Merge remote-tracking branch 'upstream/master' into trustless-remote-builder-simple

39 files changed, 858 insertions, 622 deletions

diff --git a/src/libutil/abstract-setting-to-json.hh b/src/libutil/abstract-setting-to-json.hh
index b3fbc84f7..2d82b54e7 100644
--- a/src/libutil/abstract-setting-to-json.hh
+++ b/src/libutil/abstract-setting-to-json.hh
@@ -10,6 +10,7 @@ std::map<std::string, nlohmann::json> BaseSetting<T>::toJSONObject()
     auto obj = AbstractSetting::toJSONObject();
     obj.emplace("value", value);
     obj.emplace("defaultValue", defaultValue);
+    obj.emplace("documentDefault", documentDefault);
     return obj;
 }
 }
diff --git a/src/libutil/affinity.cc b/src/libutil/affinity.cc
deleted file mode 100644
index ac2295e4a..000000000
--- a/src/libutil/affinity.cc
+++ /dev/null
@@ -1,70 +0,0 @@
-#include "types.hh"
-#include "util.hh"
-#include "affinity.hh"
-
-#if __linux__
-#include <sched.h>
-#endif
-
-namespace nix {
-
-
-#if __linux__
-static bool didSaveAffinity = false;
-static cpu_set_t savedAffinity;
-
-std::ostream& operator<<(std::ostream &os, const cpu_set_t &cset)
-{
-    auto count = CPU_COUNT(&cset);
-    for (int i=0; i < count; ++i)
-    {
-        os << (CPU_ISSET(i,&cset) ? "1" : "0");
-    }
-
-    return os;
-}
-#endif
-
-
-void setAffinityTo(int cpu)
-{
-#if __linux__
-    if (sched_getaffinity(0, sizeof(cpu_set_t), &savedAffinity) == -1) return;
-    didSaveAffinity = true;
-    debug(format("locking this thread to CPU %1%") % cpu);
-    cpu_set_t newAffinity;
-    CPU_ZERO(&newAffinity);
-    CPU_SET(cpu, &newAffinity);
-    if (sched_setaffinity(0, sizeof(cpu_set_t), &newAffinity) == -1)
-        printError("failed to lock thread to CPU %1%", cpu);
-#endif
-}
-
-
-int lockToCurrentCPU()
-{
-#if __linux__
-    int cpu = sched_getcpu();
-    if (cpu != -1) setAffinityTo(cpu);
-    return cpu;
-#else
-    return -1;
-#endif
-}
-
-
-void restoreAffinity()
-{
-#if __linux__
-    if (!didSaveAffinity) return;
-    if (sched_setaffinity(0, sizeof(cpu_set_t), &savedAffinity) == -1)
-    {
-        std::ostringstream oss;
-        oss << savedAffinity;
-        printError("failed to restore CPU affinity %1%", oss.str());
-    }
-#endif
-}
-
-
-}
diff --git a/src/libutil/affinity.hh b/src/libutil/affinity.hh
deleted file mode 100644
index c1bd28e13..000000000
--- a/src/libutil/affinity.hh
+++ /dev/null
@@ -1,9 +0,0 @@
-#pragma once
-
-namespace nix {
-
-void setAffinityTo(int cpu);
-int lockToCurrentCPU();
-void restoreAffinity();
-
-}
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index d78ec2b93..eda004756 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -37,7 +37,7 @@ static GlobalConfig::Register rArchiveSettings(&archiveSettings);
 
 const std::string narVersionMagic1 = "nix-archive-1";
 
-static string caseHackSuffix = "~nix~case~hack~";
+static std::string caseHackSuffix = "~nix~case~hack~";
 
 PathFilter defaultPathFilter = [](const Path &) { return true; };
 
@@ -84,22 +84,21 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
 
         /* If we're on a case-insensitive system like macOS, undo
            the case hack applied by restorePath(). */
-        std::map<string, string> unhacked;
+        std::map<std::string, std::string> unhacked;
         for (auto & i : readDirectory(path))
             if (archiveSettings.useCaseHack) {
-                string name(i.name);
+                std::string name(i.name);
                 size_t pos = i.name.find(caseHackSuffix);
-                if (pos != string::npos) {
+                if (pos != std::string::npos) {
                     debug(format("removing case hack suffix from '%1%'") % (path + "/" + i.name));
                     name.erase(pos);
                 }
-                if (unhacked.find(name) != unhacked.end())
+                if (!unhacked.emplace(name, i.name).second)
                     throw Error("file name collision in between '%1%' and '%2%'",
                        (path + "/" + unhacked[name]),
                        (path + "/" + i.name));
-                unhacked[name] = i.name;
             } else
-                unhacked[i.name] = i.name;
+                unhacked.emplace(i.name, i.name);
 
         for (auto & i : unhacked)
             if (filter(path + "/" + i.first)) {
@@ -125,13 +124,13 @@ void dumpPath(const Path & path, Sink & sink, PathFilter & filter)
 }
 
 
-void dumpString(const std::string & s, Sink & sink)
+void dumpString(std::string_view s, Sink & sink)
 {
     sink << narVersionMagic1 << "(" << "type" << "regular" << "contents" << s << ")";
 }
 
 
-static SerialisationError badArchive(string s)
+static SerialisationError badArchive(const std::string & s)
 {
     return SerialisationError("bad archive: " + s);
 }
@@ -172,7 +171,7 @@ static void parseContents(ParseSink & sink, Source & source, const Path & path)
 
 struct CaseInsensitiveCompare
 {
-    bool operator() (const string & a, const string & b) const
+    bool operator() (const std::string & a, const std::string & b) const
     {
         return strcasecmp(a.c_str(), b.c_str()) < 0;
     }
@@ -181,7 +180,7 @@ struct CaseInsensitiveCompare
 
 static void parse(ParseSink & sink, Source & source, const Path & path)
 {
-    string s;
+    std::string s;
 
     s = readString(source);
     if (s != "(") throw badArchive("expected open tag");
@@ -202,7 +201,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
         else if (s == "type") {
             if (type != tpUnknown)
                 throw badArchive("multiple type fields");
-            string t = readString(source);
+            std::string t = readString(source);
 
             if (t == "regular") {
                 type = tpRegular;
@@ -233,7 +232,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
         }
 
         else if (s == "entry" && type == tpDirectory) {
-            string name, prevName;
+            std::string name, prevName;
 
             s = readString(source);
             if (s != "(") throw badArchive("expected open tag");
@@ -247,7 +246,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                     break;
                 } else if (s == "name") {
                     name = readString(source);
-                    if (name.empty() || name == "." || name == ".." || name.find('/') != string::npos || name.find((char) 0) != string::npos)
+                    if (name.empty() || name == "." || name == ".." || name.find('/') != std::string::npos || name.find((char) 0) != std::string::npos)
                         throw Error("NAR contains invalid file name '%1%'", name);
                     if (name <= prevName)
                         throw Error("NAR directory is not sorted");
@@ -270,7 +269,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
         }
 
         else if (s == "target" && type == tpSymlink) {
-            string target = readString(source);
+            std::string target = readString(source);
             sink.createSymlink(path, target);
         }
 
@@ -282,7 +281,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
 
 void parseDump(ParseSink & sink, Source & source)
 {
-    string version;
+    std::string version;
     try {
         version = readString(source, narVersionMagic1.size());
     } catch (SerialisationError & e) {
@@ -346,7 +345,7 @@ struct RestoreSink : ParseSink
         writeFull(fd.get(), data);
     }
 
-    void createSymlink(const Path & path, const string & target) override
+    void createSymlink(const Path & path, const std::string & target) override
     {
         Path p = dstPath + path;
         nix::createSymlink(target, p);
diff --git a/src/libutil/archive.hh b/src/libutil/archive.hh
index 9e9e11b1a..fca351605 100644
--- a/src/libutil/archive.hh
+++ b/src/libutil/archive.hh
@@ -48,7 +48,7 @@ namespace nix {
 void dumpPath(const Path & path, Sink & sink,
     PathFilter & filter = defaultPathFilter);
 
-void dumpString(const std::string & s, Sink & sink);
+void dumpString(std::string_view s, Sink & sink);
 
 /* FIXME: fix this API, it sucks. */
 struct ParseSink
@@ -60,7 +60,7 @@ struct ParseSink
     virtual void preallocateContents(uint64_t size) { };
     virtual void receiveContents(std::string_view data) { };
 
-    virtual void createSymlink(const Path & path, const string & target) { };
+    virtual void createSymlink(const Path & path, const std::string & target) { };
 };
 
 /* If the NAR archive contains a single file at top-level, then save
@@ -82,7 +82,7 @@ struct RetrieveRegularNARSink : ParseSink
         sink(data);
     }
 
-    void createSymlink(const Path & path, const string & target) override
+    void createSymlink(const Path & path, const std::string & target) override
     {
         regular = false;
     }
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index 9df279faf..f970c0e9e 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -39,7 +39,7 @@ void Completions::add(std::string completion, std::string description)
 bool Completion::operator<(const Completion & other) const
 { return completion < other.completion || (completion == other.completion && description < other.description); }
 
-bool pathCompletions = false;
+CompletionType completionType = ctNormal;
 std::shared_ptr<Completions> completions;
 
 std::string completionMarker = "___COMPLETE___";
@@ -76,13 +76,13 @@ void Args::parseCmdline(const Strings & _cmdline)
         /* Expand compound dash options (i.e., `-qlf' -> `-q -l -f',
            `-j3` -> `-j 3`). */
         if (!dashDash && arg.length() > 2 && arg[0] == '-' && arg[1] != '-' && isalpha(arg[1])) {
-            *pos = (string) "-" + arg[1];
+            *pos = (std::string) "-" + arg[1];
             auto next = pos; ++next;
             for (unsigned int j = 2; j < arg.length(); j++)
                 if (isalpha(arg[j]))
-                    cmdline.insert(next, (string) "-" + arg[j]);
+                    cmdline.insert(next, (std::string) "-" + arg[j]);
                 else {
-                    cmdline.insert(next, string(arg, j));
+                    cmdline.insert(next, std::string(arg, j));
                     break;
                 }
             arg = *pos;
@@ -139,7 +139,7 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         return true;
     };
 
-    if (string(*pos, 0, 2) == "--") {
+    if (std::string(*pos, 0, 2) == "--") {
         if (auto prefix = needsCompletion(*pos)) {
             for (auto & [name, flag] : longFlags) {
                 if (!hiddenCategories.count(flag->category)
@@ -147,12 +147,12 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
                     completions->add("--" + name, flag->description);
             }
         }
-        auto i = longFlags.find(string(*pos, 2));
+        auto i = longFlags.find(std::string(*pos, 2));
         if (i == longFlags.end()) return false;
         return process("--" + i->first, *i->second);
     }
 
-    if (string(*pos, 0, 1) == "-" && pos->size() == 2) {
+    if (std::string(*pos, 0, 1) == "-" && pos->size() == 2) {
         auto c = (*pos)[1];
         auto i = shortFlags.find(c);
         if (i == shortFlags.end()) return false;
@@ -277,7 +277,7 @@ Args::Flag Args::Flag::mkHashTypeOptFlag(std::string && longName, std::optional<
 
 static void _completePath(std::string_view prefix, bool onlyDirs)
 {
-    pathCompletions = true;
+    completionType = ctFilenames;
     glob_t globbuf;
     int flags = GLOB_NOESCAPE | GLOB_TILDE;
     #ifdef GLOB_ONLYDIR
diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index 7521b3065..fdd036f9a 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -158,7 +158,7 @@ public:
     }
 
     /* Expect a string argument. */
-    void expectArg(const std::string & label, string * dest, bool optional = false)
+    void expectArg(const std::string & label, std::string * dest, bool optional = false)
     {
         expectArgs({
             .label = label,
@@ -237,7 +237,13 @@ public:
     void add(std::string completion, std::string description = "");
 };
 extern std::shared_ptr<Completions> completions;
-extern bool pathCompletions;
+
+enum CompletionType {
+    ctNormal,
+    ctFilenames,
+    ctAttrs
+};
+extern CompletionType completionType;
 
 std::optional<std::string> needsCompletion(std::string_view s);
 
diff --git a/src/libutil/compression.cc b/src/libutil/compression.cc
index f80ca664c..89180e7a7 100644
--- a/src/libutil/compression.cc
+++ b/src/libutil/compression.cc
@@ -190,13 +190,13 @@ struct BrotliDecompressionSink : ChunkedCompressionSink
     }
 };
 
