Merge branch 'enum-class' into no-hash-type-unknown

28 files changed, 1057 insertions, 448 deletions

diff --git a/src/libutil/affinity.cc b/src/libutil/affinity.cc
index 98f8287ad..ac2295e4a 100644
--- a/src/libutil/affinity.cc
+++ b/src/libutil/affinity.cc
@@ -12,6 +12,17 @@ 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
 
 
@@ -25,7 +36,7 @@ void setAffinityTo(int cpu)
     CPU_ZERO(&newAffinity);
     CPU_SET(cpu, &newAffinity);
     if (sched_setaffinity(0, sizeof(cpu_set_t), &newAffinity) == -1)
-        printError(format("failed to lock thread to CPU %1%") % cpu);
+        printError("failed to lock thread to CPU %1%", cpu);
 #endif
 }
 
@@ -47,7 +58,11 @@ void restoreAffinity()
 #if __linux__
     if (!didSaveAffinity) return;
     if (sched_setaffinity(0, sizeof(cpu_set_t), &savedAffinity) == -1)
-        printError("failed to restore affinity %1%");
+    {
+        std::ostringstream oss;
+        oss << savedAffinity;
+        printError("failed to restore CPU affinity %1%", oss.str());
+    }
 #endif
 }
 
diff --git a/src/libutil/ansicolor.hh b/src/libutil/ansicolor.hh
index 8ae07b092..a38c2d798 100644
--- a/src/libutil/ansicolor.hh
+++ b/src/libutil/ansicolor.hh
@@ -11,5 +11,7 @@ namespace nix {
 #define ANSI_GREEN "\e[32;1m"
 #define ANSI_YELLOW "\e[33;1m"
 #define ANSI_BLUE "\e[34;1m"
+#define ANSI_MAGENTA "\e[35m;1m"
+#define ANSI_CYAN "\e[36m;1m"
 
 }
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index db544a212..6a8484705 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -46,7 +46,7 @@ static void dumpContents(const Path & path, size_t size,
     sink << "contents" << size;
 
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
-    if (!fd) throw SysError(format("opening file '%1%'") % path);
+    if (!fd) throw SysError("opening file '%1%'", path);
 
     std::vector<unsigned char> buf(65536);
     size_t left = size;
@@ -68,7 +68,7 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
 
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
 
     sink << "(";
 
@@ -94,8 +94,9 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
                     name.erase(pos);
                 }
                 if (unhacked.find(name) != unhacked.end())
-                    throw Error(format("file name collision in between '%1%' and '%2%'")
-                        % (path + "/" + unhacked[name]) % (path + "/" + i.name));
+                    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;
@@ -111,7 +112,7 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
     else if (S_ISLNK(st.st_mode))
         sink << "type" << "symlink" << "target" << readLink(path);
 
-    else throw Error(format("file '%1%' has an unsupported type") % path);
+    else throw Error("file '%1%' has an unsupported type", path);
 
     sink << ")";
 }
@@ -247,7 +248,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                 } else if (s == "name") {
                     name = readString(source);
                     if (name.empty() || name == "." || name == ".." || name.find('/') != string::npos || name.find((char) 0) != string::npos)
-                        throw Error(format("NAR contains invalid file name '%1%'") % name);
+                        throw Error("NAR contains invalid file name '%1%'", name);
                     if (name <= prevName)
                         throw Error("NAR directory is not sorted");
                     prevName = name;
@@ -303,14 +304,14 @@ struct RestoreSink : ParseSink
     {
         Path p = dstPath + path;
         if (mkdir(p.c_str(), 0777) == -1)
-            throw SysError(format("creating directory '%1%'") % p);
+            throw SysError("creating directory '%1%'", p);
     };
 
     void createRegularFile(const Path & path)
     {
         Path p = dstPath + path;
         fd = open(p.c_str(), O_CREAT | O_EXCL | O_WRONLY | O_CLOEXEC, 0666);
-        if (!fd) throw SysError(format("creating file '%1%'") % p);
+        if (!fd) throw SysError("creating file '%1%'", p);
     }
 
     void isExecutable()
@@ -332,7 +333,7 @@ struct RestoreSink : ParseSink
                OpenSolaris).  Since preallocation is just an
                optimisation, ignore it. */
             if (errno && errno != EINVAL && errno != EOPNOTSUPP && errno != ENOSYS)
-                throw SysError(format("preallocating file of %1% bytes") % len);
+                throw SysError("preallocating file of %1% bytes", len);
         }
 #endif
     }
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index 4fe9539e4..1f0e4f803 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -45,7 +45,7 @@ void Args::parseCmdline(const Strings & _cmdline)
         }
         else if (!dashDash && std::string(arg, 0, 1) == "-") {
             if (!processFlag(pos, cmdline.end()))
-                throw UsageError(format("unrecognised flag '%1%'") % arg);
+                throw UsageError("unrecognised flag '%1%'", arg);
         }
         else {
             pendingArgs.push_back(*pos++);
@@ -130,7 +130,7 @@ bool Args::processArgs(const Strings & args, bool finish)
 {
     if (expectedArgs.empty()) {
         if (!args.empty())
-            throw UsageError(format("unexpected argument '%1%'") % args.front());
+            throw UsageError("unexpected argument '%1%'", args.front());
         return true;
     }
 
