeeSource tee { from, saved };
-            ParseSink ether;
-            parseDump(ether, tee);
-            source = std::make_unique<StringSource>(std::move(*saved.s));
+        if (GET_PROTOCOL_MINOR(clientVersion) >= 23) {
+
+            struct FramedSource : Source
+            {
+                Source & from;
+                bool eof = false;
+                std::vector<unsigned char> pending;
+                size_t pos = 0;
+
+                FramedSource(Source & from) : from(from)
+                { }
+
+                ~FramedSource()
+                {
+                    if (!eof) {
+                        while (true) {
+                            auto n = readInt(from);
+                            if (!n) break;
+                            std::vector<unsigned char> data(n);
+                            from(data.data(), n);
+                        }
+                    }
+                }
+
+                size_t read(unsigned char * data, size_t len) override
+                {
+                    if (eof) throw EndOfFile("reached end of FramedSource");
+
+                    if (pos >= pending.size()) {
+                        size_t len = readInt(from);
+                        if (!len) {
+                            eof = true;
+                            return 0;
+                        }
+                        pending = std::vector<unsigned char>(len);
+                        pos = 0;
+                        from(pending.data(), len);
+                    }
+
+                    auto n = std::min(len, pending.size() - pos);
+                    memcpy(data, pending.data() + pos, n);
+                    pos += n;
+                    return n;
+                }
+            };
+
+            logger->startWork();
+
+            {
+                FramedSource source(from);
+                store->addToStore(info, source, (RepairFlag) repair,
+                    dontCheckSigs ? NoCheckSigs : CheckSigs);
+            }
+
+            logger->stopWork();
         }
 
-        logger->startWork();
+        else {
+            std::unique_ptr<Source> source;
+            if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
+                source = std::make_unique<TunnelSource>(from, to);
+            else {
+                StringSink saved;
+                TeeSource tee { from, saved };
+                ParseSink ether;
+                parseDump(ether, tee);
+                source = std::make_unique<StringSource>(std::move(*saved.s));
+            }
 
-        // FIXME: race if addToStore doesn't read source?
-        store->addToStore(info, *source, (RepairFlag) repair,
-            dontCheckSigs ? NoCheckSigs : CheckSigs);
+            logger->startWork();
+
+            // FIXME: race if addToStore doesn't read source?
+            store->addToStore(info, *source, (RepairFlag) repair,
+                dontCheckSigs ? NoCheckSigs : CheckSigs);
+
+            logger->stopWork();
+        }
 
-        logger->stopWork();
         break;
     }
 
@@ -730,7 +796,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             targets.push_back(store->parsePathWithOutputs(s));
         logger->startWork();
         StorePathSet willBuild, willSubstitute, unknown;
-        unsigned long long downloadSize, narSize;
+        uint64_t downloadSize, narSize;
         store->queryMissing(targets, willBuild, willSubstitute, unknown, downloadSize, narSize);
         logger->stopWork();
         writeStorePaths(*store, to, willBuild);
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index aaed5c218..e74382ed2 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -500,7 +500,7 @@ struct LocalStore::GCState
     StorePathSet alive;
     bool gcKeepOutputs;
     bool gcKeepDerivations;
-    unsigned long long bytesInvalidated;
+    uint64_t bytesInvalidated;
     bool moveToTrash = true;
     bool shouldDelete;
     GCState(const GCOptions & options, GCResults & results)
@@ -518,7 +518,7 @@ bool LocalStore::isActiveTempFile(const GCState & state,
 
 void LocalStore::deleteGarbage(GCState & state, const Path & path)
 {
-    unsigned long long bytesFreed;
+    uint64_t bytesFreed;
     deletePath(path, bytesFreed);
     state.results.bytesFreed += bytesFreed;
 }
@@ -528,7 +528,7 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
 {
     checkInterrupt();
 
-    unsigned long long size = 0;
+    uint64_t size = 0;
 
     auto storePath = maybeParseStorePath(path);
     if (storePath && isValidPath(*storePath)) {
@@ -687,7 +687,7 @@ void LocalStore::removeUnusedLinks(const GCState & state)
     AutoCloseDir dir(opendir(linksDir.c_str()));
     if (!dir) throw SysError("opening directory '%1%'", linksDir);
 
-    long long actualSize = 0, unsharedSize = 0;
+    int64_t actualSize = 0, unsharedSize = 0;
 
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir.get())) {
@@ -717,10 +717,10 @@ void LocalStore::removeUnusedLinks(const GCState & state)
     struct stat st;
     if (stat(linksDir.c_str(), &st) == -1)
         throw SysError("statting '%1%'", linksDir);
-    long long overhead = st.st_blocks * 512ULL;
+    auto overhead = st.st_blocks * 512ULL;
 
-    printInfo(format("note: currently hard linking saves %.2f MiB")
-        % ((unsharedSize - actualSize - overhead) / (1024.0 * 1024.0)));
+    printInfo("note: currently hard linking saves %.2f MiB",
+        ((unsharedSize - actualSize - overhead) / (1024.0 * 1024.0)));
 }
 
 
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index d47e0b6b5..3406a9331 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -335,9 +335,6 @@ public:
         "setuid/setgid bits or with file capabilities."};
 #endif
 
-    Setting<Strings> hashedMirrors{this, {"http://tarballs.nixos.org/"}, "hashed-mirrors",
-        "A list of servers used by builtins.fetchurl to fetch files by hash."};
-
     Setting<uint64_t> minFree{this, 0, "min-free",
         "Automatically run the garbage collector when free disk space drops below the specified amount."};
 
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 91613747b..4c4d0bdc2 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -839,20 +839,32 @@ StorePathSet LocalStore::querySubstitutablePaths(const StorePathSet & paths)
 }
 
