diff --git a/doc/manual/src/command-ref/nix-prefetch-url.md b/doc/manual/src/command-ref/nix-prefetch-url.md
index 1cd1063cd..1307c7c37 100644
--- a/doc/manual/src/command-ref/nix-prefetch-url.md
+++ b/doc/manual/src/command-ref/nix-prefetch-url.md
@@ -51,6 +51,9 @@ Nix store is also printed.
     result to the Nix store. The resulting hash can be used with
     functions such as Nixpkgs’s `fetchzip` or `fetchFromGitHub`.
 
+  - `--executable`  
+    Set the executable bit on the downloaded file.
+
   - `--name` *name*  
     Override the name of the file in the Nix store. By default, this is
     `hash-basename`, where *basename* is the last component of *url*.
diff --git a/flake.lock b/flake.lock
index f4368b170..822a73332 100644
--- a/flake.lock
+++ b/flake.lock
@@ -3,16 +3,15 @@
     "lowdown-src": {
       "flake": false,
       "locked": {
-        "lastModified": 1598296217,
-        "narHash": "sha256-ha7lyNY1d8m+osmDpPc9f/bfZ3ZC1IVIXwfyklSWg8I=",
-        "owner": "edolstra",
+        "lastModified": 1598695561,
+        "narHash": "sha256-gyH/5j+h/nWw0W8AcR2WKvNBUsiQ7QuxqSJNXAwV+8E=",
+        "owner": "kristapsdz",
         "repo": "lowdown",
-        "rev": "c7a4e715af1e233080842db82d15b261cb74cb28",
+        "rev": "1705b4a26fbf065d9574dce47a94e8c7c79e052f",
         "type": "github"
       },
       "original": {
-        "owner": "edolstra",
-        "ref": "no-structs-in-anonymous-unions",
+        "owner": "kristapsdz",
         "repo": "lowdown",
         "type": "github"
       }
diff --git a/flake.nix b/flake.nix
index a50533a29..0304557e8 100644
--- a/flake.nix
+++ b/flake.nix
@@ -2,7 +2,7 @@
   description = "The purely functional package manager";
 
   inputs.nixpkgs.url = "nixpkgs/nixos-20.03-small";
-  inputs.lowdown-src = { url = "github:edolstra/lowdown/no-structs-in-anonymous-unions"; flake = false; };
+  inputs.lowdown-src = { url = "github:kristapsdz/lowdown"; flake = false; };
 
   outputs = { self, nixpkgs, lowdown-src }:
 
@@ -136,7 +136,7 @@
 
           enableParallelBuilding = true;
 
-          makeFlags = "profiledir=$(out)/etc/profile.d";
+          makeFlags = "profiledir=$(out)/etc/profile.d PRECOMPILE_HEADERS=1";
 
           doCheck = true;
 
@@ -334,9 +334,6 @@
             # syntax-check generated dot files, it still requires some
             # fonts.  So provide those.
             FONTCONFIG_FILE = texFunctions.fontsConf;
-
-            # To test building without precompiled headers.
-            makeFlagsArray = [ "PRECOMPILE_HEADERS=0" ];
           };
 
         # System tests.
diff --git a/misc/launchd/org.nixos.nix-daemon.plist.in b/misc/launchd/org.nixos.nix-daemon.plist.in
index 9f26296a9..c334639e2 100644
--- a/misc/launchd/org.nixos.nix-daemon.plist.in
+++ b/misc/launchd/org.nixos.nix-daemon.plist.in
@@ -4,6 +4,8 @@
   <dict>
     <key>EnvironmentVariables</key>
     <dict>
+      <key>NIX_SSL_CERT_FILE</key>
+      <string>/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt</string>
       <key>OBJC_DISABLE_INITIALIZE_FORK_SAFETY</key>
       <string>YES</string>
     </dict>
diff --git a/mk/precompiled-headers.mk b/mk/precompiled-headers.mk
index 500c99e4a..1fdb4b3a4 100644
--- a/mk/precompiled-headers.mk
+++ b/mk/precompiled-headers.mk
@@ -1,4 +1,4 @@
-PRECOMPILE_HEADERS ?= 1
+PRECOMPILE_HEADERS ?= 0
 