@@ -227,10 +227,15 @@ MultiCommand::MultiCommand(const Commands & commands)
 {
     expectedArgs.push_back(ExpectedArg{"command", 1, true, [=](std::vector<std::string> ss) {
         assert(!command);
-        auto i = commands.find(ss[0]);
+        auto cmd = ss[0];
+        if (auto alias = get(deprecatedAliases, cmd)) {
+            warn("'%s' is a deprecated alias for '%s'", cmd, *alias);
+            cmd = *alias;
+        }
+        auto i = commands.find(cmd);
         if (i == commands.end())
-            throw UsageError("'%s' is not a recognised command", ss[0]);
-        command = {ss[0], i->second()};
+            throw UsageError("'%s' is not a recognised command", cmd);
+        command = {cmd, i->second()};
     }});
 
     categories[Command::catDefault] = "Available commands";
diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index f2315f67a..73991881d 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -235,6 +235,8 @@ public:
 
     std::map<Command::Category, std::string> categories;
 
+    std::map<std::string, std::string> deprecatedAliases;
+
     // Selected command, if any.
     std::optional<std::pair<std::string, ref<Command>>> command;
 
diff --git a/src/libutil/compression.cc b/src/libutil/compression.cc
index 75e889f41..9f270c8f3 100644
--- a/src/libutil/compression.cc
+++ b/src/libutil/compression.cc
@@ -481,7 +481,7 @@ ref<CompressionSink> makeCompressionSink(const std::string & method, Sink & next
     else if (method == "br")
         return make_ref<BrotliCompressionSink>(nextSink);
     else
-        throw UnknownCompressionMethod(format("unknown compression method '%s'") % method);
+        throw UnknownCompressionMethod("unknown compression method '%s'", method);
 }
 
 ref<std::string> compress(const std::string & method, const std::string & in, const bool parallel)
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index 5c7a70a2e..66073546e 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -1,3 +1,4 @@
+#include <cassert>
 #include <map>
 #include <set>
 
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index a5571d4ec..5c2f9ed3f 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -2,9 +2,38 @@
 
 #include <iostream>
 #include <optional>
+#include "serialise.hh"
+#include <sstream>
 
-namespace nix
+namespace nix {
+
+
+const std::string nativeSystem = SYSTEM;
+
+// addPrefix is used for show-trace.  Strings added with addPrefix
+// will print ahead of the error itself.
+BaseError & BaseError::addPrefix(const FormatOrString & fs)
+{
+    prefix_ = fs.s + prefix_;
+    return *this;
+}
+
+// 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
 {
+    if (what_.has_value())
+        return *what_;
+    else {
+        err.name = sname();
+
+        std::ostringstream oss;
+        oss << err;
+        what_ = oss.str();
+
+        return *what_;
+    }
+}
 
 std::optional<string> ErrorInfo::programName = std::nullopt;
 
@@ -15,132 +44,178 @@ std::ostream& operator<<(std::ostream &os, const hintformat &hf)
 
 string showErrPos(const ErrPos &errPos)
 {
-    if (errPos.column > 0) {
-        return fmt("(%1%:%2%)", errPos.lineNumber, errPos.column);
-    } else {
-        return fmt("(%1%)", errPos.lineNumber);
-    };
+    if (errPos.line > 0) {
+        if (errPos.column > 0) {
+            return fmt("(%1%:%2%)", errPos.line, errPos.column);
+        } else {
+            return fmt("(%1%)", errPos.line);
+        }
+    }
+    else {
+        return "";
+    }
 }
 
-void printCodeLines(const string &prefix, const NixCode &nixCode)
+// if nixCode contains lines of code, print them to the ostream, indicating the error column.
+void printCodeLines(std::ostream &out, const string &prefix, const NixCode &nixCode)
 {
     // previous line of code.
     if (nixCode.prevLineOfCode.has_value()) {
-        std::cout << fmt("%1% %|2$5d|| %3%",
-                         prefix,
-                         (nixCode.errPos.lineNumber - 1),
-                         *nixCode.prevLineOfCode)
-                  << std::endl;
+        out << std::endl
+            << fmt("%1% %|2$5d|| %3%",
+                prefix,
+                (nixCode.errPos.line - 1),
+                *nixCode.prevLineOfCode);
     }
 
-    // line of code containing the error.%2$+5d%
-    std::cout << fmt("%1% %|2$5d|| %3%",
-                     prefix,
-                     (nixCode.errPos.lineNumber),
-                     nixCode.errLineOfCode)
-              << std::endl;
-
-    // error arrows for the column range.
-    if (nixCode.errPos.column > 0) {
-        int start = nixCode.errPos.column;
-        std::string spaces;
-        for (int i = 0; i < start; ++i) {
-            spaces.append(" ");
+    if (nixCode.errLineOfCode.has_value()) {
+        // line of code containing the error.
+        out << std::endl
+            << fmt("%1% %|2$5d|| %3%",
+                prefix,
+                (nixCode.errPos.line),
+                *nixCode.errLineOfCode);
+        // error arrows for the column range.
+        if (nixCode.errPos.column > 0) {
+            int start = nixCode.errPos.column;
+            std::string spaces;
+            for (int i = 0; i < start; ++i) {
+                spaces.append(" ");
+            }
+
+            std::string arrows("^");
+
+            out << std::endl
+                << fmt("%1%      |%2%" ANSI_RED "%3%" ANSI_NORMAL,
+                    prefix,
+                    spaces,
+                    arrows);
         }
-
-        std::string arrows("^");
-
-        std::cout << fmt("%1%      |%2%" ANSI_RED "%3%" ANSI_NORMAL,
-                         prefix,
-                         spaces,
-                         arrows) << std::endl;
     }
 