 
-void LocalStore::querySubstitutablePathInfos(const StorePathSet & paths,
-    SubstitutablePathInfos & infos)
+void LocalStore::querySubstitutablePathInfos(const StorePathCAMap & paths, SubstitutablePathInfos & infos)
 {
     if (!settings.useSubstitutes) return;
     for (auto & sub : getDefaultSubstituters()) {
-        if (sub->storeDir != storeDir) continue;
         for (auto & path : paths) {
-            if (infos.count(path)) continue;
-            debug("checking substituter '%s' for path '%s'", sub->getUri(), printStorePath(path));
+            auto subPath(path.first);
+
+            // recompute store path so that we can use a different store root
+            if (path.second) {
+                subPath = makeFixedOutputPathFromCA(path.first.name(), *path.second);
+                if (sub->storeDir == storeDir)
+                    assert(subPath == path.first);
+                if (subPath != path.first)
+                    debug("replaced path '%s' with '%s' for substituter '%s'", printStorePath(path.first), sub->printStorePath(subPath), sub->getUri());
+            } else if (sub->storeDir != storeDir) continue;
+
+            debug("checking substituter '%s' for path '%s'", sub->getUri(), sub->printStorePath(subPath));
             try {
-                auto info = sub->queryPathInfo(path);
+                auto info = sub->queryPathInfo(subPath);
+
+                if (sub->storeDir != storeDir && !(info->isContentAddressed(*sub) && info->references.empty()))
+                    continue;
+
                 auto narInfo = std::dynamic_pointer_cast<const NarInfo>(
                     std::shared_ptr<const ValidPathInfo>(info));
-                infos.insert_or_assign(path, SubstitutablePathInfo{
+                infos.insert_or_assign(path.first, SubstitutablePathInfo{
                     info->deriver,
                     info->references,
                     narInfo ? narInfo->fileSize : 0,
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 355c2814f..5a87ec2f8 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -29,8 +29,8 @@ struct Derivation;
 struct OptimiseStats
 {
     unsigned long filesLinked = 0;
-    unsigned long long bytesFreed = 0;
-    unsigned long long blocksFreed = 0;
+    uint64_t bytesFreed = 0;
+    uint64_t blocksFreed = 0;
 };
 
 
@@ -139,7 +139,7 @@ public:
 
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
 
-    void querySubstitutablePathInfos(const StorePathSet & paths,
+    void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
     void addToStore(const ValidPathInfo & info, Source & source,
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index c4d22a634..7f1b62f26 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -108,9 +108,19 @@ void Store::computeFSClosure(const StorePath & startPath,
 }
 
 
+std::optional<ContentAddress> getDerivationCA(const BasicDerivation & drv)
+{
+    auto out = drv.outputs.find("out");
+    if (out != drv.outputs.end()) {
+        if (auto v = std::get_if<DerivationOutputFixed>(&out->second.output))
+            return v->hash;
+    }
+    return std::nullopt;
+}
+
 void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
     StorePathSet & willBuild_, StorePathSet & willSubstitute_, StorePathSet & unknown_,
-    unsigned long long & downloadSize_, unsigned long long & narSize_)
+    uint64_t & downloadSize_, uint64_t & narSize_)
 {
     Activity act(*logger, lvlDebug, actUnknown, "querying info about missing paths");
 
@@ -122,8 +132,8 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
     {
         std::unordered_set<std::string> done;
         StorePathSet & unknown, & willSubstitute, & willBuild;
-        unsigned long long & downloadSize;
-        unsigned long long & narSize;
+        uint64_t & downloadSize;
+        uint64_t & narSize;
     };
 
     struct DrvState
@@ -157,7 +167,7 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
         auto outPath = parseStorePath(outPathS);
 
         SubstitutablePathInfos infos;
-        querySubstitutablePathInfos({outPath}, infos);
+        querySubstitutablePathInfos({{outPath, getDerivationCA(*drv)}}, infos);
 
         if (infos.empty()) {
             drvState_->lock()->done = true;
@@ -214,7 +224,7 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
             if (isValidPath(path.path)) return;
 
             SubstitutablePathInfos infos;
-            querySubstitutablePathInfos({path.path}, infos);
+            querySubstitutablePathInfos({{path.path, std::nullopt}}, infos);
 
             if (infos.empty()) {
                 auto state(state_.lock());
diff --git a/src/libstore/nar-accessor.cc b/src/libstore/nar-accessor.cc
index d884a131e..59ec164b6 100644
--- a/src/libstore/nar-accessor.cc
+++ b/src/libstore/nar-accessor.cc
@@ -79,14 +79,14 @@ struct NarAccessor : public FSAccessor
             parents.top()->isExecutable = true;
         }
 
-        void preallocateContents(unsigned long long size) override
+        void preallocateContents(uint64_t size) override
         {
             assert(size <= std::numeric_limits<uint64_t>::max());
             parents.top()->size = (uint64_t) size;
             parents.top()->start = pos;
         }
 
-        void receiveContents(unsigned char * data, unsigned int len) override
+        void receiveContents(unsigned char * data, size_t len) override
         { }
 
         void createSymlink(const Path & path, const string & target) override
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index b2b2412a3..e4b4b6213 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -282,7 +282,7 @@ void LocalStore::optimiseStore(OptimiseStats & stats)
     }
 }
 
-static string showBytes(unsigned long long bytes)
+static string showBytes(uint64_t bytes)
 {
     return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
 }
diff --git a/src/libstore/path.hh b/src/libstore/path.hh
index e43a8b50c..b03a0f69d 100644
--- a/src/libstore/path.hh
+++ b/src/libstore/path.hh
@@ -64,6 +64,8 @@ typedef std::set<StorePath> StorePathSet;
 typedef std::vector<StorePath> StorePaths;
 typedef std::map<string, StorePath> OutputPathMap;
 
+typedef std::map<StorePath, std::optional<ContentAddress>> StorePathCAMap;
+
 /* Extension of derivations in the Nix store. */
 const std::string drvExtension = ".drv";
 
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index f65e1babe..33d1e431b 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -39,6 +39,24 @@ void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths
         out << store.printStorePath(i);
 }
 