 print-var-help += \
   echo "  PRECOMPILE_HEADERS ($(PRECOMPILE_HEADERS)): Whether to use precompiled headers to speed up the build";
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 139067f20..883fc27a7 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -356,6 +356,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sEpsilon(symbols.create(""))
     , repair(NoRepair)
     , store(store)
+    , regexCache(makeRegexCache())
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)
 {
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 80078d8a5..0e1f61baa 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -6,7 +6,6 @@
 #include "symbol-table.hh"
 #include "config.hh"
 
-#include <regex>
 #include <map>
 #include <optional>
 #include <unordered_map>
@@ -65,6 +64,11 @@ typedef std::list<SearchPathElem> SearchPath;
 void initGC();
 
 
+struct RegexCache;
+
+std::shared_ptr<RegexCache> makeRegexCache();
+
+
 class EvalState
 {
 public:
@@ -120,7 +124,7 @@ private:
     std::unordered_map<Path, Path> resolvedPaths;
 
     /* Cache used by prim_match(). */
-    std::unordered_map<std::string, std::regex> regexCache;
+    std::shared_ptr<RegexCache> regexCache;
 
 public:
 
diff --git a/src/libexpr/flake/flakeref.cc b/src/libexpr/flake/flakeref.cc
index 6363446f6..d5c2ffe66 100644
--- a/src/libexpr/flake/flakeref.cc
+++ b/src/libexpr/flake/flakeref.cc
@@ -1,6 +1,7 @@
 #include "flakeref.hh"
 #include "store-api.hh"
 #include "url.hh"
+#include "url-parts.hh"
 #include "fetchers.hh"
 #include "registry.hh"
 
diff --git a/src/libexpr/flake/lockfile.cc b/src/libexpr/flake/lockfile.cc
index a74846944..78431f000 100644
--- a/src/libexpr/flake/lockfile.cc
+++ b/src/libexpr/flake/lockfile.cc
@@ -1,5 +1,6 @@
 #include "lockfile.hh"
 #include "store-api.hh"
+#include "url-parts.hh"
 
 #include <nlohmann/json.hpp>
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 7e8526ea1..9cfe3f402 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -3085,17 +3085,25 @@ static RegisterPrimOp primop_hashString({
     .fun = prim_hashString,
 });
 
-/* Match a regular expression against a string and return either
-   ‘null’ or a list containing substring matches. */
+struct RegexCache
+{
+    std::unordered_map<std::string, std::regex> cache;
+};
+
+std::shared_ptr<RegexCache> makeRegexCache()
+{
+    return std::make_shared<RegexCache>();
+}
+
 void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     auto re = state.forceStringNoCtx(*args[0], pos);
 
     try {
 
-        auto regex = state.regexCache.find(re);
-        if (regex == state.regexCache.end())
-            regex = state.regexCache.emplace(re, std::regex(re, std::regex::extended)).first;
+        auto regex = state.regexCache->cache.find(re);
+        if (regex == state.regexCache->cache.end())
+            regex = state.regexCache->cache.emplace(re, std::regex(re, std::regex::extended)).first;
 
         PathSet context;
         const std::string str = state.forceString(*args[1], context, pos);
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index cef85cfef..1a064ed5c 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -3,8 +3,7 @@
 #include "store-api.hh"
 #include "fetchers.hh"
 #include "url.hh"
-
-#include <regex>
+#include "url-parts.hh"
 
 namespace nix {
 
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 5ca0f8521..ad7638d73 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -3,6 +3,7 @@
 #include "globals.hh"
 #include "tarfile.hh"
 #include "store-api.hh"
+#include "url-parts.hh"
 