-ref<std::string> decompress(const std::string & method, const std::string & in)
+std::string decompress(const std::string & method, std::string_view in)
 {
     StringSink ssink;
     auto sink = makeDecompressionSink(method, ssink);
     (*sink)(in);
     sink->finish();
-    return ssink.s;
+    return std::move(ssink.s);
 }
 
 std::unique_ptr<FinishSink> makeDecompressionSink(const std::string & method, Sink & nextSink)
@@ -281,13 +281,13 @@ ref<CompressionSink> makeCompressionSink(const std::string & method, Sink & next
         throw UnknownCompressionMethod("unknown compression method '%s'", method);
 }
 
-ref<std::string> compress(const std::string & method, const std::string & in, const bool parallel, int level)
+std::string compress(const std::string & method, std::string_view in, const bool parallel, int level)
 {
     StringSink ssink;
     auto sink = makeCompressionSink(method, ssink, parallel, level);
     (*sink)(in);
     sink->finish();
-    return ssink.s;
+    return std::move(ssink.s);
 }
 
 }
diff --git a/src/libutil/compression.hh b/src/libutil/compression.hh
index 9b1e4a9d4..c470b82a5 100644
--- a/src/libutil/compression.hh
+++ b/src/libutil/compression.hh
@@ -15,11 +15,11 @@ struct CompressionSink : BufferedSink, FinishSink
     using FinishSink::finish;
 };
 
-ref<std::string> decompress(const std::string & method, const std::string & in);
+std::string decompress(const std::string & method, std::string_view in);
 
 std::unique_ptr<FinishSink> makeDecompressionSink(const std::string & method, Sink & nextSink);
 
-ref<std::string> compress(const std::string & method, const std::string & in, const bool parallel = false, int level = -1);
+std::string compress(const std::string & method, std::string_view in, const bool parallel = false, int level = -1);
 
 ref<CompressionSink> makeCompressionSink(const std::string & method, Sink & nextSink, const bool parallel = false, int level = -1);
 
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index c247c7dae..9bb412b4f 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -1,6 +1,7 @@
 #include "config.hh"
 #include "args.hh"
 #include "abstract-setting-to-json.hh"
+#include "experimental-features.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -80,16 +81,16 @@ void AbstractConfig::applyConfig(const std::string & contents, const std::string
     unsigned int pos = 0;
 
     while (pos < contents.size()) {
-        string line;
+        std::string line;
         while (pos < contents.size() && contents[pos] != '\n')
             line += contents[pos++];
         pos++;
 
-        string::size_type hash = line.find('#');
-        if (hash != string::npos)
-            line = string(line, 0, hash);
+        auto hash = line.find('#');
+        if (hash != std::string::npos)
+            line = std::string(line, 0, hash);
 
-        vector<string> tokens = tokenizeString<vector<string> >(line);
+        auto tokens = tokenizeString<std::vector<std::string>>(line);
         if (tokens.empty()) continue;
 
         if (tokens.size() < 2)
@@ -119,9 +120,9 @@ void AbstractConfig::applyConfig(const std::string & contents, const std::string
         if (tokens[1] != "=")
             throw UsageError("illegal configuration line '%1%' in '%2%'", line, path);
 
-        string name = tokens[0];
+        std::string name = tokens[0];
 
-        vector<string>::iterator i = tokens.begin();
+        auto i = tokens.begin();
         advance(i, 2);
 
         set(name, concatStringsSep(" ", Strings(i, tokens.end()))); // FIXME: slow
@@ -131,7 +132,7 @@ void AbstractConfig::applyConfig(const std::string & contents, const std::string
 void AbstractConfig::applyConfigFile(const Path & path)
 {
     try {
-        string contents = readFile(path);
+        std::string contents = readFile(path);
         applyConfig(contents, path);
     } catch (SysError &) { }
 }
@@ -313,6 +314,31 @@ template<> std::string BaseSetting<StringSet>::to_string() const
     return concatStringsSep(" ", value);
 }
 
+template<> void BaseSetting<std::set<ExperimentalFeature>>::set(const std::string & str, bool append)
+{
+    if (!append) value.clear();
+    for (auto & s : tokenizeString<StringSet>(str)) {
+        auto thisXpFeature = parseExperimentalFeature(s);
+        if (thisXpFeature)
+            value.insert(thisXpFeature.value());
+        else
+            warn("unknown experimental feature '%s'", s);
+    }
+}
+
+template<> bool BaseSetting<std::set<ExperimentalFeature>>::isAppendable()
+{
+    return true;
+}
+
+template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() const
+{
+    StringSet stringifiedXpFeatures;
+    for (auto & feature : value)
+        stringifiedXpFeatures.insert(std::string(showExperimentalFeature(feature)));
+    return concatStringsSep(" ", stringifiedXpFeatures);
+}
+
 template<> void BaseSetting<StringMap>::set(const std::string & str, bool append)
 {
     if (!append) value.clear();
@@ -348,6 +374,7 @@ template class BaseSetting<std::string>;
 template class BaseSetting<Strings>;
 template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
+template class BaseSetting<std::set<ExperimentalFeature>>;
 
 void PathSetting::set(const std::string & str, bool append)
 {
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index 736810bf3..79ec0f9cf 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -232,16 +232,19 @@ protected:
 
     T value;
     const T defaultValue;
+    const bool documentDefault;
 
 public:
 
     BaseSetting(const T & def,
+        const bool documentDefault,
         const std::string & name,
         const std::string & description,
         const std::set<std::string> & aliases = {})
         : AbstractSetting(name, description, aliases)
         , value(def)
         , defaultValue(def)
+        , documentDefault(documentDefault)
     { }
 
     operator const T &() const { return value; }
@@ -288,8 +291,9 @@ public:
         const T & def,
         const std::string & name,
         const std::string & description,
-        const std::set<std::string> & aliases = {})
-        : BaseSetting<T>(def, name, description, aliases)
+        const std::set<std::string> & aliases = {},
+        const bool documentDefault = true)
+        : BaseSetting<T>(def, documentDefault, name, description, aliases)
     {
         options->addSetting(this);
     }
@@ -311,7 +315,7 @@ public:
         const std::string & name,
         const std::string & description,
         const std::set<std::string> & aliases = {})
-        : BaseSetting<Path>(def, name, description, aliases)
+        : BaseSetting<Path>(def, true, name, description, aliases)
         , allowEmpty(allowEmpty)
     {
         options->addSetting(this);
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 203d79087..dcd2f82a5 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -17,7 +17,7 @@ BaseError & BaseError::addTrace(std::optional<ErrPos> e, hintformat hint)
 
 // c++ std::exception descendants must have a 'const char* what()' function.
 // This stringifies the error and caches it for use by what(), or similarly by msg().
-const string & BaseError::calcWhat() const
+const std::string & BaseError::calcWhat() const
 {
     if (what_.has_value())
         return *what_;
@@ -25,21 +25,21 @@ const string & BaseError::calcWhat() const
         err.name = sname();
 
         std::ostringstream oss;
-        showErrorInfo(oss, err, false);
+        showErrorInfo(oss, err, loggerSettings.showTrace);
         what_ = oss.str();
 
         return *what_;
     }
 }
 
-std::optional<string> ErrorInfo::programName = std::nullopt;
+std::optional<std::string> ErrorInfo::programName = std::nullopt;
 
 std::ostream & operator<<(std::ostream & os, const hintformat & hf)
 {
     return os << hf.str();
 }
 
-string showErrPos(const ErrPos & errPos)
+std::string showErrPos(const ErrPos & errPos)
 {
     if (errPos.line > 0) {
         if (errPos.column > 0) {
@@ -68,7 +68,7 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
 
             // count the newlines.
             int count = 0;
-            string line;
+            std::string line;
             int pl = errPos.line - 1;
             do
             {
@@ -100,7 +100,7 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
         std::istringstream iss(errPos.file);
         // count the newlines.
         int count = 0;
-        string line;
+        std::string line;
         int pl = errPos.line - 1;
 
         LinesOfCode loc;
@@ -132,7 +132,7 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
 
 // print lines of code to the ostream, indicating the error column.
 void printCodeLines(std::ostream & out,
-    const string & prefix,
+    const std::string & prefix,
     const ErrPos & errPos,
     const LinesOfCode & loc)
 {
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index ff58d3e00..d55e1d701 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -61,16 +61,16 @@ typedef enum {
 
 // the lines of code surrounding an error.
 struct LinesOfCode {
-    std::optional<string> prevLineOfCode;
-    std::optional<string> errLineOfCode;
-    std::optional<string> nextLineOfCode;
+    std::optional<std::string> prevLineOfCode;
+    std::optional<std::string> errLineOfCode;
+    std::optional<std::string> nextLineOfCode;
 };
 
 // ErrPos indicates the location of an error in a nix file.
 struct ErrPos {
     int line = 0;
     int column = 0;
-    string file;
+    std::string file;
     FileOrigin origin;
 
     operator bool() const
@@ -80,7 +80,7 @@ struct ErrPos {
 
     // convert from the Pos struct, found in libexpr.
     template <class P>
-    ErrPos& operator=(const P &pos)
+    ErrPos & operator=(const P & pos)
     {
         origin = pos.origin;
         line = pos.line;
@@ -94,7 +94,7 @@ struct ErrPos {
     }
 
     template <class P>
-    ErrPos(const P &p)
+    ErrPos(const P & p)
     {
         *this = p;
     }
@@ -107,15 +107,15 @@ struct Trace {
 
 struct ErrorInfo {
     Verbosity level;
-    string name; // FIXME: rename
+    std::string name; // FIXME: rename
     hintformat msg;
     std::optional<ErrPos> errPos;
     std::list<Trace> traces;
 
-    static std::optional<string> programName;
+    static std::optional<std::string> programName;
 };
 
-std::ostream& showErrorInfo(std::ostream &out, const ErrorInfo &einfo, bool showTrace);
+std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool showTrace);
 
 /* BaseError should generally not be caught, as it has Interrupted as
    a subclass. Catch Error instead. */
@@ -124,8 +124,8 @@ class BaseError : public std::exception
 protected:
     mutable ErrorInfo err;
 
-    mutable std::optional<string> what_;
-    const string& calcWhat() const;
+    mutable std::optional<std::string> what_;
+    const std::string & calcWhat() const;
 
 public:
     unsigned int status = 1; // exit status
@@ -137,7 +137,7 @@ public:
     { }
 
     template<typename... Args>
-    BaseError(const std::string & fs, const Args & ... args)
+    explicit BaseError(const std::string & fs, const Args & ... args)
         : err { .level = lvlError, .msg = hintfmt(fs, args...) }
     { }
 
@@ -162,11 +162,11 @@ public:
     const char * what() const noexcept override { return calcWhat().c_str(); }
 #endif
 