+StorePathCAMap readStorePathCAMap(const Store & store, Source & from)
+{
+    StorePathCAMap paths;
+    auto count = readNum<size_t>(from);
+    while (count--)
+        paths.insert_or_assign(store.parseStorePath(readString(from)), parseContentAddressOpt(readString(from)));
+    return paths;
+}
+
+void writeStorePathCAMap(const Store & store, Sink & out, const StorePathCAMap & paths)
+{
+    out << paths.size();
+    for (auto & i : paths) {
+        out << store.printStorePath(i.first);
+        out << renderContentAddress(i.second);
+    }
+}
+
 std::map<string, StorePath> readOutputPathMap(const Store & store, Source & from)
 {
     std::map<string, StorePath> pathMap;
@@ -332,18 +350,17 @@ StorePathSet RemoteStore::querySubstitutablePaths(const StorePathSet & paths)
 }
 
 
-void RemoteStore::querySubstitutablePathInfos(const StorePathSet & paths,
-    SubstitutablePathInfos & infos)
+void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, SubstitutablePathInfos & infos)
 {
-    if (paths.empty()) return;
+    if (pathsMap.empty()) return;
 
     auto conn(getConnection());
 
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
 
-        for (auto & i : paths) {
+        for (auto & i : pathsMap) {
             SubstitutablePathInfo info;
-            conn->to << wopQuerySubstitutablePathInfo << printStorePath(i);
+            conn->to << wopQuerySubstitutablePathInfo << printStorePath(i.first);
             conn.processStderr();
             unsigned int reply = readInt(conn->from);
             if (reply == 0) continue;
@@ -353,13 +370,19 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathSet & paths,
             info.references = readStorePaths<StorePathSet>(*this, conn->from);
             info.downloadSize = readLongLong(conn->from);
             info.narSize = readLongLong(conn->from);
-            infos.insert_or_assign(i, std::move(info));
+            infos.insert_or_assign(i.first, std::move(info));
         }
 
     } else {
 
         conn->to << wopQuerySubstitutablePathInfos;
-        writeStorePaths(*this, conn->to, paths);
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 22) {
+            StorePathSet paths;
+            for (auto & path : pathsMap)
+                paths.insert(path.first);
+            writeStorePaths(*this, conn->to, paths);
+        } else
+            writeStorePathCAMap(*this, conn->to, pathsMap);
         conn.processStderr();
         size_t count = readNum<size_t>(conn->from);
         for (size_t n = 0; n < count; n++) {
@@ -503,9 +526,84 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
         conn->to << info.registrationTime << info.narSize
                  << info.ultimate << info.sigs << renderContentAddress(info.ca)
                  << repair << !checkSigs;
-        bool tunnel = GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21;
-        if (!tunnel) copyNAR(source, conn->to);
-        conn.processStderr(0, tunnel ? &source : nullptr);
+
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 23) {
+
+            std::exception_ptr ex;
+
+            struct FramedSink : BufferedSink
+            {
+                ConnectionHandle & conn;
+                std::exception_ptr & ex;
+
+                FramedSink(ConnectionHandle & conn, std::exception_ptr & ex) : conn(conn), ex(ex)
+                { }
+
+                ~FramedSink()
+                {
+                    try {
+                        conn->to << 0;
+                        conn->to.flush();
+                    } catch (...) {
+                        ignoreException();
+                    }
+                }
+
+                void write(const unsigned char * data, size_t len) override
+                {
+                    /* Don't send more data if the remote has
+                       encountered an error. */
+                    if (ex) {
+                        auto ex2 = ex;
+                        ex = nullptr;
+                        std::rethrow_exception(ex2);
+                    }
+                    conn->to << len;
+                    conn->to(data, len);
+                };
+            };
+
+            /* Handle log messages / exceptions from the remote on a
+               separate thread. */
+            std::thread stderrThread([&]()
+            {
+                try {
+                    conn.processStderr();
+                } catch (...) {
+                    ex = std::current_exception();
+                }
+            });
+
+            Finally joinStderrThread([&]()
+            {
+                if (stderrThread.joinable()) {
+                    stderrThread.join();
+                    if (ex) {
+                        try {
+                            std::rethrow_exception(ex);
+                        } catch (...) {
+                            ignoreException();
+                        }
+                    }
+                }
+            });
+
+            {
+                FramedSink sink(conn, ex);
+                copyNAR(source, sink);
+                sink.flush();
+            }
+
+            stderrThread.join();
+            if (ex)
+                std::rethrow_exception(ex);
+
+        } else if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21) {
+            conn.processStderr(0, &source);
+        } else {
+            copyNAR(source, conn->to);
+            conn.processStderr(0, nullptr);
+        }
     }
 }
 
@@ -707,7 +805,7 @@ void RemoteStore::addSignatures(const StorePath & storePath, const StringSet & s
 
 void RemoteStore::queryMissing(const std::vector<StorePathWithOutputs> & targets,
     StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-    unsigned long long & downloadSize, unsigned long long & narSize)
+    uint64_t & downloadSize, uint64_t & narSize)
 {
     {
         auto conn(getConnection());
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 3c1b78b6a..72d2a6689 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -56,7 +56,7 @@ public:
 
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
 
-    void querySubstitutablePathInfos(const StorePathSet & paths,
+    void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
     void addToStore(const ValidPathInfo & info, Source & nar,
@@ -94,7 +94,7 @@ public:
 
     void queryMissing(const std::vector<StorePathWithOutputs> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-        unsigned long long & downloadSize, unsigned long long & narSize) override;
+        uint64_t & downloadSize, uint64_t & narSize) override;
 
     void connect() override;
 
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 1b7dff085..67935f3ba 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -343,13 +343,10 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
             std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1)
                 .count();
 
-        auto size = istream->tellg();
-
-        printInfo("uploaded 's3://%s/%s' (%d bytes) in %d ms",
-            bucketName, path, size, duration);
+        printInfo("uploaded 's3://%s/%s' in %d ms",
+            bucketName, path, duration);
 
         stats.putTimeMs += duration;
-        stats.putBytes += size;
         stats.put++;
     }
 
diff --git a/src/libstore/s3-binary-cache-store.hh b/src/libstore/s3-binary-cache-store.hh
index 4d43fe4d2..b2b75d498 100644
--- a/src/libstore/s3-binary-cache-store.hh
+++ b/src/libstore/s3-binary-cache-store.hh
@@ -19,7 +19,6 @@ public:
     struct Stats
     {
         std::atomic<uint64_t> put{0};
-        std::atomic<uint64_t> putBytes{0};
         std::atomic<uint64_t> putTimeMs{0};
         std::atomic<uint64_t> get{0};
         std::atomic<uint64_t> getBytes{0};
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 5939c03b8..76324cf08 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -193,6 +193,23 @@ StorePath Store::makeFixedOutputPath(
     }
 }
 