 #include <sys/time.h>
 
diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index 1cc0c5e2e..1737658a7 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -3,14 +3,15 @@
 #include "fetchers.hh"
 #include "globals.hh"
 #include "store-api.hh"
+#include "url-parts.hh"
 
 #include <nlohmann/json.hpp>
 
 namespace nix::fetchers {
 
-// A github or gitlab url
-const static std::string urlRegexS = "[a-zA-Z0-9.]*"; // FIXME: check
-std::regex urlRegex(urlRegexS, std::regex::ECMAScript);
+// A github or gitlab host
+const static std::string hostRegexS = "[a-zA-Z0-9.]*"; // FIXME: check
+std::regex hostRegex(hostRegexS, std::regex::ECMAScript);
 
 struct GitArchiveInputScheme : InputScheme
 {
@@ -50,9 +51,9 @@ struct GitArchiveInputScheme : InputScheme
                     throw BadURL("URL '%s' contains multiple branch/tag names", url.url);
                 ref = value;
             }
-            else if (name == "url") {
-                if (!std::regex_match(value, urlRegex))
-                    throw BadURL("URL '%s' contains an invalid instance url", url.url);
+            else if (name == "host") {
+                if (!std::regex_match(value, hostRegex))
+                    throw BadURL("URL '%s' contains an invalid instance host", url.url);
                 host_url = value;
             }
             // FIXME: barf on unsupported attributes
@@ -67,7 +68,7 @@ struct GitArchiveInputScheme : InputScheme
         input.attrs.insert_or_assign("repo", path[1]);
         if (rev) input.attrs.insert_or_assign("rev", rev->gitRev());
         if (ref) input.attrs.insert_or_assign("ref", *ref);
-        if (host_url) input.attrs.insert_or_assign("url", *host_url);
+        if (host_url) input.attrs.insert_or_assign("host", *host_url);
 
         return input;
     }
@@ -77,7 +78,7 @@ struct GitArchiveInputScheme : InputScheme
         if (maybeGetStrAttr(attrs, "type") != type()) return {};
 
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "owner" && name != "repo" && name != "ref" && name != "rev" && name != "narHash" && name != "lastModified")
+            if (name != "type" && name != "owner" && name != "repo" && name != "ref" && name != "rev" && name != "narHash" && name != "lastModified" && name != "host")
                 throw Error("unsupported input attribute '%s'", name);
 
         getStrAttr(attrs, "owner");
@@ -210,7 +211,7 @@ struct GitHubInputScheme : GitArchiveInputScheme
     {
         // FIXME: use regular /archive URLs instead? api.github.com
         // might have stricter rate limits.
-        auto host_url = maybeGetStrAttr(input.attrs, "url").value_or("github.com");
+        auto host_url = maybeGetStrAttr(input.attrs, "host").value_or("github.com");
         auto url = fmt("https://api.%s/repos/%s/%s/tarball/%s", // FIXME: check if this is correct for self hosted instances
             host_url, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo"),
             input.getRev()->to_string(Base16, false));
@@ -236,7 +237,7 @@ struct GitLabInputScheme : GitArchiveInputScheme
 
     Hash getRevFromRef(nix::ref<Store> store, const Input & input) const override
     {
-        auto host_url = maybeGetStrAttr(input.attrs, "url").value_or("gitlab.com");
+        auto host_url = maybeGetStrAttr(input.attrs, "host").value_or("gitlab.com");
         auto url = fmt("https://%s/api/v4/projects/%s%%2F%s/repository/commits?ref_name=%s",
             host_url, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo"), *input.getRef());
         auto json = nlohmann::json::parse(
diff --git a/src/libfetchers/indirect.cc b/src/libfetchers/indirect.cc
index b981d4d8e..74332ae3d 100644
--- a/src/libfetchers/indirect.cc
+++ b/src/libfetchers/indirect.cc
@@ -1,4 +1,5 @@
 #include "fetchers.hh"
+#include "url-parts.hh"
 
 namespace nix::fetchers {
 
diff --git a/src/libfetchers/mercurial.cc b/src/libfetchers/mercurial.cc
index 3e76ffc4d..d80c2ea7a 100644
--- a/src/libfetchers/mercurial.cc
+++ b/src/libfetchers/mercurial.cc
@@ -3,6 +3,7 @@
 #include "globals.hh"
 #include "tarfile.hh"
 #include "store-api.hh"
+#include "url-parts.hh"
 