     // next line of code.
     if (nixCode.nextLineOfCode.has_value()) {
-        std::cout << fmt("%1% %|2$5d|| %3%",
-                         prefix,
-                         (nixCode.errPos.lineNumber + 1),
-                         *nixCode.nextLineOfCode)
-                  << std::endl;
+        out << std::endl
+            << fmt("%1% %|2$5d|| %3%",
+                prefix,
+                (nixCode.errPos.line + 1),
+                *nixCode.nextLineOfCode);
     }
 }
 
-void printErrorInfo(const ErrorInfo &einfo)
+std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
 {
-    int errwidth = 80;
-    string prefix = "    ";
+    auto errwidth = std::max<size_t>(getWindowSize().second, 20);
+    string prefix = "";
 
     string levelString;
     switch (einfo.level) {
-    case ErrLevel::elError: {
-        levelString = ANSI_RED;
-        levelString += "error:";
-        levelString += ANSI_NORMAL;
-        break;
-    }
-    case ErrLevel::elWarning: {
-        levelString = ANSI_YELLOW;
-        levelString += "warning:";
-        levelString += ANSI_NORMAL;
-        break;
-    }
-    default: {
-        levelString = fmt("invalid error level: %1%", einfo.level);
-        break;
-    }
+        case Verbosity::Error: {
+            levelString = ANSI_RED;
+            levelString += "error:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        case Verbosity::Warn: {
+            levelString = ANSI_YELLOW;
+            levelString += "warning:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        case Verbosity::Info: {
+            levelString = ANSI_GREEN;
+            levelString += "info:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        case Verbosity::Talkative: {
+            levelString = ANSI_GREEN;
+            levelString += "talk:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        case Verbosity::Chatty: {
+            levelString = ANSI_GREEN;
+            levelString += "chat:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        case Verbosity::Vomit: {
+            levelString = ANSI_GREEN;
+            levelString += "vomit:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        case Verbosity::Debug: {
+            levelString = ANSI_YELLOW;
+            levelString += "debug:";
+            levelString += ANSI_NORMAL;
+            break;
+        }
+        default: {
+            levelString = fmt("invalid error level: %d", (uint8_t)einfo.level);
+            break;
+        }
     }
 
-    int ndl = prefix.length() + levelString.length() + 3 + einfo.name.length() + einfo.programName.value_or("").length();
-    int dashwidth = ndl > (errwidth - 3) ? 3 : errwidth - ndl;
+    auto ndl = prefix.length() + levelString.length() + 3 + einfo.name.length() + einfo.programName.value_or("").length();
+    auto dashwidth = ndl > (errwidth - 3) ? 3 : errwidth - ndl;
 
-    string dashes;
-    for (int i = 0; i < dashwidth; ++i)
-        dashes.append("-");
+    std::string dashes(dashwidth, '-');
 
     // divider.
-    std::cout << fmt("%1%%2%" ANSI_BLUE " %3% %4% %5% %6%" ANSI_NORMAL,
-                     prefix,
-                     levelString,
-                     "---",
-                     einfo.name,
-                     dashes,
-                     einfo.programName.value_or(""))
-              << std::endl;
-
-    // filename.
+    if (einfo.name != "")
+        out << fmt("%1%%2%" ANSI_BLUE " --- %3% %4% %5%" ANSI_NORMAL,
+            prefix,
+            levelString,
+            einfo.name,
+            dashes,
+            einfo.programName.value_or(""));
+    else
+        out << fmt("%1%%2%" ANSI_BLUE " -----%3% %4%" ANSI_NORMAL,
+            prefix,
+            levelString,
+            dashes,
+            einfo.programName.value_or(""));
+
+    bool nl = false;  // intersperse newline between sections.
     if (einfo.nixCode.has_value()) {
-        if (einfo.nixCode->errPos.nixFile != "") {
-            string eline = einfo.nixCode->errLineOfCode != ""
-                           ? string(" ") + showErrPos(einfo.nixCode->errPos)
-                           : "";
-
-            std::cout << fmt("%1%in file: " ANSI_BLUE "%2%%3%" ANSI_NORMAL,
-                             prefix, 
-                             einfo.nixCode->errPos.nixFile,
-                             eline) << std::endl;
-            std::cout << prefix << std::endl;
+        if (einfo.nixCode->errPos.file != "") {
+            // filename, line, column.
+            out << std::endl << fmt("%1%in file: " ANSI_BLUE "%2% %3%" ANSI_NORMAL,
+                prefix,
+                einfo.nixCode->errPos.file,
+                showErrPos(einfo.nixCode->errPos));
         } else {
-            std::cout << fmt("%1%from command line argument", prefix) << std::endl;
-            std::cout << prefix << std::endl;
+            out << std::endl << fmt("%1%from command line argument", prefix);
         }
+        nl = true;
     }
 
     // description
-    std::cout << prefix << einfo.description << std::endl;
-    std::cout << prefix << std::endl;
+    if (einfo.description != "") {
+        if (nl)
+            out << std::endl << prefix;
+        out << std::endl << prefix << einfo.description;
+        nl = true;
+    }
 
     // lines of code.
-    if (einfo.nixCode->errLineOfCode != "") {
-        printCodeLines(prefix, *einfo.nixCode);
-        std::cout << prefix << std::endl;
+    if (einfo.nixCode.has_value() && einfo.nixCode->errLineOfCode.has_value()) {
+        if (nl)
+            out << std::endl << prefix;
+        printCodeLines(out, prefix, *einfo.nixCode);
+        nl = true;
     }
 