+// FIXME Put this somewhere?
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
+
+StorePath Store::makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
+    const StorePathSet & references, bool hasSelfReference) const
+{
+    // New template
+    return std::visit(overloaded {
+        [&](TextHash th) {
+            return makeTextPath(name, th.hash, references);
+        },
+        [&](FixedOutputHash fsh) {
+            return makeFixedOutputPath(fsh.method, fsh.hash, name, references, hasSelfReference);
+        }
+    }, ca);
+}
 
 StorePath Store::makeTextPath(std::string_view name, const Hash & hash,
     const StorePathSet & references) const
@@ -689,6 +706,15 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 
     uint64_t total = 0;
 
+    // recompute store path on the chance dstStore does it differently
+    if (info->ca && info->references.empty()) {
+        auto info2 = make_ref<ValidPathInfo>(*info);
+        info2->path = dstStore->makeFixedOutputPathFromCA(info->path.name(), *info->ca);
+        if (dstStore->storeDir == srcStore->storeDir)
+            assert(info->path == info2->path);
+        info = info2;
+    }
+
     if (!info->narHash) {
         StringSink sink;
         srcStore->narFromPath({storePath}, sink);
@@ -724,16 +750,20 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 }
 
 
-void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
+std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
     RepairFlag repair, CheckSigsFlag checkSigs, SubstituteFlag substitute)
 {
     auto valid = dstStore->queryValidPaths(storePaths, substitute);
 
-    PathSet missing;
+    StorePathSet missing;
     for (auto & path : storePaths)
-        if (!valid.count(path)) missing.insert(srcStore->printStorePath(path));
+        if (!valid.count(path)) missing.insert(path);
 
-    if (missing.empty()) return;
+    std::map<StorePath, StorePath> pathsMap;
+    for (auto & path : storePaths)
+        pathsMap.insert_or_assign(path, path);
+
+    if (missing.empty()) return pathsMap;
 
     Activity act(*logger, lvlInfo, actCopyPaths, fmt("copying %d paths", missing.size()));
 
@@ -748,30 +778,49 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
 
     ThreadPool pool;
 
-    processGraph<Path>(pool,
-        PathSet(missing.begin(), missing.end()),
+    processGraph<StorePath>(pool,
+        StorePathSet(missing.begin(), missing.end()),
 
-        [&](const Path & storePath) {
-            if (dstStore->isValidPath(dstStore->parseStorePath(storePath))) {
+        [&](const StorePath & storePath) {
+            auto info = srcStore->queryPathInfo(storePath);
+            auto storePathForDst = storePath;
+            if (info->ca && info->references.empty()) {
+                storePathForDst = dstStore->makeFixedOutputPathFromCA(storePath.name(), *info->ca);
+                if (dstStore->storeDir == srcStore->storeDir)
+                    assert(storePathForDst == storePath);
+                if (storePathForDst != storePath)
+                    debug("replaced path '%s' to '%s' for substituter '%s'", srcStore->printStorePath(storePath), dstStore->printStorePath(storePathForDst), dstStore->getUri());
+            }
+            pathsMap.insert_or_assign(storePath, storePathForDst);
+
+            if (dstStore->isValidPath(storePath)) {
                 nrDone++;
                 showProgress();
-                return PathSet();
+                return StorePathSet();
             }
 
-            auto info = srcStore->queryPathInfo(srcStore->parseStorePath(storePath));
-
             bytesExpected += info->narSize;
             act.setExpected(actCopyPath, bytesExpected);
 
-            return srcStore->printStorePathSet(info->references);
+            return info->references;
         },
 
-        [&](const Path & storePathS) {
+        [&](const StorePath & storePath) {
             checkInterrupt();
 
-            auto storePath = dstStore->parseStorePath(storePathS);
+            auto info = srcStore->queryPathInfo(storePath);
 
-            if (!dstStore->isValidPath(storePath)) {
+            auto storePathForDst = storePath;
+            if (info->ca && info->references.empty()) {
+                storePathForDst = dstStore->makeFixedOutputPathFromCA(storePath.name(), *info->ca);
+                if (dstStore->storeDir == srcStore->storeDir)
+                    assert(storePathForDst == storePath);
+                if (storePathForDst != storePath)
+                    debug("replaced path '%s' to '%s' for substituter '%s'", srcStore->printStorePath(storePath), dstStore->printStorePath(storePathForDst), dstStore->getUri());
+            }
+            pathsMap.insert_or_assign(storePath, storePathForDst);
+
+            if (!dstStore->isValidPath(storePathForDst)) {
                 MaintainCount<decltype(nrRunning)> mc(nrRunning);
                 showProgress();
                 try {
@@ -780,7 +829,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
                     nrFailed++;
                     if (!settings.keepGoing)
                         throw e;
-                    logger->log(lvlError, fmt("could not copy %s: %s", storePathS, e.what()));
+                    logger->log(lvlError, fmt("could not copy %s: %s", dstStore->printStorePath(storePath), e.what()));
                     showProgress();
                     return;
                 }
@@ -789,6 +838,8 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
             nrDone++;
             showProgress();
         });
+
+    return pathsMap;
 }
 
 
@@ -865,10 +916,6 @@ void ValidPathInfo::sign(const Store & store, const SecretKey & secretKey)
     sigs.insert(secretKey.signDetached(fingerprint(store)));
 }
 
-// FIXME Put this somewhere?
-template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
-template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
-
 bool ValidPathInfo::isContentAddressed(const Store & store) const
 {
     if (! ca) return false;
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index a7288d0cc..49da19496 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -85,7 +85,7 @@ struct GCOptions
     StorePathSet pathsToDelete;
 