-    const string & msg() const { return calcWhat(); }
+    const std::string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }
 
     template<typename... Args>
-    BaseError & addTrace(std::optional<ErrPos> e, const string &fs, const Args & ... args)
+    BaseError & addTrace(std::optional<ErrPos> e, const std::string & fs, const Args & ... args)
     {
         return addTrace(e, hintfmt(fs, args...));
     }
diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
new file mode 100644
index 000000000..687857869
--- /dev/null
+++ b/src/libutil/experimental-features.cc
@@ -0,0 +1,60 @@
+#include "experimental-features.hh"
+#include "util.hh"
+
+#include "nlohmann/json.hpp"
+
+namespace nix {
+
+std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
+    { Xp::CaDerivations, "ca-derivations" },
+    { Xp::Flakes, "flakes" },
+    { Xp::NixCommand, "nix-command" },
+    { Xp::RecursiveNix, "recursive-nix" },
+    { Xp::NoUrlLiterals, "no-url-literals" },
+    { Xp::NixTesting, "nix-testing" },
+};
+
+const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)
+{
+    using ReverseXpMap = std::map<std::string_view, ExperimentalFeature>;
+
+    static auto reverseXpMap = []()
+    {
+        auto reverseXpMap = std::make_unique<ReverseXpMap>();
+        for (auto & [feature, name] : stringifiedXpFeatures)
+            (*reverseXpMap)[name] = feature;
+        return reverseXpMap;
+    }();
+
+    if (auto feature = get(*reverseXpMap, name))
+        return *feature;
+    else
+        return std::nullopt;
+}
+
+std::string_view showExperimentalFeature(const ExperimentalFeature feature)
+{
+    return stringifiedXpFeatures.at(feature);
+}
+
+std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> & rawFeatures)
+{
+    std::set<ExperimentalFeature> res;
+    for (auto & rawFeature : rawFeatures) {
+        if (auto feature = parseExperimentalFeature(rawFeature))
+            res.insert(*feature);
+    }
+    return res;
+}
+
+MissingExperimentalFeature::MissingExperimentalFeature(ExperimentalFeature feature)
+    : Error("experimental Nix feature '%1%' is disabled; use '--extra-experimental-features %1%' to override", showExperimentalFeature(feature))
+    , missingFeature(feature)
+{}
+
+std::ostream & operator <<(std::ostream & str, const ExperimentalFeature & feature)
+{
+    return str << showExperimentalFeature(feature);
+}
+
+}
diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
new file mode 100644
index 000000000..6c5f55bd6
--- /dev/null
+++ b/src/libutil/experimental-features.hh
@@ -0,0 +1,62 @@
+#pragma once
+
+#include "comparator.hh"
+#include "error.hh"
+#include "nlohmann/json_fwd.hpp"
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * The list of available experimental features.
+ *
+ * If you update this, don’t forget to also change the map defining their
+ * string representation in the corresponding `.cc` file.
+ **/
+enum struct ExperimentalFeature
+{
+    CaDerivations,
+    Flakes,
+    NixCommand,
+    RecursiveNix,
+    NoUrlLiterals,
+
+    /**
+     * A "permanent" experimental feature for extra features we just
+     * need for testing.
+     **/
+    NixTesting,
+};
+
+/**
+ * Just because writing `ExperimentalFeature::CaDerivations` is way too long
+ */
+using Xp = ExperimentalFeature;
+
+const std::optional<ExperimentalFeature> parseExperimentalFeature(
+        const std::string_view & name);
+std::string_view showExperimentalFeature(const ExperimentalFeature);
+
+std::ostream & operator<<(
+        std::ostream & str,
+        const ExperimentalFeature & feature);
+
+/**
+ * Parse a set of strings to the corresponding set of experimental features,
+ * ignoring (but warning for) any unkwown feature.
+ */
+std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> &);
+
+class MissingExperimentalFeature : public Error
+{
+public:
+    ExperimentalFeature missingFeature;
+
+    MissingExperimentalFeature(ExperimentalFeature);
+    virtual const char * sname() const override
+    {
+        return "MissingExperimentalFeature";
+    }
+};
+
+}
diff --git a/src/libutil/fmt.cc b/src/libutil/fmt.cc
new file mode 100644
index 000000000..3dd93d73e
--- /dev/null
+++ b/src/libutil/fmt.cc
@@ -0,0 +1,46 @@
+#include "fmt.hh"
+
+#include <regex>
+
+namespace nix {
+
+std::string hiliteMatches(
+    std::string_view s,
+    std::vector<std::smatch> matches,
+    std::string_view prefix,
+    std::string_view postfix)
+{
+    // Avoid copy on zero matches
+    if (matches.size() == 0)
+        return (std::string) s;
+
+    std::sort(matches.begin(), matches.end(), [](const auto & a, const auto & b) {
+        return a.position() < b.position();
+    });
+
+    std::string out;
+    ssize_t last_end = 0;
+
+    for (auto it = matches.begin(); it != matches.end();) {
+        auto m = *it;
+        size_t start = m.position();
+        out.append(s.substr(last_end, m.position() - last_end));
+        // Merge continous matches
+        ssize_t end = start + m.length();
+        while (++it != matches.end() && (*it).position() <= end) {
+            auto n = *it;
+            ssize_t nend = start + (n.position() - start + n.length());
+            if (nend > end)
+                end = nend;
+        }
+        out.append(prefix);
+        out.append(s.substr(start, end - start));
+        out.append(postfix);
+        last_end = end;
+    }
+
+    out.append(s.substr(last_end));
+    return out;
+}
+
+}
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index fd335b811..0821b3b74 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -2,6 +2,7 @@
 
 #include <boost/format.hpp>
 #include <string>
+#include <regex>
 #include "ansicolor.hh"
 
 
@@ -9,7 +10,6 @@ namespace nix {
 
 
 /* Inherit some names from other namespaces for convenience. */
-using std::string;
 using boost::format;
 
 
@@ -20,8 +20,8 @@ struct nop { template<typename... T> nop(T...) {} };
 
 struct FormatOrString
 {
-    string s;
-    FormatOrString(const string & s) : s(s) { };
+    std::string s;
+    FormatOrString(std::string s) : s(std::move(s)) { };
     template<class F>
     FormatOrString(const F & f) : s(f.str()) { };
     FormatOrString(const char * s) : s(s) { };
@@ -101,7 +101,7 @@ std::ostream & operator<<(std::ostream & out, const normaltxt<T> & y)
 class hintformat
 {
 public:
-    hintformat(const string & format) : fmt(format)
+    hintformat(const std::string & format) : fmt(format)
     {
         fmt.exceptions(boost::io::all_error_bits ^
                        boost::io::too_many_args_bit ^
@@ -154,4 +154,16 @@ inline hintformat hintfmt(std::string plain_string)
     // we won't be receiving any args in this case, so just print the original string
     return hintfmt("%s", normaltxt(plain_string));
 }
+
+/* Highlight all the given matches in the given string `s` by wrapping
+   them between `prefix` and `postfix`.
+
+   If some matches overlap, then their union will be wrapped rather
+   than the individual matches. */
+std::string hiliteMatches(
+    std::string_view s,
+    std::vector<std::smatch> matches,
+    std::string_view prefix,
+    std::string_view postfix);
+
 }
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 4df8b4ecb..a4d632161 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -66,31 +66,31 @@ bool Hash::operator < (const Hash & h) const
 }
 
 
-const string base16Chars = "0123456789abcdef";
+const std::string base16Chars = "0123456789abcdef";
 
 
-static string printHash16(const Hash & hash)
+static std::string printHash16(const Hash & hash)
 {
     char buf[hash.hashSize * 2];
     for (unsigned int i = 0; i < hash.hashSize; i++) {
         buf[i * 2] = base16Chars[hash.hash[i] >> 4];
         buf[i * 2 + 1] = base16Chars[hash.hash[i] & 0x0f];
     }
-    return string(buf, hash.hashSize * 2);
+    return std::string(buf, hash.hashSize * 2);
 }
 
 
 // omitted: E O U T
-const string base32Chars = "0123456789abcdfghijklmnpqrsvwxyz";
+const std::string base32Chars = "0123456789abcdfghijklmnpqrsvwxyz";
 
 
-static string printHash32(const Hash & hash)
+static std::string printHash32(const Hash & hash)
 {
     assert(hash.hashSize);
     size_t len = hash.base32Len();
     assert(len);
 
-    string s;
+    std::string s;
     s.reserve(len);
 
     for (int n = (int) len - 1; n >= 0; n--) {
@@ -107,7 +107,7 @@ static string printHash32(const Hash & hash)
 }
 
 
-string printHash16or32(const Hash & hash)
+std::string printHash16or32(const Hash & hash)
 {
     assert(hash.type);
     return hash.to_string(hash.type == htMD5 ? Base16 : Base32, false);
@@ -151,7 +151,8 @@ Hash Hash::parseSRI(std::string_view original) {
 }
 
 // Mutates the string to eliminate the prefixes when found
-static std::pair<std::optional<HashType>, bool> getParsedTypeAndSRI(std::string_view & rest) {
+static std::pair<std::optional<HashType>, bool> getParsedTypeAndSRI(std::string_view & rest)
+{
     bool isSRI = false;
 
     // Parse the has type before the separater, if there was one.
@@ -259,7 +260,7 @@ Hash::Hash(std::string_view rest, HashType type, bool isSRI)
         throw BadHash("hash '%s' has wrong length for hash type '%s'", rest, printHashType(this->type));
 }
 
-Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht)
+Hash newHashAllowEmpty(std::string_view hashStr, std::optional<HashType> ht)
 {
     if (hashStr.empty()) {
         if (!ht)
@@ -402,7 +403,7 @@ HashType parseHashType(std::string_view s)
         throw UsageError("unknown hash algorithm '%1%'", s);
 }
 
-string printHashType(HashType ht)
+std::string printHashType(HashType ht)
 {
     switch (ht) {
     case htMD5: return "md5";
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 1b626dd85..56b5938b3 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -20,7 +20,7 @@ const int sha512HashSize = 64;
 
 extern std::set<std::string> hashTypes;
 
-extern const string base32Chars;
+extern const std::string base32Chars;
 
 enum Base : int { Base64, Base32, Base16, SRI };
 
@@ -107,10 +107,10 @@ public:
 };
 
 /* Helper that defaults empty hashes to the 0 hash. */
-Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht);
+Hash newHashAllowEmpty(std::string_view hashStr, std::optional<HashType> ht);
 
 /* Print a hash in base-16 if it's MD5, or base-32 otherwise. */
-string printHash16or32(const Hash & hash);
+std::string printHash16or32(const Hash & hash);
 
 /* Compute the hash of the given string. */
 Hash hashString(HashType ht, std::string_view s);
@@ -135,7 +135,7 @@ HashType parseHashType(std::string_view s);
 std::optional<HashType> parseHashTypeOpt(std::string_view s);
 
 /* And the reverse. */
-string printHashType(HashType ht);
+std::string printHashType(HashType ht);
 
 
 union Ctx;
diff --git a/src/libutil/json.cc b/src/libutil/json.cc
index 01331947e..3a981376f 100644
--- a/src/libutil/json.cc
+++ b/src/libutil/json.cc
@@ -7,16 +7,38 @@ namespace nix {
 
 void toJSON(std::ostream & str, const char * start, const char * end)
 {
-    str << '"';
-    for (auto i = start; i != end; i++)
-        if (*i == '\"' || *i == '\\') str << '\\' << *i;
-        else if (*i == '\n') str << "\\n";
-        else if (*i == '\r') str << "\\r";
-        else if (*i == '\t') str << "\\t";
-        else if (*i >= 0 && *i < 32)
-            str << "\\u" << std::setfill('0') << std::setw(4) << std::hex << (uint16_t) *i << std::dec;
-        else str << *i;
-    str << '"';
+    constexpr size_t BUF_SIZE = 4096;
+    char buf[BUF_SIZE + 7]; // BUF_SIZE + largest single sequence of puts
+    size_t bufPos = 0;
+
+    const auto flush = [&] {
+        str.write(buf, bufPos);
+        bufPos = 0;
+    };
+    const auto put = [&] (char c) {
+        buf[bufPos++] = c;
+    };
+
+    put('"');
+    for (auto i = start; i != end; i++) {
+        if (bufPos >= BUF_SIZE) flush();
+        if (*i == '\"' || *i == '\\') { put('\\'); put(*i); }
+        else if (*i == '\n') { put('\\'); put('n'); }
+        else if (*i == '\r') { put('\\'); put('r'); }
+        else if (*i == '\t') { put('\\'); put('t'); }
+        else if (*i >= 0 && *i < 32) {
+            const char hex[17] = "0123456789abcdef";
+            put('\\');
+            put('u');
+            put(hex[(uint16_t(*i) >> 12) & 0xf]);
+            put(hex[(uint16_t(*i) >>  8) & 0xf]);
+            put(hex[(uint16_t(*i) >>  4) & 0xf]);
+            put(hex[(uint16_t(*i) >>  0) & 0xf]);
+        }
+        else put(*i);
+    }
+    put('"');
+    flush();
 }
 
 void toJSON(std::ostream & str, const char * s)
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index f8a121ed1..74ee2f063 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -113,7 +113,7 @@ void warnOnce(bool & haveWarned, const FormatOrString & fs)
     }
 }
 
-void writeToStderr(const string & s)
+void writeToStderr(std::string_view s)
 {
     try {
         writeFull(STDERR_FILENO, s, false);
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 96ad69790..bd28036ae 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -40,7 +40,7 @@ struct LoggerSettings : Config
     Setting<bool> showTrace{
         this, false, "show-trace",
         R"(
-          Where Nix should print out a stack trace in case of Nix
+          Whether Nix should print out a stack trace in case of Nix
           expression evaluation errors.
         )"};
 };
@@ -189,13 +189,14 @@ extern Verbosity verbosity; /* suppress msgs > this */
 /* Print a string message if the current log level is at least the specified
    level. Note that this has to be implemented as a macro to ensure that the
    arguments are evaluated lazily. */
-#define printMsg(level, args...) \
+#define printMsgUsing(loggerParam, level, args...) \
     do { \
         auto __lvl = level; \
         if (__lvl <= nix::verbosity) { \
-            logger->log(__lvl, fmt(args)); \
+            loggerParam->log(__lvl, fmt(args)); \
         } \
     } while (0)
+#define printMsg(level, args...) printMsgUsing(logger, level, args)
 