     // hint
     if (einfo.hint.has_value()) {
-        std::cout << prefix << *einfo.hint << std::endl;
-        std::cout << prefix << std::endl;
+        if (nl)
+            out << std::endl << prefix;
+        out << std::endl << prefix << *einfo.hint;
+        nl = true;
     }
-}
 
+    return out;
+}
 }
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index f402b692e..4f9df826f 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -1,121 +1,186 @@
-#ifndef error_hh
-#define error_hh
+#pragma once
 
-#include "ansicolor.hh"
-#include <string>
-#include <optional>
-#include <iostream>
+
+#include "ref.hh"
 #include "types.hh"
 
-namespace nix
-{
+#include <cstring>
+#include <list>
+#include <memory>
+#include <map>
+#include <optional>
 
-typedef enum {
-    elWarning,
-    elError
-} ErrLevel;
+#include "fmt.hh"
 
-struct ErrPos
-{
-    int lineNumber;
-    int column;
-    string nixFile;
+/* Before 4.7, gcc's std::exception uses empty throw() specifiers for
+ * its (virtual) destructor and what() in c++11 mode, in violation of spec
+ */
+#ifdef __GNUC__
+#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
+#define EXCEPTION_NEEDS_THROW_SPEC
+#endif
+#endif
+
+namespace nix {
+
+/*
+
+This file defines two main structs/classes used in nix error handling.
+
+ErrorInfo provides a standard payload of error information, with conversion to string
+happening in the logger rather than at the call site.
+
+BaseError is the ancestor of nix specific exceptions (and Interrupted), and contains
+an ErrorInfo.
+
+ErrorInfo structs are sent to the logger as part of an exception, or directly with the
+logError or logWarning macros.
+
+See the error-demo.cc program for usage examples.
 
+*/
+
+enum struct Verbosity {
+    Error = 0,
+    Warn,
+    Info,
+    Talkative,
+    Chatty,
+    Debug,
+    Vomit,
+};
+
+// ErrPos indicates the location of an error in a nix file.
+struct ErrPos {
+    int line = 0;
+    int column = 0;
+    string file;
+
+    operator bool() const
+    {
+        return line != 0;
+    }
+
+    // convert from the Pos struct, found in libexpr.
     template <class P>
     ErrPos& operator=(const P &pos)
     {
-        lineNumber = pos.line;
+        line = pos.line;
         column = pos.column;
-        nixFile = pos.file;
+        file = pos.file;
         return *this;
     }
 
     template <class P>
     ErrPos(const P &p)
     {
-      *this = p;
+        *this = p;
     }
 };
 
-struct NixCode
-{
+struct NixCode {
     ErrPos errPos;
     std::optional<string> prevLineOfCode;
-    string errLineOfCode;
+    std::optional<string> errLineOfCode;
     std::optional<string> nextLineOfCode;
 };
 
-// ----------------------------------------------------------------
-// format function for hints.  same as fmt, except templated values
-// are always in yellow.
+struct ErrorInfo {
+    Verbosity level;
+    string name;
+    string description;
+    std::optional<hintformat> hint;
+    std::optional<NixCode> nixCode;
 
-template <class T>
-struct yellowify
-{
-    yellowify(T &s) : value(s) {}
-    T &value;
+    static std::optional<string> programName;
 };
 
-template <class T>
-std::ostream& operator<<(std::ostream &out, const yellowify<T> &y)
-{
-    return out << ANSI_YELLOW << y.value << ANSI_NORMAL;
-}
+std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo);
 
-class hintformat
+/* BaseError should generally not be caught, as it has Interrupted as
+   a subclass. Catch Error instead. */
+class BaseError : public std::exception
 {
+protected:
+    string prefix_; // used for location traces etc.
+    mutable ErrorInfo err;
+
+    mutable std::optional<string> what_;
+    const string& calcWhat() const;
+
 public:
-    hintformat(string format) :fmt(format)
-    {
-        fmt.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
-    }
-    template<class T>
-    hintformat& operator%(const T &value)
-    {
-        fmt % yellowify(value);
-        return *this;
-    }
+    unsigned int status = 1; // exit status
+
+    template<typename... Args>
+    BaseError(unsigned int status, const Args & ... args)
+        : err { .level = Verbosity::Error,
+                .hint = hintfmt(args...)
+              }
+        , status(status)
+    { }
+
+    template<typename... Args>
+    BaseError(const std::string & fs, const Args & ... args)
+        : err { .level = Verbosity::Error,
+                .hint = hintfmt(fs, args...)
+              }
+    { }
+
+    BaseError(hintformat hint)
+        : err { .level = Verbosity::Error,
+                .hint = hint
+              }
+    { }
+
+    BaseError(ErrorInfo && e)
+        : err(std::move(e))
+    { }
+
+    BaseError(const ErrorInfo & e)
+        : err(e)
+    { }
+
+    virtual const char* sname() const { return "BaseError"; }
+
+#ifdef EXCEPTION_NEEDS_THROW_SPEC
+    ~BaseError() throw () { };
+    const char * what() const throw () { return calcWhat().c_str(); }
+#else
+    const char * what() const noexcept override { return calcWhat().c_str(); }
+#endif
 
-    std::string str() const
-    {
-        return fmt.str();
-    }
+    const string & msg() const { return calcWhat(); }
+    const string & prefix() const { return prefix_; }
+    BaseError & addPrefix(const FormatOrString & fs);
 
-    template <typename U>
-    friend class AddHint;
-private:
-    format fmt;
+    const ErrorInfo & info() { calcWhat(); return err; }
 };
 