     /* Stop after at least `maxFreed' bytes have been freed. */
-    unsigned long long maxFreed{std::numeric_limits<unsigned long long>::max()};
+    uint64_t maxFreed{std::numeric_limits<uint64_t>::max()};
 };
 
 
@@ -97,7 +97,7 @@ struct GCResults
 
     /* For `gcReturnDead', `gcDeleteDead' and `gcDeleteSpecific', the
        number of bytes that would be or was freed. */
-    unsigned long long bytesFreed = 0;
+    uint64_t bytesFreed = 0;
 };
 
 
@@ -105,8 +105,8 @@ struct SubstitutablePathInfo
 {
     std::optional<StorePath> deriver;
     StorePathSet references;
-    unsigned long long downloadSize; /* 0 = unknown or inapplicable */
-    unsigned long long narSize; /* 0 = unknown */
+    uint64_t downloadSize; /* 0 = unknown or inapplicable */
+    uint64_t narSize; /* 0 = unknown */
 };
 
 typedef std::map<StorePath, SubstitutablePathInfo> SubstitutablePathInfos;
@@ -344,7 +344,11 @@ public:
         bool hasSelfReference = false) const;
 
     StorePath makeTextPath(std::string_view name, const Hash & hash,
-        const StorePathSet & references) const;
+        const StorePathSet & references = {}) const;
+
+    StorePath makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
+        const StorePathSet & references = {},
+        bool hasSelfReference = false) const;
 
     /* This is the preparatory part of addToStore(); it computes the
        store path to which srcPath is to be copied.  Returns the store
@@ -436,9 +440,10 @@ public:
     virtual StorePathSet querySubstitutablePaths(const StorePathSet & paths) { return {}; };
 
     /* Query substitute info (i.e. references, derivers and download
-       sizes) of a set of paths.  If a path does not have substitute
-       info, it's omitted from the resulting ‘infos’ map. */
-    virtual void querySubstitutablePathInfos(const StorePathSet & paths,
+       sizes) of a map of paths to their optional ca values. If a path
+       does not have substitute info, it's omitted from the resulting
+       ‘infos’ map. */
+    virtual void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) { return; };
 
     /* Import a path into the store. */
@@ -610,7 +615,7 @@ public:
        that will be substituted. */
     virtual void queryMissing(const std::vector<StorePathWithOutputs> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-        unsigned long long & downloadSize, unsigned long long & narSize);
+        uint64_t & downloadSize, uint64_t & narSize);
 
     /* Sort a set of paths topologically under the references
        relation.  If p refers to q, then p precedes q in this list. */
@@ -740,11 +745,13 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 
 
 /* Copy store paths from one store to another. The paths may be copied
-   in parallel. They are copied in a topologically sorted order
-   (i.e. if A is a reference of B, then A is copied before B), but
-   the set of store paths is not automatically closed; use
-   copyClosure() for that. */
-void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
+   in parallel. They are copied in a topologically sorted order (i.e.
+   if A is a reference of B, then A is copied before B), but the set
+   of store paths is not automatically closed; use copyClosure() for
+   that. Returns a map of what each path was copied to the dstStore
+   as. */
+std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStore,
+    const StorePathSet & storePaths,
     RepairFlag repair = NoRepair,
     CheckSigsFlag checkSigs = CheckSigs,
     SubstituteFlag substitute = NoSubstitute);
@@ -843,4 +850,6 @@ std::optional<ValidPathInfo> decodeValidPathInfo(
 /* Split URI into protocol+hierarchy part and its parameter set. */
 std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri);
 
+std::optional<ContentAddress> getDerivationCA(const BasicDerivation & drv);
+
 }
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index 8b538f6da..f76b13fb4 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x116
+#define PROTOCOL_VERSION 0x117
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -70,6 +70,10 @@ template<class T> T readStorePaths(const Store & store, Source & from);
 
 void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths);
 
+StorePathCAMap readStorePathCAMap(const Store & store, Source & from);
+
+void writeStorePathCAMap(const Store & store, Sink & out, const StorePathCAMap & paths);
+
 void writeOutputPathMap(const Store & store, Sink & out, const OutputPathMap & paths);
 
 }
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 51c88537e..ce7cf9754 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -150,17 +150,17 @@ static void skipGeneric(Source & source)
 
 static void parseContents(ParseSink & sink, Source & source, const Path & path)
 {
-    unsigned long long size = readLongLong(source);
+    uint64_t size = readLongLong(source);
 
     sink.preallocateContents(size);
 
-    unsigned long long left = size;
+    uint64_t left = size;
     std::vector<unsigned char> buf(65536);
 
     while (left) {
         checkInterrupt();
         auto n = buf.size();
-        if ((unsigned long long)n > left) n = left;
+        if ((uint64_t)n > left) n = left;
         source(buf.data(), n);
         sink.receiveContents(buf.data(), n);
         left -= n;
@@ -323,7 +323,7 @@ struct RestoreSink : ParseSink
             throw SysError("fchmod");
     }
 
-    void preallocateContents(unsigned long long len)
+    void preallocateContents(uint64_t len)
     {
 #if HAVE_POSIX_FALLOCATE
         if (len) {
@@ -338,7 +338,7 @@ struct RestoreSink : ParseSink
 #endif
     }
 
-    void receiveContents(unsigned char * data, unsigned int len)
+    void receiveContents(unsigned char * data, size_t len)
     {
         writeFull(fd.get(), data, len);
     }
diff --git a/src/libutil/archive.hh b/src/libutil/archive.hh
index 57780d16a..5665732d2 100644
--- a/src/libutil/archive.hh
+++ b/src/libutil/archive.hh
@@ -57,8 +57,8 @@ struct ParseSink
 
     virtual void createRegularFile(const Path & path) { };
     virtual void isExecutable() { };
-    virtual void preallocateContents(unsigned long long size) { };
-    virtual void receiveContents(unsigned char * data, unsigned int len) { };
+    virtual void preallocateContents(uint64_t size) { };
+    virtual void receiveContents(unsigned char * data, size_t len) { };
 
     virtual void createSymlink(const Path & path, const string & target) { };
 };
@@ -77,7 +77,7 @@ struct RetrieveRegularNARSink : ParseSink
         regular = false;
     }
 