 #include <sys/time.h>
 
diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index 34f844a18..ebc0bd6a4 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -11,6 +11,7 @@
 #include "nar-accessor.hh"
 #include "json.hh"
 #include "thread-pool.hh"
+#include "callback.hh"
 
 #include <chrono>
 #include <future>
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index bff8a8327..b17ccd7d0 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -17,6 +17,7 @@
 #include "daemon.hh"
 #include "worker-protocol.hh"
 #include "topo-sort.hh"
+#include "callback.hh"
 
 #include <algorithm>
 #include <iostream>
diff --git a/src/libstore/dummy-store.cc b/src/libstore/dummy-store.cc
index 128832e60..49641c2ac 100644
--- a/src/libstore/dummy-store.cc
+++ b/src/libstore/dummy-store.cc
@@ -1,4 +1,5 @@
 #include "store-api.hh"
+#include "callback.hh"
 
 namespace nix {
 
diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index 4149f8155..6241b5e00 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -5,6 +5,7 @@
 #include "s3.hh"
 #include "compression.hh"
 #include "finally.hh"
+#include "callback.hh"
 
 #ifdef ENABLE_S3
 #include <aws/core/client/ClientConfiguration.h>
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 491c664db..0beb9b2b7 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -2,6 +2,7 @@
 #include "util.hh"
 #include "archive.hh"
 #include "args.hh"
+#include "abstract-setting-to-json.hh"
 
 #include <algorithm>
 #include <map>
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 02721285a..8a2d3ff75 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -2,7 +2,6 @@
 
 #include "types.hh"
 #include "config.hh"
-#include "abstractsettingtojson.hh"
 #include "util.hh"
 
 #include <map>
diff --git a/src/libstore/http-binary-cache-store.cc b/src/libstore/http-binary-cache-store.cc
index f4ab15a10..86be7c006 100644
--- a/src/libstore/http-binary-cache-store.cc
+++ b/src/libstore/http-binary-cache-store.cc
@@ -2,6 +2,7 @@
 #include "filetransfer.hh"
 #include "globals.hh"
 #include "nar-info-disk-cache.hh"
+#include "callback.hh"
 
 namespace nix {
 
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index e9478c1d5..5af75669a 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -6,6 +6,7 @@
 #include "worker-protocol.hh"
 #include "ssh.hh"
 #include "derivations.hh"
+#include "callback.hh"
 
 namespace nix {
 
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 8d9a202ce..b0ffa58c4 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -6,6 +6,7 @@
 #include "derivations.hh"
 #include "nar-info.hh"
 #include "references.hh"
+#include "callback.hh"
 
 #include <iostream>
 #include <algorithm>
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index da3981696..ad4dccef9 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -5,7 +5,7 @@
 #include "store-api.hh"
 #include "thread-pool.hh"
 #include "topo-sort.hh"
-
+#include "callback.hh"
 
 namespace nix {
 
diff --git a/src/libstore/names.cc b/src/libstore/names.cc
index d1c8a6101..41e28dc99 100644
--- a/src/libstore/names.cc
+++ b/src/libstore/names.cc
@@ -1,10 +1,18 @@
 #include "names.hh"
 #include "util.hh"
 
+#include <regex>
+
 
 namespace nix {
 
 
+struct Regex
+{
+    std::regex regex;
+};
+
+
 DrvName::DrvName()
 {
     name = "";
@@ -30,11 +38,18 @@ DrvName::DrvName(std::string_view s) : hits(0)
 }
 