 #define printError(args...) printMsg(lvlError, args)
 #define notice(args...) printMsg(lvlNotice, args)
@@ -215,6 +216,6 @@ inline void warn(const std::string & fs, const Args & ... args)
 
 void warnOnce(bool & haveWarned, const FormatOrString & fs);
 
-void writeToStderr(const string & s);
+void writeToStderr(std::string_view s);
 
 }
diff --git a/src/libutil/ref.hh b/src/libutil/ref.hh
index d6bf53bb8..347b81f73 100644
--- a/src/libutil/ref.hh
+++ b/src/libutil/ref.hh
@@ -17,7 +17,7 @@ private:
 
 public:
 
-    ref<T>(const ref<T> & r)
+    ref(const ref<T> & r)
         : p(r.p)
     { }
 
diff --git a/src/libutil/rust-ffi.cc b/src/libutil/rust-ffi.cc
deleted file mode 100644
index 67924568f..000000000
--- a/src/libutil/rust-ffi.cc
+++ /dev/null
@@ -1,24 +0,0 @@
-#if 0
-#include "logging.hh"
-#include "rust-ffi.hh"
-
-extern "C" std::exception_ptr * make_error(rust::StringSlice s)
-{
-    return new std::exception_ptr(std::make_exception_ptr(nix::Error(std::string(s.ptr, s.size))));
-}
-
-extern "C" void destroy_error(std::exception_ptr * ex)
-{
-    free(ex);
-}
-
-namespace rust {
-
-std::ostream & operator << (std::ostream & str, const String & s)
-{
-    str << (std::string_view) s;
-    return str;
-}
-
-}
-#endif
diff --git a/src/libutil/rust-ffi.hh b/src/libutil/rust-ffi.hh
deleted file mode 100644
index cfbaf9dec..000000000
--- a/src/libutil/rust-ffi.hh
+++ /dev/null
@@ -1,189 +0,0 @@
-#pragma once
-#if 0
-
-#include "serialise.hh"
-
-#include <string_view>
-#include <cstring>
-#include <array>
-
-namespace rust {
-
-typedef void (*DropFun)(void *);
-
-/* A Rust value of N bytes. It can be moved but not copied. When it
-   goes out of scope, the C++ destructor will run the drop
-   function. */
-template<std::size_t N, DropFun drop>
-struct Value
-{
-protected:
-
-    std::array<char, N> raw;
-
-    ~Value()
-    {
-        if (!isEvacuated()) {
-            drop(this);
-            evacuate();
-        }
-    }
-
-    // Must not be called directly.
-    Value()
-    { }
-
-    Value(Value && other)
-        : raw(other.raw)
-    {
-        other.evacuate();
-    }
-
-    void operator =(Value && other)
-    {
-        if (!isEvacuated())
-            drop(this);
-        raw = other.raw;
-        other.evacuate();
-    }
-
-private:
-
-    /* FIXME: optimize these (ideally in such a way that the compiler
-       can elide most calls to evacuate() / isEvacuated(). */
-    inline void evacuate()
-    {
-        for (auto & i : raw) i = 0;
-    }
-
-    inline bool isEvacuated()
-    {
-        for (auto & i : raw)
-            if (i != 0) return false;
-        return true;
-    }
-};
-
-/* A Rust vector. */
-template<typename T, DropFun drop>
-struct Vec : Value<3 * sizeof(void *), drop>
-{
-    inline size_t size() const
-    {
-        return ((const size_t *) &this->raw)[2];
-    }
-
-    const T * data() const
-    {
-        return ((const T * *) &this->raw)[0];
-    }
-};
-
-/* A Rust slice. */
-template<typename T>
-struct Slice
-{
-    const T * ptr;
-    size_t size;
-
-    Slice(const T * ptr, size_t size) : ptr(ptr), size(size)
-    {
-        assert(ptr);
-    }
-};
-
-struct StringSlice : Slice<char>
-{
-    StringSlice(const std::string & s): Slice(s.data(), s.size()) {}
-    explicit StringSlice(std::string_view s): Slice(s.data(), s.size()) {}
-    StringSlice(const char * s): Slice(s, strlen(s)) {}
-
-    operator std::string_view() const
-    {
-        return std::string_view(ptr, size);
-    }
-};
-
-/* A Rust string. */
-struct String;
-
-extern "C" {
-    void ffi_String_new(StringSlice s, String * out);
-    void ffi_String_drop(void * s);
-}
-
-struct String : Vec<char, ffi_String_drop>
-{
-    String() = delete;
-
-    String(std::string_view s)
-    {
-        ffi_String_new(StringSlice(s), this);
-    }
-
-    String(const char * s)
-        : String({s, std::strlen(s)})
-    {
-    }
-
-    operator std::string_view() const
-    {
-        return std::string_view(data(), size());
-    }
-};
-
-std::ostream & operator << (std::ostream & str, const String & s);
-
-/* C++ representation of Rust's Result<T, CppException>. */
-template<typename T>
-struct Result
-{
-    enum { Ok = 0, Err = 1, Uninit = 2 } tag;
-
-    union {
-        T data;
-        std::exception_ptr * exc;
-    };
-
-    Result() : tag(Uninit) { }; // FIXME: remove
-
-    Result(const Result &) = delete;
-
-    Result(Result && other)
-        : tag(other.tag)
-    {
-        other.tag = Uninit;
-        if (tag == Ok)
-            data = std::move(other.data);
-        else if (tag == Err)
-            exc = other.exc;
-    }
-
-    ~Result()
-    {
-        if (tag == Ok)
-            data.~T();
-        else if (tag == Err)
-            free(exc);
-        else if (tag == Uninit)
-            ;
-        else
-            abort();
-    }
-
-    /* Rethrow the wrapped exception or return the wrapped value. */
-    T unwrap()
-    {
-        if (tag == Ok) {
-            tag = Uninit;
-            return std::move(data);
-        }
-        else if (tag == Err)
-            std::rethrow_exception(*exc);
-        else
-            abort();
-    }
-};
-
-}
-#endif
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 16f3476c2..6445b3f1b 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -110,7 +110,7 @@ std::string Source::drain()
 {
     StringSink s;
     drainInto(s);
-    return *s.s;
+    return std::move(s.s);
 }
 
 
@@ -325,7 +325,7 @@ void writeString(std::string_view data, Sink & sink)
 }
 
 
-Sink & operator << (Sink & sink, const string & s)
+Sink & operator << (Sink & sink, std::string_view s)
 {
     writeString(s, sink);
     return sink;
@@ -391,7 +391,7 @@ size_t readString(char * buf, size_t max, Source & source)
 }
 
 
-string readString(Source & source, size_t max)
+std::string readString(Source & source, size_t max)
 {
     auto len = readNum<size_t>(source);
     if (len > max) throw SerialisationError("string is too long");
@@ -401,7 +401,7 @@ string readString(Source & source, size_t max)
     return res;
 }
 
-Source & operator >> (Source & in, string & s)
+Source & operator >> (Source & in, std::string & s)
 {
     s = readString(in);
     return in;
@@ -450,11 +450,11 @@ Error readError(Source & source)
 void StringSink::operator () (std::string_view data)
 {
     static bool warned = false;
-    if (!warned && s->size() > threshold) {
+    if (!warned && s.size() > threshold) {
         warnLargeDump();
         warned = true;
     }
-    s->append(data);
+    s.append(data);
 }
 
 size_t ChainSource::read(char * data, size_t len)
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 0fe6e8332..13da26c6a 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -154,12 +154,13 @@ private:
 /* A sink that writes data to a string. */
 struct StringSink : Sink
 {
-    ref<std::string> s;
-    StringSink() : s(make_ref<std::string>()) { };
-    explicit StringSink(const size_t reservedSize) : s(make_ref<std::string>()) {
-      s->reserve(reservedSize);
+    std::string s;
+    StringSink() { }
+    explicit StringSink(const size_t reservedSize)
+    {
+      s.reserve(reservedSize);
     };
-    StringSink(ref<std::string> s) : s(s) { };
+    StringSink(std::string && s) : s(std::move(s)) { };
     void operator () (std::string_view data) override;
 };
 
@@ -167,9 +168,9 @@ struct StringSink : Sink
 /* A source that reads data from a string. */
 struct StringSource : Source
 {
-    const string & s;
+    std::string_view s;
     size_t pos;
-    StringSource(const string & _s) : s(_s), pos(0) { }
+    StringSource(std::string_view s) : s(s), pos(0) { }
     size_t read(char * data, size_t len) override;
 };
 
@@ -317,10 +318,10 @@ inline Sink & operator << (Sink & sink, uint64_t n)
     return sink;
 }
 
-Sink & operator << (Sink & sink, const string & s);
+Sink & operator << (Sink & in, const Error & ex);
+Sink & operator << (Sink & sink, std::string_view s);
 Sink & operator << (Sink & sink, const Strings & s);
 Sink & operator << (Sink & sink, const StringSet & s);
-Sink & operator << (Sink & in, const Error & ex);
 
 
 MakeError(SerialisationError, Error);
@@ -363,10 +364,10 @@ inline uint64_t readLongLong(Source & source)
 
 void readPadding(size_t len, Source & source);
 size_t readString(char * buf, size_t max, Source & source);
-string readString(Source & source, size_t max = std::numeric_limits<size_t>::max());
+std::string readString(Source & source, size_t max = std::numeric_limits<size_t>::max());
 template<class T> T readStrings(Source & source);
 
-Source & operator >> (Source & in, string & s);
+Source & operator >> (Source & in, std::string & s);
 
 template<typename T>
 Source & operator >> (Source & in, T & n)
diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index 50e691a3d..790bc943a 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -93,9 +93,16 @@ static void extract_archive(TarArchive & archive, const Path & destDir)
         else
             archive.check(r);
 
-        archive_entry_set_pathname(entry,
+        archive_entry_copy_pathname(entry,
             (destDir + "/" + name).c_str());
 
+        // Patch hardlink path
+        const char *original_hardlink = archive_entry_hardlink(entry);
+        if (original_hardlink) {
+            archive_entry_copy_hardlink(entry,
+                (destDir + "/" + original_hardlink).c_str());
+        }
+
         archive.check(archive_read_extract(archive.archive, entry, flags));
     }
 
diff --git a/src/libutil/tests/compression.cc b/src/libutil/tests/compression.cc
index 2efa3266b..bbbf3500f 100644
--- a/src/libutil/tests/compression.cc
+++ b/src/libutil/tests/compression.cc
@@ -12,17 +12,17 @@ namespace nix {
     }
 
     TEST(compress, noneMethodDoesNothingToTheInput) {
-        ref<std::string> o = compress("none", "this-is-a-test");
+        auto o = compress("none", "this-is-a-test");
 
-        ASSERT_EQ(*o, "this-is-a-test");
+        ASSERT_EQ(o, "this-is-a-test");
     }
 
     TEST(decompress, decompressNoneCompressed) {
         auto method = "none";
         auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
-        ref<std::string> o = decompress(method, str);
+        auto o = decompress(method, str);
 
-        ASSERT_EQ(*o, str);
+        ASSERT_EQ(o, str);
     }
 
     TEST(decompress, decompressEmptyCompressed) {
@@ -30,33 +30,33 @@ namespace nix {
         // (Content-Encoding == "").
         auto method = "";
         auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
-        ref<std::string> o = decompress(method, str);
+        auto o = decompress(method, str);
 
-        ASSERT_EQ(*o, str);
+        ASSERT_EQ(o, str);
     }
 
     TEST(decompress, decompressXzCompressed) {
         auto method = "xz";
         auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
-        ref<std::string> o = decompress(method, *compress(method, str));
+        auto o = decompress(method, compress(method, str));
 
-        ASSERT_EQ(*o, str);
+        ASSERT_EQ(o, str);
     }
 
     TEST(decompress, decompressBzip2Compressed) {
         auto method = "bzip2";
         auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
-        ref<std::string> o = decompress(method, *compress(method, str));
+        auto o = decompress(method, compress(method, str));
 
-        ASSERT_EQ(*o, str);
+        ASSERT_EQ(o, str);
     }
 
     TEST(decompress, decompressBrCompressed) {
         auto method = "br";
         auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
-        ref<std::string> o = decompress(method, *compress(method, str));
+        auto o = decompress(method, compress(method, str));
 