-std::ostream& operator<<(std::ostream &os, const hintformat &hf);
+#define MakeError(newClass, superClass) \
+    class newClass : public superClass                  \
+    {                                                   \
+    public:                                             \
+        using superClass::superClass;                   \
+        virtual const char* sname() const override { return #newClass; } \
+    }
 
-template<typename... Args>
-inline hintformat hintfmt(const std::string & fs, const Args & ... args)
-{
-    hintformat f(fs);
-    formatHelper(f, args...);
-    return f;
-}
+MakeError(Error, BaseError);
 
-// -------------------------------------------------
-// ErrorInfo.
-struct ErrorInfo
+class SysError : public Error
 {
-    ErrLevel level;
-    string name;
-    string description;
-    std::optional<hintformat> hint;
-    std::optional<NixCode> nixCode;
-
-    static std::optional<string> programName;
-};
+public:
+    int errNo;
 
-// --------------------------------------------------------
-// error printing
+    template<typename... Args>
+    SysError(const Args & ... args)
+      :Error("")
+    {
+        errNo = errno;
+        auto hf = hintfmt(args...);
+        err.hint = hintfmt("%1%: %2%", normaltxt(hf.str()), strerror(errNo));
+    }
 
-// just to cout for now.
-void printErrorInfo(const ErrorInfo &einfo);
+    virtual const char* sname() const override { return "SysError"; }
+};
 
 }
-
-#endif
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
new file mode 100644
index 000000000..12ab9c407
--- /dev/null
+++ b/src/libutil/fmt.hh
@@ -0,0 +1,139 @@
+#pragma once
+
+#include <boost/format.hpp>
+#include <string>
+#include "ansicolor.hh"
+
+
+namespace nix {
+
+
+/* Inherit some names from other namespaces for convenience. */
+using std::string;
+using boost::format;
+
+
+/* A variadic template that does nothing. Useful to call a function
+   for all variadic arguments but ignoring the result. */
+struct nop { template<typename... T> nop(T...) {} };
+
+
+struct FormatOrString
+{
+    string s;
+    FormatOrString(const string & s) : s(s) { };
+    template<class F>
+    FormatOrString(const F & f) : s(f.str()) { };
+    FormatOrString(const char * s) : s(s) { };
+};
+
+
+/* A helper for formatting strings. ‘fmt(format, a_0, ..., a_n)’ is
+   equivalent to ‘boost::format(format) % a_0 % ... %
+   ... a_n’. However, ‘fmt(s)’ is equivalent to ‘s’ (so no %-expansion
+   takes place). */
+
+template<class F>
+inline void formatHelper(F & f)
+{
+}
+
+template<class F, typename T, typename... Args>
+inline void formatHelper(F & f, const T & x, const Args & ... args)
+{
+    formatHelper(f % x, args...);
+}
+
+inline std::string fmt(const std::string & s)
+{
+    return s;
+}
+
+inline std::string fmt(const char * s)
+{
+    return s;
+}
+
+inline std::string fmt(const FormatOrString & fs)
+{
+    return fs.s;
+}
+
+template<typename... Args>
+inline std::string fmt(const std::string & fs, const Args & ... args)
+{
+    boost::format f(fs);
+    f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
+    formatHelper(f, args...);
+    return f.str();
+}
+
+// -----------------------------------------------------------------------------
+// format function for hints in errors.  same as fmt, except templated values
+// are always in yellow.
+
+template <class T>
+struct yellowtxt
+{
+    yellowtxt(const T &s) : value(s) {}
+    const T &value;
+};
+
+template <class T>
+std::ostream& operator<<(std::ostream &out, const yellowtxt<T> &y)
+{
+    return out << ANSI_YELLOW << y.value << ANSI_NORMAL;
+}
+
+template <class T>
+struct normaltxt
+{
+    normaltxt(const T &s) : value(s) {}
+    const T &value;
+};
+
+template <class T>
+std::ostream& operator<<(std::ostream &out, const normaltxt<T> &y)
+{
+    return out << ANSI_NORMAL << y.value;
+}
+
+class hintformat
+{
+public:
+    hintformat(const string &format) :fmt(format)
+    {
+        fmt.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
+    }
+
+    hintformat(const hintformat &hf)
+    : fmt(hf.fmt)
+    {}
+
+    template<class T>
+    hintformat& operator%(const T &value)
+    {
+        fmt % yellowtxt(value);
+        return *this;
+    }
+
+    std::string str() const
+    {
+        return fmt.str();
+    }
+
+private:
+    format fmt;
+};
+
+std::ostream& operator<<(std::ostream &os, const hintformat &hf);
+
+template<typename... Args>
+inline hintformat hintfmt(const std::string & fs, const Args & ... args)
+{
+    hintformat f(fs);
+    formatHelper(f, args...);
+    return f;
+}
+
+}
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 6b9effdd2..106b47ae3 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -134,10 +134,10 @@ std::string Hash::to_string(Base base, bool includeType) const
     return s;
 }
 
-Hash::Hash(const std::string & s, HashType type) : Hash(s, std::optional { type }) { }
-Hash::Hash(const std::string & s) : Hash(s, std::optional<HashType>{}) { }
+Hash::Hash(std::string_view s, HashType type) : Hash(s, std::optional { type }) { }
+Hash::Hash(std::string_view s) : Hash(s, std::optional<HashType>{}) { }
 