 
+DrvName::~DrvName()
+{ }
+
+
 bool DrvName::matches(DrvName & n)
 {
     if (name != "*") {
-        if (!regex) regex = std::unique_ptr<std::regex>(new std::regex(name, std::regex::extended));
-        if (!std::regex_match(n.name, *regex)) return false;
+        if (!regex) {
+            regex = std::make_unique<Regex>();
+            regex->regex = std::regex(name, std::regex::extended);
+        }
+        if (!std::regex_match(n.name, regex->regex)) return false;
     }
     if (version != "" && version != n.version) return false;
     return true;
@@ -99,7 +114,7 @@ DrvNames drvNamesFromArgs(const Strings & opArgs)
 {
     DrvNames result;
     for (auto & i : opArgs)
-        result.push_back(DrvName(i));
+        result.emplace_back(i);
     return result;
 }
 
diff --git a/src/libstore/names.hh b/src/libstore/names.hh
index 00e14b8c7..bc62aac93 100644
--- a/src/libstore/names.hh
+++ b/src/libstore/names.hh
@@ -3,10 +3,11 @@
 #include <memory>
 
 #include "types.hh"
-#include <regex>
 
 namespace nix {
 
+struct Regex;
+
 struct DrvName
 {
     string fullName;
@@ -16,10 +17,12 @@ struct DrvName
 
     DrvName();
     DrvName(std::string_view s);
+    ~DrvName();
+
     bool matches(DrvName & n);
 
 private:
-    std::unique_ptr<std::regex> regex;
+    std::unique_ptr<Regex> regex;
 };
 
 typedef list<DrvName> DrvNames;
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index e92b94975..27535f1d0 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -10,6 +10,7 @@
 #include "pool.hh"
 #include "finally.hh"
 #include "logging.hh"
+#include "callback.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 2d5077ed0..1bbc74db8 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -8,9 +8,7 @@
 #include "json.hh"
 #include "url.hh"
 #include "archive.hh"
-
-#include <future>
-
+#include "callback.hh"
 
 namespace nix {
 
diff --git a/src/libutil/abstractsettingtojson.hh b/src/libutil/abstract-setting-to-json.hh
similarity index 100%
rename from src/libutil/abstractsettingtojson.hh
rename to src/libutil/abstract-setting-to-json.hh
diff --git a/src/libutil/callback.hh b/src/libutil/callback.hh
new file mode 100644
index 000000000..ef31794be
--- /dev/null
+++ b/src/libutil/callback.hh
@@ -0,0 +1,46 @@
+#pragma once
+
+#include <future>
+#include <functional>
+
+namespace nix {
+
+/* A callback is a wrapper around a lambda that accepts a valid of
+   type T or an exception. (We abuse std::future<T> to pass the value or
+   exception.) */
+template<typename T>
+class Callback
+{
+    std::function<void(std::future<T>)> fun;
+    std::atomic_flag done = ATOMIC_FLAG_INIT;
+
+public:
+
+    Callback(std::function<void(std::future<T>)> fun) : fun(fun) { }
+
+    Callback(Callback && callback) : fun(std::move(callback.fun))
+    {
+        auto prev = callback.done.test_and_set();
+        if (prev) done.test_and_set();
+    }
+
+    void operator()(T && t) noexcept
+    {
+        auto prev = done.test_and_set();
+        assert(!prev);
+        std::promise<T> promise;
+        promise.set_value(std::move(t));
+        fun(promise.get_future());
+    }
+
+    void rethrow(const std::exception_ptr & exc = std::current_exception()) noexcept
+    {
+        auto prev = done.test_and_set();
+        assert(!prev);
+        std::promise<T> promise;
+        promise.set_exception(exc);
+        fun(promise.get_future());
+    }
+};
+
+}
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 309d23b40..5e6a211df 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -1,6 +1,6 @@
 #include "config.hh"
 #include "args.hh"
-#include "abstractsettingtojson.hh"
+#include "abstract-setting-to-json.hh"
 