-    void receiveContents(unsigned char * data, unsigned int len)
+    void receiveContents(unsigned char * data, size_t len)
     {
         sink(data, len);
     }
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 026e80e71..d4badbab3 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -116,7 +116,7 @@ Hash hashFile(HashType ht, const Path & path);
 
 /* Compute the hash of the given path.  The hash is defined as
    (essentially) hashString(ht, dumpPath(path)). */
-typedef std::pair<Hash, unsigned long long> HashResult;
+typedef std::pair<Hash, uint64_t> HashResult;
 HashResult hashPath(HashType ht, const Path & path,
     PathFilter & filter = defaultPathFilter);
 
@@ -146,7 +146,7 @@ class HashSink : public BufferedSink, public AbstractHashSink
 private:
     HashType ht;
     Ctx * ctx;
-    unsigned long long bytes;
+    uint64_t bytes;
 
 public:
     HashSink(HashType ht);
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 5d9acf887..c29c6b29b 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -312,14 +312,14 @@ T readNum(Source & source)
     source(buf, sizeof(buf));
 
     uint64_t n =
-        ((unsigned long long) buf[0]) |
-        ((unsigned long long) buf[1] << 8) |
-        ((unsigned long long) buf[2] << 16) |
-        ((unsigned long long) buf[3] << 24) |
-        ((unsigned long long) buf[4] << 32) |
-        ((unsigned long long) buf[5] << 40) |
-        ((unsigned long long) buf[6] << 48) |
-        ((unsigned long long) buf[7] << 56);
+        ((uint64_t) buf[0]) |
+        ((uint64_t) buf[1] << 8) |
+        ((uint64_t) buf[2] << 16) |
+        ((uint64_t) buf[3] << 24) |
+        ((uint64_t) buf[4] << 32) |
+        ((uint64_t) buf[5] << 40) |
+        ((uint64_t) buf[6] << 48) |
+        ((uint64_t) buf[7] << 56);
 
     if (n > std::numeric_limits<T>::max())
         throw SerialisationError("serialised integer %d is too large for type '%s'", n, typeid(T).name());
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 2c9373ad6..690169184 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -374,7 +374,7 @@ void writeLine(int fd, string s)
 }
 
 
-static void _deletePath(int parentfd, const Path & path, unsigned long long & bytesFreed)
+static void _deletePath(int parentfd, const Path & path, uint64_t & bytesFreed)
 {
     checkInterrupt();
 
@@ -414,7 +414,7 @@ static void _deletePath(int parentfd, const Path & path, unsigned long long & by
     }
 }
 
-static void _deletePath(const Path & path, unsigned long long & bytesFreed)
+static void _deletePath(const Path & path, uint64_t & bytesFreed)
 {
     Path dir = dirOf(path);
     if (dir == "")
@@ -435,12 +435,12 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
 
 void deletePath(const Path & path)
 {
-    unsigned long long dummy;
+    uint64_t dummy;
     deletePath(path, dummy);
 }
 
 
-void deletePath(const Path & path, unsigned long long & bytesFreed)
+void deletePath(const Path & path, uint64_t & bytesFreed)
 {
     //Activity act(*logger, lvlDebug, format("recursively deleting path '%1%'") % path);
     bytesFreed = 0;
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 42130f6dc..6850b5a7a 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -125,7 +125,7 @@ void writeLine(int fd, string s);
    second variant returns the number of bytes and blocks freed. */
 void deletePath(const Path & path);
 
-void deletePath(const Path & path, unsigned long long & bytesFreed);
+void deletePath(const Path & path, uint64_t & bytesFreed);
 
 std::string getUserName();
 
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index f77de56ea..94412042f 100755
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -174,7 +174,7 @@ static void _main(int argc, char * * argv)
         else if (*arg == "--run-env") // obsolete
             runEnv = true;
 
-        else if (*arg == "--command" || *arg == "--run") {
+        else if (runEnv && (*arg == "--command" || *arg == "--run")) {
             if (*arg == "--run")
                 interactive = false;
             envCommand = getArg(*arg, arg, end) + "\nexit";
@@ -192,7 +192,7 @@ static void _main(int argc, char * * argv)
         else if (*arg == "--pure") pure = true;
         else if (*arg == "--impure") pure = false;
 
-        else if (*arg == "--packages" || *arg == "-p")
+        else if (runEnv && (*arg == "--packages" || *arg == "-p"))
             packages = true;
 
         else if (inShebang && *arg == "-i") {
@@ -325,7 +325,7 @@ static void _main(int argc, char * * argv)
     auto buildPaths = [&](const std::vector<StorePathWithOutputs> & paths) {
         /* Note: we do this even when !printMissing to efficiently
            fetch binary cache data. */
-        unsigned long long downloadSize, narSize;
+        uint64_t downloadSize, narSize;
         StorePathSet willBuild, willSubstitute, unknown;
         store->queryMissing(paths,
             willBuild, willSubstitute, unknown, downloadSize, narSize);
diff --git a/src/nix-collect-garbage/nix-collect-garbage.cc b/src/nix-collect-garbage/nix-collect-garbage.cc
index aa5ada3a6..bcf1d8518 100644
--- a/src/nix-collect-garbage/nix-collect-garbage.cc
+++ b/src/nix-collect-garbage/nix-collect-garbage.cc
@@ -67,10 +67,8 @@ static int _main(int argc, char * * argv)
                 deleteOlderThan = getArg(*arg, arg, end);
             }
             else if (*arg == "--dry-run") dryRun = true;
-            else if (*arg == "--max-freed") {
-                long long maxFreed = getIntArg<long long>(*arg, arg, end, true);
-                options.maxFreed = maxFreed >= 0 ? maxFreed : 0;
-            }
+            else if (*arg == "--max-freed")
+                options.maxFreed = std::max(getIntArg<int64_t>(*arg, arg, end, true), (int64_t) 0);
             else
                 return false;
             return true;
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index b974dcc2d..2996e36c4 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -130,7 +130,7 @@ static void opRealise(Strings opFlags, Strings opArgs)
     for (auto & i : opArgs)
         paths.push_back(store->followLinksToStorePathWithOutputs(i));
 