-        ASSERT_EQ(*o, str);
+        ASSERT_EQ(o, str);
     }
 
     TEST(decompress, decompressInvalidInputThrowsCompressionError) {
@@ -77,7 +77,7 @@ namespace nix {
         (*sink)(inputString);
         sink->finish();
 
-        ASSERT_STREQ((*strSink.s).c_str(), inputString);
+        ASSERT_STREQ(strSink.s.c_str(), inputString);
     }
 
     TEST(makeCompressionSink, compressAndDecompress) {
@@ -90,7 +90,7 @@ namespace nix {
         sink->finish();
         decompressionSink->finish();
 
-        ASSERT_STREQ((*strSink.s).c_str(), inputString);
+        ASSERT_STREQ(strSink.s.c_str(), inputString);
     }
 
 }
diff --git a/src/libutil/tests/config.cc b/src/libutil/tests/config.cc
index 0ebdaf3db..8be6730dd 100644
--- a/src/libutil/tests/config.cc
+++ b/src/libutil/tests/config.cc
@@ -161,7 +161,7 @@ namespace nix {
         Setting<std::string> setting{&config, "", "name-of-the-setting", "description"};
         setting.assign("value");
 
-        ASSERT_EQ(config.toJSON().dump(), R"#({"name-of-the-setting":{"aliases":[],"defaultValue":"","description":"description\n","value":"value"}})#");
+        ASSERT_EQ(config.toJSON().dump(), R"#({"name-of-the-setting":{"aliases":[],"defaultValue":"","description":"description\n","documentDefault":true,"value":"value"}})#");
     }
 
     TEST(Config, setSettingAlias) {
diff --git a/src/libutil/tests/fmt.cc b/src/libutil/tests/fmt.cc
new file mode 100644
index 000000000..33772162c
--- /dev/null
+++ b/src/libutil/tests/fmt.cc
@@ -0,0 +1,68 @@
+#include "fmt.hh"
+
+#include <gtest/gtest.h>
+
+#include <regex>
+
+namespace nix {
+/* ----------- tests for fmt.hh -------------------------------------------------*/
+
+    TEST(hiliteMatches, noHighlight) {
+        ASSERT_STREQ(hiliteMatches("Hello, world!", std::vector<std::smatch>(), "(", ")").c_str(), "Hello, world!");
+    }
+
+    TEST(hiliteMatches, simpleHighlight) {
+        std::string str = "Hello, world!";
+        std::regex re = std::regex("world");
+        auto matches = std::vector(std::sregex_iterator(str.begin(), str.end(), re), std::sregex_iterator());
+        ASSERT_STREQ(
+                    hiliteMatches(str, matches, "(", ")").c_str(),
+                    "Hello, (world)!"
+        );
+    }
+
+    TEST(hiliteMatches, multipleMatches) {
+        std::string str = "Hello, world, world, world, world, world, world, Hello!";
+        std::regex re = std::regex("world");
+        auto matches = std::vector(std::sregex_iterator(str.begin(), str.end(), re), std::sregex_iterator());
+        ASSERT_STREQ(
+                    hiliteMatches(str, matches, "(", ")").c_str(),
+                    "Hello, (world), (world), (world), (world), (world), (world), Hello!"
+        );
+    }
+
+    TEST(hiliteMatches, overlappingMatches) {
+        std::string str = "world, Hello, world, Hello, world, Hello, world, Hello, world!";
+        std::regex re = std::regex("Hello, world");
+        std::regex re2 = std::regex("world, Hello");
+        auto v = std::vector(std::sregex_iterator(str.begin(), str.end(), re), std::sregex_iterator());
+        for(auto it = std::sregex_iterator(str.begin(), str.end(), re2); it != std::sregex_iterator(); ++it) {
+            v.push_back(*it);
+        }
+        ASSERT_STREQ(
+                    hiliteMatches(str, v, "(", ")").c_str(),
+                    "(world, Hello, world, Hello, world, Hello, world, Hello, world)!"
+        );
+    }
+
+    TEST(hiliteMatches, complexOverlappingMatches) {
+        std::string str = "legacyPackages.x86_64-linux.git-crypt";
+        std::vector regexes = {
+            std::regex("t-cry"),
+            std::regex("ux\\.git-cry"),
+            std::regex("git-c"),
+            std::regex("pt"),
+        };
+        std::vector<std::smatch> matches;
+        for(auto regex : regexes)
+        {
+            for(auto it = std::sregex_iterator(str.begin(), str.end(), regex); it != std::sregex_iterator(); ++it) {
+                matches.push_back(*it);
+            }
+        }
+        ASSERT_STREQ(
+                    hiliteMatches(str, matches, "(", ")").c_str(),
+                    "legacyPackages.x86_64-lin(ux.git-crypt)"
+        );
+    }
+}
diff --git a/src/libutil/tests/tests.cc b/src/libutil/tests/tests.cc
index 58df9c5ac..92972ed14 100644
--- a/src/libutil/tests/tests.cc
+++ b/src/libutil/tests/tests.cc
@@ -4,6 +4,8 @@
 #include <limits.h>
 #include <gtest/gtest.h>
 
+#include <numeric>
+
 namespace nix {
 
 /* ----------- tests for util.hh ------------------------------------------------*/
@@ -282,6 +284,17 @@ namespace nix {
         ASSERT_EQ(decoded, s);
     }
 
+    TEST(base64Encode, encodeAndDecodeNonPrintable) {
+        char s[256];
+        std::iota(std::rbegin(s), std::rend(s), 0);
+
+        auto encoded = base64Encode(s);
+        auto decoded = base64Decode(encoded);
+
+        EXPECT_EQ(decoded.length(), 255);
+        ASSERT_EQ(decoded, s);
+    }
+
     /* ----------------------------------------------------------------------------
      * base64Decode
      * --------------------------------------------------------------------------*/
@@ -294,6 +307,10 @@ namespace nix {
         ASSERT_EQ(base64Decode("cXVvZCBlcmF0IGRlbW9uc3RyYW5kdW0="), "quod erat demonstrandum");
     }
 
+    TEST(base64Decode, decodeThrowsOnInvalidChar) {
+        ASSERT_THROW(base64Decode("cXVvZCBlcm_0IGRlbW9uc3RyYW5kdW0="), Error);
+    }
+
     /* ----------------------------------------------------------------------------
      * toLower
      * --------------------------------------------------------------------------*/
diff --git a/src/libutil/tests/url.cc b/src/libutil/tests/url.cc
index aff58e9ee..f20e2dc41 100644
--- a/src/libutil/tests/url.cc
+++ b/src/libutil/tests/url.cc
@@ -5,9 +5,9 @@ namespace nix {
 
 /* ----------- tests for url.hh --------------------------------------------------*/
 
-    string print_map(std::map<string, string> m) {
-        std::map<string, string>::iterator it;
-        string s = "{ ";
+    std::string print_map(std::map<std::string, std::string> m) {
+        std::map<std::string, std::string>::iterator it;
+        std::string s = "{ ";
         for (it = m.begin(); it != m.end(); ++it) {
             s += "{ ";
             s += it->first;
@@ -262,21 +262,21 @@ namespace nix {
      * --------------------------------------------------------------------------*/
 
     TEST(percentDecode, decodesUrlEncodedString) {
-        string s = "==@==";
-        string d = percentDecode("%3D%3D%40%3D%3D");
+        std::string s = "==@==";
+        std::string d = percentDecode("%3D%3D%40%3D%3D");
         ASSERT_EQ(d, s);
     }
 
     TEST(percentDecode, multipleDecodesAreIdempotent) {
-        string once = percentDecode("%3D%3D%40%3D%3D");
-        string twice = percentDecode(once);
+        std::string once = percentDecode("%3D%3D%40%3D%3D");
+        std::string twice = percentDecode(once);
 
         ASSERT_EQ(once, twice);
     }
 
     TEST(percentDecode, trailingPercent) {
-        string s = "==@==%";
-        string d = percentDecode("%3D%3D%40%3D%3D%25");
+        std::string s = "==@==%";
+        std::string d = percentDecode("%3D%3D%40%3D%3D%25");
 
         ASSERT_EQ(d, s);
     }
diff --git a/src/libutil/thread-pool.cc b/src/libutil/thread-pool.cc
index 857ee91f8..dc4067f1b 100644
--- a/src/libutil/thread-pool.cc
+++ b/src/libutil/thread-pool.cc
@@ -1,13 +1,10 @@
 #include "thread-pool.hh"
-#include "affinity.hh"
 
 namespace nix {
 
 ThreadPool::ThreadPool(size_t _maxThreads)
     : maxThreads(_maxThreads)
 {
-    restoreAffinity(); // FIXME
-
     if (!maxThreads) {
         maxThreads = std::thread::hardware_concurrency();
         if (!maxThreads) maxThreads = 1;
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 9c85fef62..00ba567c6 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -6,26 +6,23 @@
 #include <set>
 #include <string>
 #include <map>
+#include <variant>
 #include <vector>
 
 namespace nix {
 
-using std::list;
-using std::set;
-using std::vector;
-using std::string;
-
-typedef list<string> Strings;
-typedef set<string> StringSet;
-typedef std::map<string, string> StringMap;
+typedef std::list<std::string> Strings;
+typedef std::set<std::string> StringSet;
+typedef std::map<std::string, std::string> StringMap;
+typedef std::map<std::string, std::string> StringPairs;
 
 /* Paths are just strings. */
+typedef std::string Path;
+typedef std::string_view PathView;
+typedef std::list<Path> Paths;
+typedef std::set<Path> PathSet;
 
-typedef string Path;
-typedef list<Path> Paths;
-typedef set<Path> PathSet;
-
-typedef vector<std::pair<string, string>> Headers;
+typedef std::vector<std::pair<std::string, std::string>> Headers;
 
 /* Helper class to run code at startup. */
 template<typename T>
@@ -46,4 +43,63 @@ struct Explicit {
     }
 };
 
+
+/* This wants to be a little bit like rust's Cow type.
+   Some parts of the evaluator benefit greatly from being able to reuse
+   existing allocations for strings, but have to be able to also use
+   newly allocated storage for values.
+
+   We do not define implicit conversions, even with ref qualifiers,
+   since those can easily become ambiguous to the reader and can degrade
+   into copying behaviour we want to avoid. */
+class BackedStringView {
+private:
+    std::variant<std::string, std::string_view> data;
+
+    /* Needed to introduce a temporary since operator-> must return
+       a pointer. Without this we'd need to store the view object
+       even when we already own a string. */
+    class Ptr {
+    private:
+        std::string_view view;
+    public:
+        Ptr(std::string_view view): view(view) {}
+        const std::string_view * operator->() const { return &view; }
+    };
+
+public:
+    BackedStringView(std::string && s): data(std::move(s)) {}
+    BackedStringView(std::string_view sv): data(sv) {}
+    template<size_t N>
+    BackedStringView(const char (& lit)[N]): data(std::string_view(lit)) {}
+
+    BackedStringView(const BackedStringView &) = delete;
+    BackedStringView & operator=(const BackedStringView &) = delete;
+
+    /* We only want move operations defined since the sole purpose of
+       this type is to avoid copies. */
+    BackedStringView(BackedStringView && other) = default;
+    BackedStringView & operator=(BackedStringView && other) = default;
+
+    bool isOwned() const
+    {
+        return std::holds_alternative<std::string>(data);
+    }
+
+    std::string toOwned() &&
+    {
+        return isOwned()
+            ? std::move(std::get<std::string>(data))
+            : std::string(std::get<std::string_view>(data));
+    }
+
+    std::string_view operator*() const
+    {
+        return isOwned()
+            ? std::get<std::string>(data)
+            : std::get<std::string_view>(data);
+    }
+    Ptr operator->() const { return Ptr(**this); }
+};
+
 }
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 563a72c12..88c5ae806 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -1,5 +1,4 @@
 #include "util.hh"
-#include "affinity.hh"
 #include "sync.hh"
 #include "finally.hh"
 #include "serialise.hh"
@@ -82,7 +81,7 @@ void replaceEnv(std::map<std::string, std::string> newEnv)
 }
 
 
-Path absPath(Path path, std::optional<Path> dir, bool resolveSymlinks)
+Path absPath(Path path, std::optional<PathView> dir, bool resolveSymlinks)
 {
     if (path[0] != '/') {
         if (!dir) {
@@ -96,28 +95,28 @@ Path absPath(Path path, std::optional<Path> dir, bool resolveSymlinks)
             if (!getcwd(buf, sizeof(buf)))
 #endif
                 throw SysError("cannot get cwd");
-            dir = buf;
+            path = concatStrings(buf, "/", path);
 #ifdef __GNU__
             free(buf);
 #endif
-        }
-        path = *dir + "/" + path;
+        } else
+            path = concatStrings(*dir, "/", path);
     }
     return canonPath(path, resolveSymlinks);
 }
 