 #include <nlohmann/json.hpp>
 
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index a39de041f..6e69bdce2 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -1,7 +1,6 @@
 #pragma once
 
 #include <boost/format.hpp>
-#include <boost/algorithm/string/replace.hpp>
 #include <string>
 #include "ansicolor.hh"
 
diff --git a/src/libutil/url-parts.hh b/src/libutil/url-parts.hh
new file mode 100644
index 000000000..64e06cfbc
--- /dev/null
+++ b/src/libutil/url-parts.hh
@@ -0,0 +1,44 @@
+#pragma once
+
+#include <string>
+#include <regex>
+
+namespace nix {
+
+// URI stuff.
+const static std::string pctEncoded = "(?:%[0-9a-fA-F][0-9a-fA-F])";
+const static std::string schemeRegex = "(?:[a-z+.-]+)";
+const static std::string ipv6AddressRegex = "(?:\\[[0-9a-fA-F:]+\\])";
+const static std::string unreservedRegex = "(?:[a-zA-Z0-9-._~])";
+const static std::string subdelimsRegex = "(?:[!$&'\"()*+,;=])";
+const static std::string hostnameRegex = "(?:(?:" + unreservedRegex + "|" + pctEncoded + "|" + subdelimsRegex + ")*)";
+const static std::string hostRegex = "(?:" + ipv6AddressRegex + "|" + hostnameRegex + ")";
+const static std::string userRegex = "(?:(?:" + unreservedRegex + "|" + pctEncoded + "|" + subdelimsRegex + "|:)*)";
+const static std::string authorityRegex = "(?:" + userRegex + "@)?" + hostRegex + "(?::[0-9]+)?";
+const static std::string pcharRegex = "(?:" + unreservedRegex + "|" + pctEncoded + "|" + subdelimsRegex + "|[:@])";
+const static std::string queryRegex = "(?:" + pcharRegex + "|[/? \"])*";
+const static std::string segmentRegex = "(?:" + pcharRegex + "+)";
+const static std::string absPathRegex = "(?:(?:/" + segmentRegex + ")*/?)";
+const static std::string pathRegex = "(?:" + segmentRegex + "(?:/" + segmentRegex + ")*/?)";
+
+// A Git ref (i.e. branch or tag name).
+const static std::string refRegexS = "[a-zA-Z0-9][a-zA-Z0-9_.-]*"; // FIXME: check
+extern std::regex refRegex;
+
+// Instead of defining what a good Git Ref is, we define what a bad Git Ref is
+// This is because of the definition of a ref in refs.c in https://github.com/git/git
+// See tests/fetchGitRefs.sh for the full definition
+const static std::string badGitRefRegexS = "//|^[./]|/\\.|\\.\\.|[[:cntrl:][:space:]:?^~\[]|\\\\|\\*|\\.lock$|\\.lock/|@\\{|[/.]$|^@$|^$";
+extern std::regex badGitRefRegex;
+
+// A Git revision (a SHA-1 commit hash).
+const static std::string revRegexS = "[0-9a-fA-F]{40}";
+extern std::regex revRegex;
+
+// A ref or revision, or a ref followed by a revision.
+const static std::string refAndOrRevRegex = "(?:(" + revRegexS + ")|(?:(" + refRegexS + ")(?:/(" + revRegexS + "))?))";
+
+const static std::string flakeIdRegexS = "[a-zA-Z][a-zA-Z0-9_-]*";
+extern std::regex flakeIdRegex;
+
+}
diff --git a/src/libutil/url.cc b/src/libutil/url.cc
index 88c09eef9..c1bab866c 100644
--- a/src/libutil/url.cc
+++ b/src/libutil/url.cc
@@ -1,4 +1,5 @@
 #include "url.hh"
+#include "url-parts.hh"
 #include "util.hh"
 
 namespace nix {
diff --git a/src/libutil/url.hh b/src/libutil/url.hh
index 1f716ba10..6e77142e3 100644
--- a/src/libutil/url.hh
+++ b/src/libutil/url.hh
@@ -2,8 +2,6 @@
 