-    unsigned long long downloadSize, narSize;
+    uint64_t downloadSize, narSize;
     StorePathSet willBuild, willSubstitute, unknown;
     store->queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
 
@@ -572,10 +572,8 @@ static void opGC(Strings opFlags, Strings opArgs)
         if (*i == "--print-roots") printRoots = true;
         else if (*i == "--print-live") options.action = GCOptions::gcReturnLive;
         else if (*i == "--print-dead") options.action = GCOptions::gcReturnDead;
-        else if (*i == "--max-freed") {
-            long long maxFreed = getIntArg<long long>(*i, i, opFlags.end(), true);
-            options.maxFreed = maxFreed >= 0 ? maxFreed : 0;
-        }
+        else if (*i == "--max-freed")
+            options.maxFreed = std::max(getIntArg<int64_t>(*i, i, opFlags.end(), true), (int64_t) 0);
         else throw UsageError("bad sub-operation '%1%' in GC", *i);
 
     if (!opArgs.empty()) throw UsageError("no arguments expected");
@@ -831,7 +829,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                     for (auto & path : paths)
                         if (!path.isDerivation())
                             paths2.push_back({path});
-                    unsigned long long downloadSize, narSize;
+                    uint64_t downloadSize, narSize;
                     StorePathSet willBuild, willSubstitute, unknown;
                     store->queryMissing(paths2,
                         willBuild, willSubstitute, unknown, downloadSize, narSize);
diff --git a/src/nix/add-to-store.cc b/src/nix/add-to-store.cc
index ad1f9e91f..e183cb8b5 100644
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@@ -9,6 +9,7 @@ struct CmdAddToStore : MixDryRun, StoreCommand
 {
     Path path;
     std::optional<std::string> namePart;
+    FileIngestionMethod ingestionMethod = FileIngestionMethod::Recursive;
 
     CmdAddToStore()
     {
@@ -21,6 +22,13 @@ struct CmdAddToStore : MixDryRun, StoreCommand
             .labels = {"name"},
             .handler = {&namePart},
         });
+
+        addFlag({
+            .longName = "flat",
+            .shortName = 0,
+            .description = "add flat file to the Nix store",
+            .handler = {&ingestionMethod, FileIngestionMethod::Flat},
+        });
     }
 
     std::string description() override
@@ -45,12 +53,19 @@ struct CmdAddToStore : MixDryRun, StoreCommand
 
         auto narHash = hashString(htSHA256, *sink.s);
 
-        ValidPathInfo info(store->makeFixedOutputPath(FileIngestionMethod::Recursive, narHash, *namePart));
+        Hash hash = narHash;
+        if (ingestionMethod == FileIngestionMethod::Flat) {
+            HashSink hsink(htSHA256);
+            readFile(path, hsink);
+            hash = hsink.finish().first;
+        }
+
+        ValidPathInfo info(store->makeFixedOutputPath(ingestionMethod, hash, *namePart));
         info.narHash = narHash;
         info.narSize = sink.s->size();
         info.ca = std::optional { FixedOutputHash {
-            .method = FileIngestionMethod::Recursive,
-            .hash = *info.narHash,
+            .method = ingestionMethod,
+            .hash = hash,
         } };
 
         if (!dryRun) {
diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index 2f590f53f..a0c119e43 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -68,22 +68,22 @@ BuildEnvironment readEnvironment(const Path & path)
 
         std::smatch match;
 
-        if (std::regex_search(pos, file.cend(), match, declareRegex)) {
+        if (std::regex_search(pos, file.cend(), match, declareRegex, std::regex_constants::match_continuous)) {
             pos = match[0].second;
             exported.insert(match[1]);
         }
 
-        else if (std::regex_search(pos, file.cend(), match, varRegex)) {
+        else if (std::regex_search(pos, file.cend(), match, varRegex, std::regex_constants::match_continuous)) {
             pos = match[0].second;
             res.env.insert({match[1], Var { .exported = exported.count(match[1]) > 0, .value = match[2] }});
         }
 
-        else if (std::regex_search(pos, file.cend(), match, assocArrayRegex)) {
+        else if (std::regex_search(pos, file.cend(), match, assocArrayRegex, std::regex_constants::match_continuous)) {
             pos = match[0].second;
             res.env.insert({match[1], Var { .associative = true, .value = match[2] }});
         }
 
-        else if (std::regex_search(pos, file.cend(), match, functionRegex)) {
+        else if (std::regex_search(pos, file.cend(), match, functionRegex, std::regex_constants::match_continuous)) {
             res.bashFunctions = std::string(pos, file.cend());
             break;
         }
diff --git a/src/nix/path-info.cc b/src/nix/path-info.cc
index 65f73cd94..0c12efaf0 100644
--- a/src/nix/path-info.cc
+++ b/src/nix/path-info.cc
@@ -61,7 +61,7 @@ struct CmdPathInfo : StorePathsCommand, MixJSON
         };
     }
 
-    void printSize(unsigned long long value)
+    void printSize(uint64_t value)
     {
         if (!humanReadable) {
             std::cout << fmt("\t%11d", value);
diff --git a/tests/check.sh b/tests/check.sh
index 5f25d04cb..5f4997e28 100644
--- a/tests/check.sh
+++ b/tests/check.sh
@@ -61,30 +61,30 @@ nix-build check.nix -A nondeterministic --no-out-link --repeat 1 2> $TEST_ROOT/l
 [ "$status" = "1" ]
 grep 'differs from previous round' $TEST_ROOT/log
 
-path=$(nix-build check.nix -A fetchurl --no-out-link --hashed-mirrors '')
+path=$(nix-build check.nix -A fetchurl --no-out-link)
 
 chmod +w $path
 echo foo > $path
 chmod -w $path
 
-nix-build check.nix -A fetchurl --no-out-link --check --hashed-mirrors ''
+nix-build check.nix -A fetchurl --no-out-link --check
 # Note: "check" doesn't repair anything, it just compares to the hash stored in the database.
 [[ $(cat $path) = foo ]]
 