-Hash::Hash(const std::string & s, std::optional<HashType> type)
+Hash::Hash(std::string_view s, std::optional<HashType> type)
     : type(type)
 {
     size_t pos = 0;
@@ -206,7 +206,7 @@ Hash::Hash(const std::string & s, std::optional<HashType> type)
     }
 
     else if (isSRI || size == base64Len()) {
-        auto d = base64Decode(std::string(s, pos));
+        auto d = base64Decode(s.substr(pos));
         if (d.size() != hashSize)
             throw BadHash("invalid %s hash '%s'", isSRI ? "SRI" : "base-64", s);
         assert(hashSize);
@@ -217,6 +217,18 @@ Hash::Hash(const std::string & s, std::optional<HashType> type)
         throw BadHash("hash '%s' has wrong length for hash type '%s'", s, printHashType(*type));
 }
 
+Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht)
+{
+    if (hashStr.empty()) {
+        if (!ht)
+            throw BadHash("empty hash requires explicit hash type");
+        Hash h(*ht);
+        warn("found empty hash, assuming '%s'", h.to_string(Base::SRI, true));
+        return h;
+    } else
+        return Hash(hashStr, ht);
+}
+
 
 union Ctx
 {
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 41322be67..8f9364440 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -25,7 +25,7 @@ const int sha512HashSize = 64;
 
 extern const string base32Chars;
 
-enum struct Base : int {
+enum struct Base {
     Base64,
     Base32,
     Base16,
@@ -52,11 +52,11 @@ struct Hash
        Subresource Integrity hash expression). If the 'type' argument
        is not present, then the hash type must be specified in the
        string. */
-    Hash(const std::string & s, std::optional<HashType> type);
+    Hash(std::string_view s, std::optional<HashType> type);
     // type must be provided
-    Hash(const std::string & s, HashType type);
+    Hash(std::string_view s, HashType type);
     // hash type must be part of string
-    Hash(const std::string & s);
+    Hash(std::string_view s);
 
     void init();
 
@@ -93,7 +93,7 @@ struct Hash
     /* Return a string representation of the hash, in base-16, base-32
        or base-64. By default, this is prefixed by the hash type
        (e.g. "sha256:"). */
-    std::string to_string(Base base = Base::Base32, bool includeType = true) const;
+    std::string to_string(Base base, bool includeType) const;
 
     std::string gitRev() const
     {
@@ -108,6 +108,8 @@ struct Hash
     }
 };
 
+/* Helper that defaults empty hashes to the 0 hash. */
+Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht);
 
 /* Print a hash in base-16 if it's MD5, or base-32 otherwise. */
 string printHash16or32(const Hash & hash);
diff --git a/src/libutil/json.cc b/src/libutil/json.cc
index 74e37b4c4..01331947e 100644
--- a/src/libutil/json.cc
+++ b/src/libutil/json.cc
@@ -173,7 +173,7 @@ JSONObject JSONPlaceholder::object()
 
 JSONPlaceholder::~JSONPlaceholder()
 {
-    assert(!first || std::uncaught_exception());
+    assert(!first || std::uncaught_exceptions());
 }
 
 }
diff --git a/src/libutil/local.mk b/src/libutil/local.mk
index 16c1fa03f..ae7eb67ad 100644
--- a/src/libutil/local.mk
+++ b/src/libutil/local.mk
@@ -7,5 +7,3 @@ libutil_DIR := $(d)
 libutil_SOURCES := $(wildcard $(d)/*.cc)
 
 libutil_LDFLAGS = $(LIBLZMA_LIBS) -lbz2 -pthread $(OPENSSL_LIBS) $(LIBBROTLI_LIBS) $(LIBARCHIVE_LIBS) $(BOOST_LDFLAGS) -lboost_context
-
-libutil_LIBS = libnixrust
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 5cd2524c6..04156151f 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -18,7 +18,7 @@ void setCurActivity(const ActivityId activityId)
     curActivity = activityId;
 }
 
-Logger * logger = makeDefaultLogger();
+Logger * logger = makeSimpleLogger(true);
 
 void Logger::warn(const std::string & msg)
 {
@@ -35,13 +35,19 @@ class SimpleLogger : public Logger
 public:
 
     bool systemd, tty;
+    bool printBuildLogs;
 
-    SimpleLogger()
+    SimpleLogger(bool printBuildLogs)
+        : printBuildLogs(printBuildLogs)
     {
         systemd = getEnv("IN_SYSTEMD") == "1";
         tty = isatty(STDERR_FILENO);
     }
 
+    bool isVerbose() override {
+        return printBuildLogs;
+    }
+
     void log(Verbosity lvl, const FormatOrString & fs) override
     {
         if (lvl > verbosity) return;
@@ -63,13 +69,33 @@ public:
         writeToStderr(prefix + filterANSIEscapes(fs.s, !tty) + "\n");
     }
 
+    void logEI(const ErrorInfo & ei) override
+    {
+        std::stringstream oss;
+        oss << ei;
+
+        log(ei.level, oss.str());
+    }
+
     void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
         const std::string & s, const Fields & fields, ActivityId parent)
-        override
+    override
     {
         if (lvl <= verbosity && !s.empty())
             log(lvl, s + "...");
     }
+
+    void result(ActivityId act, ResultType type, const Fields & fields) override
+    {
+        if (type == ResultType::BuildLogLine && printBuildLogs) {
+            auto lastLine = fields[0].s;
+            printError(lastLine);
+        }
+        else if (type == ResultType::PostBuildLogLine && printBuildLogs) {
+            auto lastLine = fields[0].s;
+            printError("post-build-hook: " + lastLine);
+        }
+    }
 };
 
 Verbosity verbosity = Verbosity::Info;
@@ -94,9 +120,9 @@ void writeToStderr(const string & s)
     }
 }
 