 
-Path canonPath(const Path & path, bool resolveSymlinks)
+Path canonPath(PathView path, bool resolveSymlinks)
 {
     assert(path != "");
 
-    string s;
+    std::string s;
+    s.reserve(256);
 
     if (path[0] != '/')
         throw Error("not an absolute path: '%1%'", path);
 
-    string::const_iterator i = path.begin(), end = path.end();
-    string temp;
+    std::string temp;
 
     /* Count the number of times we follow a symlink and stop at some
        arbitrary (but high) limit to prevent infinite loops. */
@@ -126,33 +125,37 @@ Path canonPath(const Path & path, bool resolveSymlinks)
     while (1) {
 
         /* Skip slashes. */
-        while (i != end && *i == '/') i++;
-        if (i == end) break;
+        while (!path.empty() && path[0] == '/') path.remove_prefix(1);
+        if (path.empty()) break;
 
         /* Ignore `.'. */
-        if (*i == '.' && (i + 1 == end || i[1] == '/'))
-            i++;
+        if (path == "." || path.substr(0, 2) == "./")
+            path.remove_prefix(1);
 
         /* If `..', delete the last component. */
-        else if (*i == '.' && i + 1 < end && i[1] == '.' &&
-            (i + 2 == end || i[2] == '/'))
+        else if (path == ".." || path.substr(0, 3) == "../")
         {
             if (!s.empty()) s.erase(s.rfind('/'));
-            i += 2;
+            path.remove_prefix(2);
         }
 
         /* Normal component; copy it. */
         else {
             s += '/';
-            while (i != end && *i != '/') s += *i++;
+            if (const auto slash = path.find('/'); slash == std::string::npos) {
+                s += path;
+                path = {};
+            } else {
+                s += path.substr(0, slash);
+                path = path.substr(slash);
+            }
 
             /* If s points to a symlink, resolve it and continue from there */
             if (resolveSymlinks && isLink(s)) {
                 if (++followCount >= maxFollow)
                     throw Error("infinite symlink recursion in path '%1%'", path);
-                temp = readLink(s) + string(i, end);
-                i = temp.begin();
-                end = temp.end();
+                temp = concatStrings(readLink(s), path);
+                path = temp;
                 if (!temp.empty() && temp[0] == '/') {
                     s.clear();  /* restart for symlinks pointing to absolute path */
                 } else {
@@ -165,14 +168,14 @@ Path canonPath(const Path & path, bool resolveSymlinks)
         }
     }
 
-    return s.empty() ? "/" : s;
+    return s.empty() ? "/" : std::move(s);
 }
 
 
-Path dirOf(const Path & path)
+Path dirOf(const PathView path)
 {
     Path::size_type pos = path.rfind('/');
-    if (pos == string::npos)
+    if (pos == std::string::npos)
         return ".";
     return pos == 0 ? "/" : Path(path, 0, pos);
 }
@@ -188,7 +191,7 @@ std::string_view baseNameOf(std::string_view path)
         last -= 1;
 
     auto pos = path.rfind('/', last);
-    if (pos == string::npos)
+    if (pos == std::string::npos)
         pos = 0;
     else
         pos += 1;
@@ -197,16 +200,16 @@ std::string_view baseNameOf(std::string_view path)
 }
 
 
-bool isInDir(const Path & path, const Path & dir)
+bool isInDir(std::string_view path, std::string_view dir)
 {
-    return path[0] == '/'
-        && string(path, 0, dir.size()) == dir
+    return path.substr(0, 1) == "/"
+        && path.substr(0, dir.size()) == dir
         && path.size() >= dir.size() + 2
         && path[dir.size()] == '/';
 }
 
 
-bool isDirOrInDir(const Path & path, const Path & dir)
+bool isDirOrInDir(std::string_view path, std::string_view dir)
 {
     return path == dir || isInDir(path, dir);
 }
@@ -246,7 +249,7 @@ Path readLink(const Path & path)
             else
                 throw SysError("reading symbolic link '%1%'", path);
         else if (rlSize < bufSize)
-            return string(buf.data(), rlSize);
+            return std::string(buf.data(), rlSize);
     }
 }
 
@@ -266,7 +269,7 @@ DirEntries readDirectory(DIR *dir, const Path & path)
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir)) { /* sic */
         checkInterrupt();
-        string name = dirent->d_name;
+        std::string name = dirent->d_name;
         if (name == "." || name == "..") continue;
         entries.emplace_back(name, dirent->d_ino,
 #ifdef HAVE_STRUCT_DIRENT_D_TYPE
@@ -300,7 +303,7 @@ unsigned char getFileType(const Path & path)
 }
 
 
-string readFile(int fd)
+std::string readFile(int fd)
 {
     struct stat st;
     if (fstat(fd, &st) == -1)
@@ -310,7 +313,7 @@ string readFile(int fd)
 }
 
 
-string readFile(const Path & path)
+std::string readFile(const Path & path)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
     if (!fd)
@@ -363,9 +366,9 @@ void writeFile(const Path & path, Source & source, mode_t mode)
     }
 }
 
-string readLine(int fd)
+std::string readLine(int fd)
 {
-    string s;
+    std::string s;
     while (1) {
         checkInterrupt();
         char ch;
@@ -384,7 +387,7 @@ string readLine(int fd)
 }
 
 
-void writeLine(int fd, string s)
+void writeLine(int fd, std::string s)
 {
     s += '\n';
     writeFull(fd, s);
@@ -395,7 +398,7 @@ static void _deletePath(int parentfd, const Path & path, uint64_t & bytesFreed)
 {
     checkInterrupt();
 
-    string name(baseNameOf(path));
+    std::string name(baseNameOf(path));
 
     struct stat st;
     if (fstatat(parentfd, name.c_str(), &st, AT_SYMLINK_NOFOLLOW) == -1) {
@@ -512,6 +515,7 @@ std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
     AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));
     if (!fd)
         throw SysError("creating temporary file '%s'", tmpl);
+    closeOnExec(fd.get());
     return {std::move(fd), tmpl};
 }
 
@@ -562,8 +566,8 @@ Path getConfigDir()
 std::vector<Path> getConfigDirs()
 {
     Path configHome = getConfigDir();
-    string configDirs = getEnv("XDG_CONFIG_DIRS").value_or("");
-    std::vector<Path> result = tokenizeString<std::vector<string>>(configDirs, ":");
+    auto configDirs = getEnv("XDG_CONFIG_DIRS").value_or("/etc/xdg");
+    std::vector<Path> result = tokenizeString<std::vector<std::string>>(configDirs, ":");
     result.insert(result.begin(), configHome);
     return result;
 }
@@ -666,11 +670,13 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts)
 }
 
 
-string drainFD(int fd, bool block, const size_t reserveSize)
+std::string drainFD(int fd, bool block, const size_t reserveSize)
 {
-    StringSink sink(reserveSize);
+    // the parser needs two extra bytes to append terminating characters, other users will
+    // not care very much about the extra memory.
+    StringSink sink(reserveSize + 2);
     drainFD(fd, sink, block);
-    return std::move(*sink.s);
+    return std::move(sink.s);
 }
 
 
@@ -713,7 +719,7 @@ void drainFD(int fd, Sink & sink, bool block)
 
 AutoDelete::AutoDelete() : del{false} {}
 
-AutoDelete::AutoDelete(const string & p, bool recursive) : path(p)
+AutoDelete::AutoDelete(const std::string & p, bool recursive) : path(p)
 {
     del = true;
     this->recursive = recursive;
@@ -1003,7 +1009,6 @@ pid_t startProcess(std::function<void()> fun, const ProcessOptions & options)
             if (options.dieWithParent && prctl(PR_SET_PDEATHSIG, SIGKILL) == -1)
                 throw SysError("setting death signal");
 #endif
-            restoreAffinity();
             fun();
         } catch (std::exception & e) {
             try {
@@ -1031,7 +1036,7 @@ std::vector<char *> stringsToCharPtrs(const Strings & ss)
     return res;
 }
 
-string runProgram(Path program, bool searchPath, const Strings & args,
+std::string runProgram(Path program, bool searchPath, const Strings & args,
     const std::optional<std::string> & input)
 {
     auto res = runProgram(RunOptions {.program = program, .searchPath = searchPath, .args = args, .input = input});
@@ -1056,7 +1061,7 @@ std::pair<int, std::string> runProgram(RunOptions && options)
         status = e.status;
     }
 
-    return {status, std::move(*sink.s)};
+    return {status, std::move(sink.s)};
 }
 
 void runProgram2(const RunOptions & options)
@@ -1169,7 +1174,7 @@ void runProgram2(const RunOptions & options)
 }
 
 
-void closeMostFDs(const set<int> & exceptions)
+void closeMostFDs(const std::set<int> & exceptions)
 {
 #if __linux__
     try {
@@ -1205,7 +1210,7 @@ void closeOnExec(int fd)
 //////////////////////////////////////////////////////////////////////
 
 
-bool _isInterrupted = false;
+std::atomic<bool> _isInterrupted = false;
 
 static thread_local bool interruptThrown = false;
 thread_local std::function<bool()> interruptCheck;
@@ -1230,45 +1235,45 @@ void _interrupted()
 //////////////////////////////////////////////////////////////////////
 
 
-template<class C> C tokenizeString(std::string_view s, const string & separators)
+template<class C> C tokenizeString(std::string_view s, std::string_view separators)
 {
     C result;
-    string::size_type pos = s.find_first_not_of(separators, 0);
-    while (pos != string::npos) {
-        string::size_type end = s.find_first_of(separators, pos + 1);
-        if (end == string::npos) end = s.size();
-        string token(s, pos, end - pos);
-        result.insert(result.end(), token);
+    auto pos = s.find_first_not_of(separators, 0);
+    while (pos != std::string::npos) {
+        auto end = s.find_first_of(separators, pos + 1);
+        if (end == std::string::npos) end = s.size();
+        result.insert(result.end(), std::string(s, pos, end - pos));
         pos = s.find_first_not_of(separators, end);
     }
     return result;
 }
 
-template Strings tokenizeString(std::string_view s, const string & separators);
-template StringSet tokenizeString(std::string_view s, const string & separators);
-template vector<string> tokenizeString(std::string_view s, const string & separators);
+template Strings tokenizeString(std::string_view s, std::string_view separators);
+template StringSet tokenizeString(std::string_view s, std::string_view separators);
+template std::vector<std::string> tokenizeString(std::string_view s, std::string_view separators);
 
 
-string chomp(std::string_view s)
+std::string chomp(std::string_view s)
 {
     size_t i = s.find_last_not_of(" \n\r\t");
-    return i == string::npos ? "" : string(s, 0, i + 1);
+    return i == std::string_view::npos ? "" : std::string(s, 0, i + 1);
 }
 
 
-string trim(const string & s, const string & whitespace)
+std::string trim(std::string_view s, std::string_view whitespace)
 {
     auto i = s.find_first_not_of(whitespace);
-    if (i == string::npos) return "";
+    if (i == std::string_view::npos) return "";
     auto j = s.find_last_not_of(whitespace);
-    return string(s, i, j == string::npos ? j : j - i + 1);
+    return std::string(s, i, j == std::string::npos ? j : j - i + 1);
 }
 
 
-string replaceStrings(std::string_view s,
-    const std::string & from, const std::string & to)
+std::string replaceStrings(
+    std::string res,
+    std::string_view from,
+    std::string_view to)
 {
-    string res(s);
     if (from.empty()) return res;
     size_t pos = 0;
     while ((pos = res.find(from, pos)) != std::string::npos) {
@@ -1279,20 +1284,19 @@ string replaceStrings(std::string_view s,
 }
 
 
-std::string rewriteStrings(const std::string & _s, const StringMap & rewrites)
+std::string rewriteStrings(std::string s, const StringMap & rewrites)
 {
-    auto s = _s;
     for (auto & i : rewrites) {
         if (i.first == i.second) continue;
         size_t j = 0;
-        while ((j = s.find(i.first, j)) != string::npos)
+        while ((j = s.find(i.first, j)) != std::string::npos)
             s.replace(j, i.first.size(), i.second);
     }
     return s;
 }
 