-nix-build check.nix -A fetchurl --no-out-link --repair --hashed-mirrors ''
+nix-build check.nix -A fetchurl --no-out-link --repair
 [[ $(cat $path) != foo ]]
 
-nix-build check.nix -A hashmismatch --no-out-link --hashed-mirrors '' || status=$?
+nix-build check.nix -A hashmismatch --no-out-link || status=$?
 [ "$status" = "102" ]
 
 echo -n > ./dummy
-nix-build check.nix -A hashmismatch --no-out-link --hashed-mirrors ''
+nix-build check.nix -A hashmismatch --no-out-link
 echo 'Hello World' > ./dummy
 
-nix-build check.nix -A hashmismatch --no-out-link --check --hashed-mirrors '' || status=$?
+nix-build check.nix -A hashmismatch --no-out-link --check || status=$?
 [ "$status" = "102" ]
 
 # Multiple failures with --keep-going
 nix-build check.nix -A nondeterministic --no-out-link
-nix-build check.nix -A nondeterministic -A hashmismatch --no-out-link --check --keep-going --hashed-mirrors '' || status=$?
+nix-build check.nix -A nondeterministic -A hashmismatch --no-out-link --check --keep-going || status=$?
 [ "$status" = "110" ]
diff --git a/tests/fetchGit.sh b/tests/fetchGit.sh
index 9faa5d9f6..cedd796f7 100644
--- a/tests/fetchGit.sh
+++ b/tests/fetchGit.sh
@@ -32,6 +32,8 @@ rev2=$(git -C $repo rev-parse HEAD)
 # Fetch a worktree
 unset _NIX_FORCE_HTTP
 path0=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$TEST_ROOT/worktree).outPath")
+path0_=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; url = file://$TEST_ROOT/worktree; }).outPath")
+[[ $path0 = $path0_ ]]
 export _NIX_FORCE_HTTP=1
 [[ $(tail -n 1 $path0/hello) = "hello" ]]
 
@@ -102,6 +104,12 @@ git -C $repo commit -m 'Bla3' -a
 path4=$(nix eval --impure --refresh --raw --expr "(builtins.fetchGit file://$repo).outPath")
 [[ $path2 = $path4 ]]
 
+nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=\"; }).outPath" || status=$?
+[[ "$status" = "102" ]]
+
+path5=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-Hr8g6AqANb3xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath")
+[[ $path = $path5 ]]
+
 # tarball-ttl should be ignored if we specify a rev
 echo delft > $repo/hello
 git -C $repo add hello
diff --git a/tests/fetchurl.sh b/tests/fetchurl.sh
index 2535651b0..0f2044342 100644
--- a/tests/fetchurl.sh
+++ b/tests/fetchurl.sh
@@ -5,7 +5,7 @@ clearStore
 # Test fetching a flat file.
 hash=$(nix-hash --flat --type sha256 ./fetchurl.sh)
 
-outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha256 $hash --no-out-link --hashed-mirrors '')
+outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha256 $hash --no-out-link)
 
 cmp $outPath fetchurl.sh
 
@@ -14,7 +14,7 @@ clearStore
 
 hash=$(nix hash-file --type sha512 --base64 ./fetchurl.sh)
 
-outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha512 $hash --no-out-link --hashed-mirrors '')
+outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha512 $hash --no-out-link)
 
 cmp $outPath fetchurl.sh
 
@@ -25,26 +25,24 @@ hash=$(nix hash-file ./fetchurl.sh)
 
 [[ $hash =~ ^sha256- ]]
 
-outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr hash $hash --no-out-link --hashed-mirrors '')
+outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr hash $hash --no-out-link)
 
 cmp $outPath fetchurl.sh
 
-# Test the hashed mirror feature.
+# Test that we can substitute from a different store dir.
 clearStore
 
-hash=$(nix hash-file --type sha512 --base64 ./fetchurl.sh)
-hash32=$(nix hash-file --type sha512 --base16 ./fetchurl.sh)
+other_store=file://$TEST_ROOT/other_store?store=/fnord/store
 
-mirror=$TEST_ROOT/hashed-mirror
-rm -rf $mirror
-mkdir -p $mirror/sha512
-ln -s $(pwd)/fetchurl.sh $mirror/sha512/$hash32
+hash=$(nix hash-file --type sha256 --base16 ./fetchurl.sh)
 
-outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file:///no-such-dir/fetchurl.sh --argstr sha512 $hash --no-out-link --hashed-mirrors "file://$mirror")
+storePath=$(nix --store $other_store add-to-store --flat ./fetchurl.sh)
+
+outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file:///no-such-dir/fetchurl.sh --argstr sha256 $hash --no-out-link --substituters $other_store)
 
 # Test hashed mirrors with an SRI hash.
-nix-build '<nix/fetchurl.nix>' --argstr url file:///no-such-dir/fetchurl.sh --argstr hash $(nix to-sri --type sha512 $hash) \
-          --argstr name bla --no-out-link --hashed-mirrors "file://$mirror"
+nix-build '<nix/fetchurl.nix>' --argstr url file:///no-such-dir/fetchurl.sh --argstr hash $(nix to-sri --type sha256 $hash) \
+          --no-out-link --substituters $other_store
 
 # Test unpacking a NAR.
 rm -rf $TEST_ROOT/archive
diff --git a/tests/tarball.sh b/tests/tarball.sh
index b3ec16d40..88a1a07a0 100644
--- a/tests/tarball.sh
+++ b/tests/tarball.sh
@@ -27,10 +27,13 @@ test_tarball() {
 
     nix-build -o $TEST_ROOT/result -E "import (fetchTarball file://$tarball)"
 
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree file://$tarball)"
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; })"
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })"
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" 2>&1 | grep 'NAR hash mismatch in input'
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree file://$tarball)"
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; })"
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })"
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" 2>&1 | grep 'NAR hash mismatch in input'
+
+    nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" >&2
+    nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" 2>&1 | grep 'true'
 
     nix-instantiate --eval -E '1 + 2' -I fnord=file://no-such-tarball.tar$ext
     nix-instantiate --eval -E 'with <fnord/xyzzy>; 1 + 2' -I fnord=file://no-such-tarball$ext