-Logger * makeDefaultLogger()
+Logger * makeSimpleLogger(bool printBuildLogs)
 {
-    return new SimpleLogger();
+    return new SimpleLogger(printBuildLogs);
 }
 
 std::atomic<uint64_t> nextId{(uint64_t) getpid() << 32};
@@ -108,12 +134,15 @@ Activity::Activity(Logger & logger, Verbosity lvl, ActivityType type,
     logger.startActivity(id, lvl, type, s, fields, parent);
 }
 
-struct JSONLogger : Logger
-{
+struct JSONLogger : Logger {
     Logger & prevLogger;
 
     JSONLogger(Logger & prevLogger) : prevLogger(prevLogger) { }
 
+    bool isVerbose() override {
+        return true;
+    }
+
     void addFields(nlohmann::json & json, const Fields & fields)
     {
         if (fields.empty()) return;
@@ -141,6 +170,19 @@ struct JSONLogger : Logger
         write(json);
     }
 
+    void logEI(const ErrorInfo & ei) override
+    {
+        std::ostringstream oss;
+        oss << ei;
+
+        nlohmann::json json;
+        json["action"] = "msg";
+        json["level"] = ei.level;
+        json["msg"] = oss.str();
+
+        write(json);
+    }
+
     void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
         const std::string & s, const Fields & fields, ActivityId parent) override
     {
@@ -231,13 +273,17 @@ bool handleJSONLogMessage(const std::string & msg,
         }
 
     } catch (std::exception & e) {
-        printError("bad log message from builder: %s", e.what());
+        logError({
+            .name = "Json log message",
+            .hint = hintfmt("bad log message from builder: %s", e.what())
+        });
     }
 
     return true;
 }
 
-Activity::~Activity() {
+Activity::~Activity()
+{
     try {
         logger.stopActivity(id);
     } catch (...) {
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 3449f5080..3b54101f0 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -1,20 +1,11 @@
 #pragma once
 
 #include "types.hh"
+#include "error.hh"
 
 namespace nix {
 
-enum struct Verbosity : uint64_t {
-    Error = 0,
-    Warn,
-    Info,
-    Talkative,
-    Chatty,
-    Debug,
-    Vomit,
-};
-
-enum struct ActivityType : uint64_t {
+enum struct ActivityType {
     Unknown = 0,
     CopyPath = 100,
     Download = 101,
@@ -27,9 +18,10 @@ enum struct ActivityType : uint64_t {
     Substitute = 108,
     QueryPathInfo = 109,
     PostBuildHook = 110,
+    BuildWaiting = 111,
 };
 
-enum struct ResultType : uint64_t {
+enum struct ResultType {
     FileLinked = 100,
     BuildLogLine = 101,
     UntrustedPath = 102,
@@ -63,6 +55,11 @@ public:
 
     virtual ~Logger() { }
 
+    virtual void stop() { };
+
+    // Whether the logger prints the whole build log
+    virtual bool isVerbose() { return false; }
+
     virtual void log(Verbosity lvl, const FormatOrString & fs) = 0;
 
     void log(const FormatOrString & fs)
@@ -70,6 +67,14 @@ public:
         log(Verbosity::Info, fs);
     }
 
+    virtual void logEI(const ErrorInfo &ei) = 0;
+
+    void logEI(Verbosity lvl, ErrorInfo ei)
+    {
+        ei.level = lvl;
+        logEI(ei);
+    }
+
     virtual void warn(const std::string & msg);
 
     virtual void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
@@ -141,7 +146,7 @@ struct PushActivity
 
 extern Logger * logger;
 
-Logger * makeDefaultLogger();
+Logger * makeSimpleLogger(bool printBuildLogs = true);
 
 Logger * makeJSONLogger(Logger & prevLogger);
 
@@ -151,9 +156,23 @@ bool handleJSONLogMessage(const std::string & msg,
 
 extern Verbosity verbosity; /* suppress msgs > this */
 
-/* Print a 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. */
+/* Print a message with the standard ErrorInfo format.
+   In general, use these 'log' macros for reporting problems that may require user
+   intervention or that need more explanation.  Use the 'print' macros for more
+   lightweight status messages. */
+#define logErrorInfo(level, errorInfo...) \
+    do { \
+        if (level <= nix::verbosity) { \
+            logger->logEI(level, errorInfo); \
+        } \
+    } while (0)
+
+#define logError(errorInfo...) logErrorInfo(Verbosity::Error, errorInfo)
+#define logWarning(errorInfo...) logErrorInfo(Verbosity::Warn, errorInfo)
+
+/* 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...) \
     do { \
         if (level <= nix::verbosity) { \
@@ -167,6 +186,7 @@ extern Verbosity verbosity; /* suppress msgs > this */
 #define debug(args...) printMsg(Verbosity::Debug, args)
 #define vomit(args...) printMsg(Verbosity::Vomit, args)
 
+/* if verbosity >= Verbosity::Warn, print a message with a yellow 'warning:' prefix. */
 template<typename... Args>
 inline void warn(const std::string & fs, const Args & ... args)
 {
diff --git a/src/libutil/lru-cache.hh b/src/libutil/lru-cache.hh
index 8b83f842c..6ef4a3e06 100644
--- a/src/libutil/lru-cache.hh
+++ b/src/libutil/lru-cache.hh
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cassert>
 #include <map>
 #include <list>
 #include <optional>
diff --git a/src/libutil/rust-ffi.cc b/src/libutil/rust-ffi.cc
index 6f36b3192..67924568f 100644
--- a/src/libutil/rust-ffi.cc
+++ b/src/libutil/rust-ffi.cc
@@ -1,3 +1,4 @@
+#if 0
 #include "logging.hh"
 #include "rust-ffi.hh"
 