 #include "error.hh"
 
-#include <regex>
-
 namespace nix {
 
 struct ParsedURL
@@ -29,40 +27,4 @@ std::map<std::string, std::string> decodeQuery(const std::string & query);
 
 ParsedURL parseURL(const std::string & url);
 
-// URI stuff.
-const static std::string pctEncoded = "(?:%[0-9a-fA-F][0-9a-fA-F])";
-const static std::string schemeRegex = "(?:[a-z+.-]+)";
-const static std::string ipv6AddressRegex = "(?:\\[[0-9a-fA-F:]+\\])";
-const static std::string unreservedRegex = "(?:[a-zA-Z0-9-._~])";
-const static std::string subdelimsRegex = "(?:[!$&'\"()*+,;=])";
-const static std::string hostnameRegex = "(?:(?:" + unreservedRegex + "|" + pctEncoded + "|" + subdelimsRegex + ")*)";
-const static std::string hostRegex = "(?:" + ipv6AddressRegex + "|" + hostnameRegex + ")";
-const static std::string userRegex = "(?:(?:" + unreservedRegex + "|" + pctEncoded + "|" + subdelimsRegex + "|:)*)";
-const static std::string authorityRegex = "(?:" + userRegex + "@)?" + hostRegex + "(?::[0-9]+)?";
-const static std::string pcharRegex = "(?:" + unreservedRegex + "|" + pctEncoded + "|" + subdelimsRegex + "|[:@])";
-const static std::string queryRegex = "(?:" + pcharRegex + "|[/? \"])*";
-const static std::string segmentRegex = "(?:" + pcharRegex + "+)";
-const static std::string absPathRegex = "(?:(?:/" + segmentRegex + ")*/?)";
-const static std::string pathRegex = "(?:" + segmentRegex + "(?:/" + segmentRegex + ")*/?)";
-
-// A Git ref (i.e. branch or tag name).
-const static std::string refRegexS = "[a-zA-Z0-9][a-zA-Z0-9_.-]*"; // FIXME: check
-extern std::regex refRegex;
-
-// Instead of defining what a good Git Ref is, we define what a bad Git Ref is
-// This is because of the definition of a ref in refs.c in https://github.com/git/git
-// See tests/fetchGitRefs.sh for the full definition
-const static std::string badGitRefRegexS = "//|^[./]|/\\.|\\.\\.|[[:cntrl:][:space:]:?^~\[]|\\\\|\\*|\\.lock$|\\.lock/|@\\{|[/.]$|^@$|^$";
-extern std::regex badGitRefRegex;
-
-// A Git revision (a SHA-1 commit hash).
-const static std::string revRegexS = "[0-9a-fA-F]{40}";
-extern std::regex revRegex;
-
-// A ref or revision, or a ref followed by a revision.
-const static std::string refAndOrRevRegex = "(?:(" + revRegexS + ")|(?:(" + refRegexS + ")(?:/(" + revRegexS + "))?))";
-
-const static std::string flakeIdRegexS = "[a-zA-Z][a-zA-Z0-9_-]*";
-extern std::regex flakeIdRegex;
-
 }
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 082e26375..b8e201203 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -12,13 +12,9 @@
 #include <signal.h>
 
 #include <functional>
-#include <limits>
-#include <cstdio>
 #include <map>
 #include <sstream>
 #include <optional>
-#include <future>
-#include <iterator>
 
 #ifndef HAVE_STRUCT_DIRENT_D_TYPE
 #define DT_UNKNOWN 0
@@ -480,43 +476,8 @@ std::optional<typename T::mapped_type> get(const T & map, const typename T::key_
 }
 