 
-string statusToString(int status)
+std::string statusToString(int status)
 {
     if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
         if (WIFEXITED(status))
@@ -1340,9 +1344,11 @@ std::string toLower(const std::string & s)
 }
 
 
-std::string shellEscape(const std::string & s)
+std::string shellEscape(const std::string_view s)
 {
-    std::string r = "'";
+    std::string r;
+    r.reserve(s.size() + 2);
+    r += "'";
     for (auto & i : s)
         if (i == '\'') r += "'\\''"; else r += i;
     r += '\'';
@@ -1352,11 +1358,15 @@ std::string shellEscape(const std::string & s)
 
 void ignoreException()
 {
+    /* Make sure no exceptions leave this function.
+       printError() also throws when remote is closed. */
     try {
-        throw;
-    } catch (std::exception & e) {
-        printError("error (ignored): %1%", e.what());
-    }
+        try {
+            throw;
+        } catch (std::exception & e) {
+            printError("error (ignored): %1%", e.what());
+        }
+    } catch (...) { }
 }
 
 bool shouldANSI()
@@ -1436,12 +1446,11 @@ std::string filterANSIEscapes(const std::string & s, bool filterAll, unsigned in
 }
 
 
-static char base64Chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
-static std::array<char, 256> base64DecodeChars;
+constexpr char base64Chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
-string base64Encode(std::string_view s)
+std::string base64Encode(std::string_view s)
 {
-    string res;
+    std::string res;
     int data = 0, nbits = 0;
 
     for (char c : s) {
@@ -1460,16 +1469,19 @@ string base64Encode(std::string_view s)
 }
 
 
-string base64Decode(std::string_view s)
+std::string base64Decode(std::string_view s)
 {
-    static std::once_flag flag;
-    std::call_once(flag, [](){
-        base64DecodeChars = { (char)-1 };
+    constexpr char npos = -1;
+    constexpr std::array<char, 256> base64DecodeChars = [&]() {
+        std::array<char, 256>  result{};
+        for (auto& c : result)
+            c = npos;
         for (int i = 0; i < 64; i++)
-            base64DecodeChars[(int) base64Chars[i]] = i;
-    });
+            result[base64Chars[i]] = i;
+        return result;
+    }();
 
-    string res;
+    std::string res;
     unsigned int d = 0, bits = 0;
 
     for (char c : s) {
@@ -1477,7 +1489,7 @@ string base64Decode(std::string_view s)
         if (c == '\n') continue;
 
         char digit = base64DecodeChars[(unsigned char) c];
-        if (digit == -1)
+        if (digit == npos)
             throw Error("invalid character in Base64 string: '%c'", c);
 
         bits += 6;
@@ -1553,7 +1565,22 @@ std::pair<unsigned short, unsigned short> getWindowSize()
 }
 
 
-static Sync<std::list<std::function<void()>>> _interruptCallbacks;
+/* We keep track of interrupt callbacks using integer tokens, so we can iterate
+   safely without having to lock the data structure while executing arbitrary
+   functions.
+ */
+struct InterruptCallbacks {
+    typedef int64_t Token;
+
+    /* We use unique tokens so that we can't accidentally delete the wrong
+       handler because of an erroneous double delete. */
+    Token nextToken = 0;
+
+    /* Used as a list, see InterruptCallbacks comment. */
+    std::map<Token, std::function<void()>> callbacks;
+};
+
+static Sync<InterruptCallbacks> _interruptCallbacks;
 
 static void signalHandlerThread(sigset_t set)
 {
@@ -1575,8 +1602,19 @@ void triggerInterrupt()
     _isInterrupted = true;
 
     {
-        auto interruptCallbacks(_interruptCallbacks.lock());
-        for (auto & callback : *interruptCallbacks) {
+        InterruptCallbacks::Token i = 0;
+        while (true) {
+            std::function<void()> callback;
+            {
+                auto interruptCallbacks(_interruptCallbacks.lock());
+                auto lb = interruptCallbacks->callbacks.lower_bound(i);
+                if (lb == interruptCallbacks->callbacks.end())
+                    break;
+
+                callback = lb->second;
+                i = lb->first + 1;
+            }
+
             try {
                 callback();
             } catch (...) {
@@ -1630,11 +1668,47 @@ void setStackSize(size_t stackSize)
     #endif
 }
 
-void restoreProcessContext()
+static AutoCloseFD fdSavedMountNamespace;
+
+void saveMountNamespace()
 {
-    restoreSignals();
+#if __linux__
+    static std::once_flag done;
+    std::call_once(done, []() {
+        AutoCloseFD fd = open("/proc/self/ns/mnt", O_RDONLY);
+        if (!fd)
+            throw SysError("saving parent mount namespace");
+        fdSavedMountNamespace = std::move(fd);
+    });
+#endif
+}
+
+void restoreMountNamespace()
+{
+#if __linux__
+    try {
+        if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
+            throw SysError("restoring parent mount namespace");
+    } catch (Error & e) {
+        debug(e.msg());
+    }
+#endif
+}
+
+void unshareFilesystem()
+{
+#ifdef __linux__
+    if (unshare(CLONE_FS) != 0 && errno != EPERM)
+        throw SysError("unsharing filesystem state in download thread");
+#endif
+}
 
-    restoreAffinity();
+void restoreProcessContext(bool restoreMounts)
+{
+    restoreSignals();
+    if (restoreMounts) {
+        restoreMountNamespace();
+    }
 
     #if __linux__
     if (savedStackSize) {
@@ -1650,27 +1724,28 @@ void restoreProcessContext()
 /* RAII helper to automatically deregister a callback. */
 struct InterruptCallbackImpl : InterruptCallback
 {
-    std::list<std::function<void()>>::iterator it;
+    InterruptCallbacks::Token token;
     ~InterruptCallbackImpl() override
     {
-        _interruptCallbacks.lock()->erase(it);
+        auto interruptCallbacks(_interruptCallbacks.lock());
+        interruptCallbacks->callbacks.erase(token);
     }
 };
 
 std::unique_ptr<InterruptCallback> createInterruptCallback(std::function<void()> callback)
 {
     auto interruptCallbacks(_interruptCallbacks.lock());
-    interruptCallbacks->push_back(callback);
+    auto token = interruptCallbacks->nextToken++;
+    interruptCallbacks->callbacks.emplace(token, callback);
 
     auto res = std::make_unique<InterruptCallbackImpl>();
-    res->it = interruptCallbacks->end();
-    res->it--;
+    res->token = token;
 
     return std::unique_ptr<InterruptCallback>(res.release());
 }
 
 
-AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
+AutoCloseFD createUnixDomainSocket()
 {
     AutoCloseFD fdSocket = socket(PF_UNIX, SOCK_STREAM
         #ifdef SOCK_CLOEXEC
@@ -1679,8 +1754,14 @@ AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
         , 0);
     if (!fdSocket)
         throw SysError("cannot create Unix domain socket");
-
     closeOnExec(fdSocket.get());
+    return fdSocket;
+}
+
+
+AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
+{
+    auto fdSocket = nix::createUnixDomainSocket();
 
     bind(fdSocket.get(), path);
 
@@ -1703,13 +1784,13 @@ void bind(int fd, const std::string & path)
 
     if (path.size() + 1 >= sizeof(addr.sun_path)) {
         Pid pid = startProcess([&]() {
-            auto dir = dirOf(path);
+            Path dir = dirOf(path);
             if (chdir(dir.c_str()) == -1)
                 throw SysError("chdir to '%s' failed", dir);
             std::string base(baseNameOf(path));
             if (base.size() + 1 >= sizeof(addr.sun_path))
                 throw Error("socket path '%s' is too long", base);
-            strcpy(addr.sun_path, base.c_str());
+            memcpy(addr.sun_path, base.c_str(), base.size() + 1);
             if (bind(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
                 throw SysError("cannot bind to socket '%s'", path);
             _exit(0);
@@ -1718,7 +1799,7 @@ void bind(int fd, const std::string & path)
         if (status != 0)
             throw Error("cannot bind to socket '%s'", path);
     } else {
-        strcpy(addr.sun_path, path.c_str());
+        memcpy(addr.sun_path, path.c_str(), path.size() + 1);
         if (bind(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
             throw SysError("cannot bind to socket '%s'", path);
     }
@@ -1732,13 +1813,13 @@ void connect(int fd, const std::string & path)
 
     if (path.size() + 1 >= sizeof(addr.sun_path)) {
         Pid pid = startProcess([&]() {
-            auto dir = dirOf(path);
+            Path dir = dirOf(path);
             if (chdir(dir.c_str()) == -1)
                 throw SysError("chdir to '%s' failed", dir);
             std::string base(baseNameOf(path));
             if (base.size() + 1 >= sizeof(addr.sun_path))
                 throw Error("socket path '%s' is too long", base);
-            strcpy(addr.sun_path, base.c_str());
+            memcpy(addr.sun_path, base.c_str(), base.size() + 1);
             if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
                 throw SysError("cannot connect to socket at '%s'", path);
             _exit(0);
@@ -1747,14 +1828,14 @@ void connect(int fd, const std::string & path)
         if (status != 0)
             throw Error("cannot connect to socket at '%s'", path);
     } else {
-        strcpy(addr.sun_path, path.c_str());
+        memcpy(addr.sun_path, path.c_str(), path.size() + 1);
         if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
             throw SysError("cannot connect to socket at '%s'", path);
     }
 }
 
 
-string showBytes(uint64_t bytes)
+std::string showBytes(uint64_t bytes)
 {
     return fmt("%.2f MiB", bytes / (1024.0 * 1024.0));
 }
@@ -1765,8 +1846,8 @@ void commonChildInit(Pipe & logPipe)
 {
     logger = makeSimpleLogger();
 
-    const static string pathNullDevice = "/dev/null";
-    restoreProcessContext();
+    const static std::string pathNullDevice = "/dev/null";
+    restoreProcessContext(false);
 
     /* Put the child in a separate session (and thus a separate
        process group) so that it has no controlling terminal (meaning
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 29232453f..20591952d 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -11,6 +11,9 @@
 #include <unistd.h>
 #include <signal.h>
 
+#include <boost/lexical_cast.hpp>
+
+#include <atomic>
 #include <functional>
 #include <map>
 #include <sstream>
@@ -46,30 +49,32 @@ void clearEnv();
    specified directory, or the current directory otherwise.  The path
    is also canonicalised. */
 Path absPath(Path path,
-    std::optional<Path> dir = {},
+    std::optional<PathView> dir = {},
     bool resolveSymlinks = false);
 
 /* Canonicalise a path by removing all `.' or `..' components and
    double or trailing slashes.  Optionally resolves all symlink
    components such that each component of the resulting path is *not*
    a symbolic link. */
-Path canonPath(const Path & path, bool resolveSymlinks = false);
+Path canonPath(PathView path, bool resolveSymlinks = false);
 
 /* Return the directory part of the given canonical path, i.e.,
    everything before the final `/'.  If the path is the root or an
    immediate child thereof (e.g., `/foo'), this means `/'
    is returned.*/
-Path dirOf(const Path & path);
+Path dirOf(const PathView path);
 
 /* Return the base name of the given canonical path, i.e., everything
    following the final `/' (trailing slashes are removed). */
 std::string_view baseNameOf(std::string_view path);
 
-/* Check whether 'path' is a descendant of 'dir'. */
-bool isInDir(const Path & path, const Path & dir);
+/* Check whether 'path' is a descendant of 'dir'. Both paths must be
+   canonicalized. */
+bool isInDir(std::string_view path, std::string_view dir);
 
-/* Check whether 'path' is equal to 'dir' or a descendant of 'dir'. */
-bool isDirOrInDir(const Path & path, const Path & dir);
+/* Check whether 'path' is equal to 'dir' or a descendant of
+   'dir'. Both paths must be canonicalized. */
+bool isDirOrInDir(std::string_view path, std::string_view dir);
 
 /* Get status of `path'. */
 struct stat lstat(const Path & path);
@@ -87,22 +92,22 @@ bool isLink(const Path & path);
    removed. */
 struct DirEntry
 {
-    string name;
+    std::string name;
     ino_t ino;
     unsigned char type; // one of DT_*
-    DirEntry(const string & name, ino_t ino, unsigned char type)
-        : name(name), ino(ino), type(type) { }
+    DirEntry(std::string name, ino_t ino, unsigned char type)
+        : name(std::move(name)), ino(ino), type(type) { }
 };
 
-typedef vector<DirEntry> DirEntries;
+typedef std::vector<DirEntry> DirEntries;
 
 DirEntries readDirectory(const Path & path);
 
 unsigned char getFileType(const Path & path);
 