 
-/* A callback is a wrapper around a lambda that accepts a valid of
-   type T or an exception. (We abuse std::future<T> to pass the value or
-   exception.) */
 template<typename T>
-class Callback
-{
-    std::function<void(std::future<T>)> fun;
-    std::atomic_flag done = ATOMIC_FLAG_INIT;
-
-public:
-
-    Callback(std::function<void(std::future<T>)> fun) : fun(fun) { }
-
-    Callback(Callback && callback) : fun(std::move(callback.fun))
-    {
-        auto prev = callback.done.test_and_set();
-        if (prev) done.test_and_set();
-    }
-
-    void operator()(T && t) noexcept
-    {
-        auto prev = done.test_and_set();
-        assert(!prev);
-        std::promise<T> promise;
-        promise.set_value(std::move(t));
-        fun(promise.get_future());
-    }
-
-    void rethrow(const std::exception_ptr & exc = std::current_exception()) noexcept
-    {
-        auto prev = done.test_and_set();
-        assert(!prev);
-        std::promise<T> promise;
-        promise.set_exception(exc);
-        fun(promise.get_future());
-    }
-};
+class Callback;
 
 
 /* Start a thread that handles various signals. Also block those signals
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index e5a433ac0..3e7c453fb 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -230,7 +230,7 @@ static DrvInfos filterBySelector(EvalState & state, const DrvInfos & allElems,
 {
     DrvNames selectors = drvNamesFromArgs(args);
     if (selectors.empty())
-        selectors.push_back(DrvName("*"));
+        selectors.emplace_back("*");
 
     DrvInfos elems;
     set<unsigned int> done;
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index 1001f27af..377ae03a8 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -57,6 +57,7 @@ static int _main(int argc, char * * argv)
         bool fromExpr = false;
         string attrPath;
         bool unpack = false;
+        bool executable = false;
         string name;
 
         struct MyArgs : LegacyArgs, MixEvalArgs
@@ -81,6 +82,8 @@ static int _main(int argc, char * * argv)
             }
             else if (*arg == "--unpack")
                 unpack = true;
+            else if (*arg == "--executable")
+                executable = true;
             else if (*arg == "--name")
                 name = getArg(*arg, arg, end);
             else if (*arg != "" && arg->at(0) == '-')
@@ -175,7 +178,11 @@ static int _main(int argc, char * * argv)
 
             /* Download the file. */
             {
-                AutoCloseFD fd = open(tmpFile.c_str(), O_WRONLY | O_CREAT | O_EXCL, 0600);
+                auto mode = 0600;
+                if (executable)
+                    mode = 0700;
+
+                AutoCloseFD fd = open(tmpFile.c_str(), O_WRONLY | O_CREAT | O_EXCL, mode);
                 if (!fd) throw SysError("creating temporary file '%s'", tmpFile);
 
                 FdSink sink(fd.get());
@@ -201,7 +208,7 @@ static int _main(int argc, char * * argv)
                     tmpFile = unpacked;
             }
 
-            const auto method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
+            const auto method = unpack || executable ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
 
             auto info = store->addToStoreSlow(name, tmpFile, method, ht, expectedHash);
             storePath = info.path;
diff --git a/src/nix/bundle.cc b/src/nix/bundle.cc
index 241c8699b..fc41da9e4 100644
--- a/src/nix/bundle.cc
+++ b/src/nix/bundle.cc
@@ -98,14 +98,14 @@ struct CmdBundle : InstallableCommand
         if (!evalState->isDerivation(*vRes))
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
-        auto attr1 = vRes->attrs->find(evalState->sDrvPath);
+        auto attr1 = vRes->attrs->get(evalState->sDrvPath);
         if (!attr1)
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
         PathSet context2;
         StorePath drvPath = store->parseStorePath(evalState->coerceToPath(*attr1->pos, *attr1->value, context2));
 
-        auto attr2 = vRes->attrs->find(evalState->sOutPath);
+        auto attr2 = vRes->attrs->get(evalState->sOutPath);
         if (!attr2)
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());