 /* Read the contents of a file into a string. */
-string readFile(int fd);
-string readFile(const Path & path);
+std::string readFile(int fd);
+std::string readFile(const Path & path);
 void readFile(const Path & path, Sink & sink);
 
 /* Write a string to a file. */
@@ -111,10 +116,10 @@ void writeFile(const Path & path, std::string_view s, mode_t mode = 0666);
 void writeFile(const Path & path, Source & source, mode_t mode = 0666);
 
 /* Read a line from a file descriptor. */
-string readLine(int fd);
+std::string readLine(int fd);
 
 /* Write a line to a file descriptor. */
-void writeLine(int fd, string s);
+void writeLine(int fd, std::string s);
 
 /* Delete a path; i.e., in the case of a directory, it is deleted
    recursively. It's not an error if the path does not exist. The
@@ -143,6 +148,9 @@ Path getDataDir();
 /* Create a directory and all its parents, if necessary.  Returns the
    list of created directories, in order of creation. */
 Paths createDirs(const Path & path);
+inline Paths createDirs(PathView path) {
+    return createDirs(Path(path));
+}
 
 /* Create a symlink. */
 void createSymlink(const Path & target, const Path & link,
@@ -162,7 +170,7 @@ MakeError(EndOfFile, Error);
 
 
 /* Read a file descriptor until EOF occurs. */
-string drainFD(int fd, bool block = true, const size_t reserveSize=0);
+std::string drainFD(int fd, bool block = true, const size_t reserveSize=0);
 
 void drainFD(int fd, Sink & sink, bool block = true);
 
@@ -182,6 +190,7 @@ public:
     void cancel();
     void reset(const Path & p, bool recursive = true);
     operator Path() const { return path; }
+    operator PathView() const { return path; }
 };
 
 
@@ -259,7 +268,7 @@ void killUser(uid_t uid);
    pid to the caller. */
 struct ProcessOptions
 {
-    string errorPrefix = "";
+    std::string errorPrefix = "";
     bool dieWithParent = true;
     bool runExitHandlers = false;
     bool allowVfork = false;
@@ -270,7 +279,7 @@ pid_t startProcess(std::function<void()> fun, const ProcessOptions & options = P
 
 /* Run a program and return its stdout in a string (i.e., like the
    shell backtick operator). */
-string runProgram(Path program, bool searchPath = false,
+std::string runProgram(Path program, bool searchPath = false,
     const Strings & args = Strings(),
     const std::optional<std::string> & input = {});
 
@@ -299,8 +308,21 @@ void setStackSize(size_t stackSize);
 
 
 /* Restore the original inherited Unix process context (such as signal
-   masks, stack size, CPU affinity). */
-void restoreProcessContext();
+   masks, stack size). */
+void restoreProcessContext(bool restoreMounts = true);
+
+/* Save the current mount namespace. Ignored if called more than
+   once. */
+void saveMountNamespace();
+
+/* Restore the mount namespace saved by saveMountNamespace(). Ignored
+   if saveMountNamespace() was never called. */
+void restoreMountNamespace();
+
+/* Cause this thread to not share any FS attributes with the main
+   thread, because this causes setns() in restoreMountNamespace() to
+   fail. */
+void unshareFilesystem();
 
 
 class ExecError : public Error
@@ -321,7 +343,7 @@ std::vector<char *> stringsToCharPtrs(const Strings & ss);
 
 /* Close all file descriptors except those listed in the given set.
    Good practice in child processes. */
-void closeMostFDs(const set<int> & exceptions);
+void closeMostFDs(const std::set<int> & exceptions);
 
 /* Set the close-on-exec flag for the given file descriptor. */
 void closeOnExec(int fd);
@@ -329,7 +351,7 @@ void closeOnExec(int fd);
 
 /* User interruption. */
 
-extern bool _isInterrupted;
+extern std::atomic<bool> _isInterrupted;
 
 extern thread_local std::function<bool()> interruptCheck;
 
@@ -350,15 +372,19 @@ MakeError(FormatError, Error);
 
 
 /* String tokenizer. */
-template<class C> C tokenizeString(std::string_view s, const string & separators = " \t\n\r");
+template<class C> C tokenizeString(std::string_view s, std::string_view separators = " \t\n\r");
 
 
 /* Concatenate the given strings with a separator between the
    elements. */
 template<class C>
-string concatStringsSep(const string & sep, const C & ss)
+std::string concatStringsSep(const std::string_view sep, const C & ss)
 {
-    string s;
+    size_t size = 0;
+    // need a cast to string_view since this is also called with Symbols
+    for (const auto & s : ss) size += sep.size() + std::string_view(s).size();
+    std::string s;
+    s.reserve(size);
     for (auto & i : ss) {
         if (s.size() != 0) s += sep;
         s += i;
@@ -366,6 +392,14 @@ string concatStringsSep(const string & sep, const C & ss)
     return s;
 }
 
+template<class ... Parts>
+auto concatStrings(Parts && ... parts)
+    -> std::enable_if_t<(... && std::is_convertible_v<Parts, std::string_view>), std::string>
+{
+    std::string_view views[sizeof...(parts)] = { parts... };
+    return concatStringsSep({}, views);
+}
+
 
 /* Add quotes around a collection of strings. */
 template<class C> Strings quoteStrings(const C & c)
@@ -379,45 +413,47 @@ template<class C> Strings quoteStrings(const C & c)
 
 /* Remove trailing whitespace from a string. FIXME: return
    std::string_view. */
-string chomp(std::string_view s);
+std::string chomp(std::string_view s);
 
 
 /* Remove whitespace from the start and end of a string. */
-string trim(const string & s, const string & whitespace = " \n\r\t");
+std::string trim(std::string_view s, std::string_view whitespace = " \n\r\t");
 
 
 /* Replace all occurrences of a string inside another string. */
-string replaceStrings(std::string_view s,
-    const std::string & from, const std::string & to);
+std::string replaceStrings(
+    std::string s,
+    std::string_view from,
+    std::string_view to);
 
 
-std::string rewriteStrings(const std::string & s, const StringMap & rewrites);
+std::string rewriteStrings(std::string s, const StringMap & rewrites);
 
 
 /* Convert the exit status of a child as returned by wait() into an
    error string. */
-string statusToString(int status);
+std::string statusToString(int status);
 
 bool statusOk(int status);
 
 
 /* Parse a string into an integer. */
 template<class N>
-std::optional<N> string2Int(const std::string & s)
+std::optional<N> string2Int(const std::string_view s)
 {
     if (s.substr(0, 1) == "-" && !std::numeric_limits<N>::is_signed)
         return std::nullopt;
-    std::istringstream str(s);
-    N n;
-    str >> n;
-    if (str && str.get() == EOF) return n;
-    return std::nullopt;
+    try {
+        return boost::lexical_cast<N>(s.data(), s.size());
+    } catch (const boost::bad_lexical_cast &) {
+        return std::nullopt;
+    }
 }
 
 /* Like string2Int(), but support an optional suffix 'K', 'M', 'G' or
    'T' denoting a binary unit prefix. */
 template<class N>
-N string2IntWithUnitPrefix(std::string s)
+N string2IntWithUnitPrefix(std::string_view s)
 {
     N multiplier = 1;
     if (!s.empty()) {
@@ -428,7 +464,7 @@ N string2IntWithUnitPrefix(std::string s)
             else if (u == 'G') multiplier = 1ULL << 30;
             else if (u == 'T') multiplier = 1ULL << 40;
             else throw UsageError("invalid unit specifier '%1%'", u);
-            s.resize(s.size() - 1);
+            s.remove_suffix(1);
         }
     }
     if (auto n = string2Int<N>(s))
@@ -438,13 +474,13 @@ N string2IntWithUnitPrefix(std::string s)
 
 /* Parse a string into a float. */
 template<class N>
-std::optional<N> string2Float(const string & s)
+std::optional<N> string2Float(const std::string_view s)
 {
-    std::istringstream str(s);
-    N n;
-    str >> n;
-    if (str && str.get() == EOF) return n;
-    return std::nullopt;
+    try {
+        return boost::lexical_cast<N>(s.data(), s.size());
+    } catch (const boost::bad_lexical_cast &) {
+        return std::nullopt;
+    }
 }
 
 
@@ -461,7 +497,7 @@ std::string toLower(const std::string & s);
 
 
 /* Escape a string as a shell word. */
-std::string shellEscape(const std::string & s);
+std::string shellEscape(const std::string_view s);
 
 
 /* Exception handling in destructors: print an error message, then
@@ -491,8 +527,8 @@ std::string filterANSIEscapes(const std::string & s,
 
 
 /* Base64 encoding/decoding. */
-string base64Encode(std::string_view s);
-string base64Decode(std::string_view s);
+std::string base64Encode(std::string_view s);
+std::string base64Decode(std::string_view s);
 
 
 /* Remove common leading whitespace from the lines in the string
@@ -511,6 +547,29 @@ std::optional<typename T::mapped_type> get(const T & map, const typename T::key_
 }
 
 
+/* Remove and return the first item from a container. */
+template <class T>
+std::optional<typename T::value_type> remove_begin(T & c)
+{
+    auto i = c.begin();
+    if (i == c.end()) return {};
+    auto v = std::move(*i);
+    c.erase(i);
+    return v;
+}
+
+
+/* Remove and return the first item from a container. */
+template <class T>
+std::optional<typename T::value_type> pop(T & c)
+{
+    if (c.empty()) return {};
+    auto v = std::move(c.front());
+    c.pop();
+    return v;
+}
+
+
 template<typename T>
 class Callback;
 
@@ -571,6 +630,9 @@ extern PathFilter defaultPathFilter;
 /* Common initialisation performed in child processes. */
 void commonChildInit(Pipe & logPipe);
 
+/* Create a Unix domain socket. */
+AutoCloseFD createUnixDomainSocket();
+
 /* Create a Unix domain socket in listen mode. */
 AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode);
 
diff --git a/src/libutil/xml-writer.cc b/src/libutil/xml-writer.cc
index 68857e34d..7993bee9a 100644
--- a/src/libutil/xml-writer.cc
+++ b/src/libutil/xml-writer.cc
@@ -31,11 +31,12 @@ void XMLWriter::close()
 void XMLWriter::indent_(size_t depth)
 {
     if (!indent) return;
-    output << string(depth * 2, ' ');
+    output << std::string(depth * 2, ' ');
 }
 
 
-void XMLWriter::openElement(const string & name,
+void XMLWriter::openElement(
+    std::string_view name,
     const XMLAttrs & attrs)
 {
     assert(!closed);
@@ -44,7 +45,7 @@ void XMLWriter::openElement(const string & name,
     writeAttrs(attrs);
     output << ">";
     if (indent) output << std::endl;
-    pendingElems.push_back(name);
+    pendingElems.push_back(std::string(name));
 }
 
 
@@ -59,7 +60,8 @@ void XMLWriter::closeElement()
 }
 
 
-void XMLWriter::writeEmptyElement(const string & name,
+void XMLWriter::writeEmptyElement(
+    std::string_view name,
     const XMLAttrs & attrs)
 {
     assert(!closed);
diff --git a/src/libutil/xml-writer.hh b/src/libutil/xml-writer.hh
index b98b44526..4c91adee6 100644
--- a/src/libutil/xml-writer.hh
+++ b/src/libutil/xml-writer.hh
@@ -8,12 +8,8 @@
 
 namespace nix {
 
-using std::string;
-using std::map;
-using std::list;
 
-
-typedef map<string, string> XMLAttrs;
+typedef std::map<std::string, std::string> XMLAttrs;
 
 
 class XMLWriter
@@ -25,7 +21,7 @@ private:
     bool indent;
     bool closed;
 
-    list<string> pendingElems;
+    std::list<std::string> pendingElems;
 
 public:
 
@@ -34,11 +30,11 @@ public:
 
     void close();
 
-    void openElement(const string & name,
+    void openElement(std::string_view name,
         const XMLAttrs & attrs = XMLAttrs());
     void closeElement();
 
-    void writeEmptyElement(const string & name,
+    void writeEmptyElement(std::string_view name,
         const XMLAttrs & attrs = XMLAttrs());
 
 private:
@@ -53,7 +49,7 @@ class XMLOpenElement
 private:
     XMLWriter & writer;
 public:
-    XMLOpenElement(XMLWriter & writer, const string & name,
+    XMLOpenElement(XMLWriter & writer, std::string_view name,
         const XMLAttrs & attrs = XMLAttrs())
         : writer(writer)
     {