@@ -20,3 +21,4 @@ std::ostream & operator << (std::ostream & str, const String & s)
 }
 
 }
+#endif
diff --git a/src/libutil/rust-ffi.hh b/src/libutil/rust-ffi.hh
index 228e2eead..cfbaf9dec 100644
--- a/src/libutil/rust-ffi.hh
+++ b/src/libutil/rust-ffi.hh
@@ -1,4 +1,5 @@
 #pragma once
+#if 0
 
 #include "serialise.hh"
 
@@ -185,3 +186,4 @@ struct Result
 };
 
 }
+#endif
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 8201549fd..c8b71188f 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -52,7 +52,10 @@ size_t threshold = 256 * 1024 * 1024;
 
 static void warnLargeDump()
 {
-    printError("warning: dumping very large path (> 256 MiB); this may run out of memory");
+    logWarning({
+        .name = "Large path",
+        .description = "dumping very large path (> 256 MiB); this may run out of memory"
+    });
 }
 
 
diff --git a/src/libutil/tests/hash.cc b/src/libutil/tests/hash.cc
index ecc0d4a03..84a50d61b 100644
--- a/src/libutil/tests/hash.cc
+++ b/src/libutil/tests/hash.cc
@@ -11,28 +11,28 @@ namespace nix {
         // values taken from: https://tools.ietf.org/html/rfc1321
         auto s1 = "";
         auto hash = hashString(HashType::MD5, s1);
-        ASSERT_EQ(hash.to_string(Base::Base16), "md5:d41d8cd98f00b204e9800998ecf8427e");
+        ASSERT_EQ(hash.to_string(Base::Base16, true), "md5:d41d8cd98f00b204e9800998ecf8427e");
     }
 
     TEST(hashString, testKnownMD5Hashes2) {
         // values taken from: https://tools.ietf.org/html/rfc1321
         auto s2 = "abc";
         auto hash = hashString(HashType::MD5, s2);
-        ASSERT_EQ(hash.to_string(Base::Base16), "md5:900150983cd24fb0d6963f7d28e17f72");
+        ASSERT_EQ(hash.to_string(Base::Base16, true), "md5:900150983cd24fb0d6963f7d28e17f72");
     }
 
     TEST(hashString, testKnownSHA1Hashes1) {
         // values taken from: https://tools.ietf.org/html/rfc3174
         auto s = "abc";
         auto hash = hashString(HashType::SHA1, s);
-        ASSERT_EQ(hash.to_string(Base::Base16),"sha1:a9993e364706816aba3e25717850c26c9cd0d89d");
+        ASSERT_EQ(hash.to_string(Base::Base16, true),"sha1:a9993e364706816aba3e25717850c26c9cd0d89d");
     }
 
     TEST(hashString, testKnownSHA1Hashes2) {
         // values taken from: https://tools.ietf.org/html/rfc3174
         auto s = "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq";
         auto hash = hashString(HashType::SHA1, s);
-        ASSERT_EQ(hash.to_string(Base::Base16),"sha1:84983e441c3bd26ebaae4aa1f95129e5e54670f1");
+        ASSERT_EQ(hash.to_string(Base::Base16, true),"sha1:84983e441c3bd26ebaae4aa1f95129e5e54670f1");
     }
 
     TEST(hashString, testKnownSHA256Hashes1) {
@@ -40,7 +40,7 @@ namespace nix {
         auto s = "abc";
 
         auto hash = hashString(HashType::SHA256, s);
-        ASSERT_EQ(hash.to_string(Base::Base16),
+        ASSERT_EQ(hash.to_string(Base::Base16, true),
                 "sha256:ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
     }
 
@@ -48,7 +48,7 @@ namespace nix {
         // values taken from: https://tools.ietf.org/html/rfc4634
         auto s = "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq";
         auto hash = hashString(HashType::SHA256, s);
-        ASSERT_EQ(hash.to_string(Base::Base16),
+        ASSERT_EQ(hash.to_string(Base::Base16, true),
                 "sha256:248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1");
     }
 
@@ -56,7 +56,7 @@ namespace nix {
         // values taken from: https://tools.ietf.org/html/rfc4634
         auto s = "abc";
         auto hash = hashString(HashType::SHA512, s);
-        ASSERT_EQ(hash.to_string(Base::Base16),
+        ASSERT_EQ(hash.to_string(Base::Base16, true),
                 "sha512:ddaf35a193617abacc417349ae20413112e6fa4e89a9"
                 "7ea20a9eeee64b55d39a2192992a274fc1a836ba3c23a3feebbd"
                 "454d4423643ce80e2a9ac94fa54ca49f");
@@ -67,7 +67,7 @@ namespace nix {
         auto s = "abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu";
 
         auto hash = hashString(HashType::SHA512, s);
-        ASSERT_EQ(hash.to_string(Base::Base16),
+        ASSERT_EQ(hash.to_string(Base::Base16, true),
                 "sha512:8e959b75dae313da8cf4f72814fc143f8f7779c6eb9f7fa1"
                 "7299aeadb6889018501d289e4900f7e4331b99dec4b5433a"
                 "c7d329eeb6dd26545e96e55b874be909");
diff --git a/src/libutil/tests/local.mk b/src/libutil/tests/local.mk
index a297edb64..815e18560 100644
--- a/src/libutil/tests/local.mk
+++ b/src/libutil/tests/local.mk
@@ -8,7 +8,7 @@ libutil-tests_INSTALL_DIR :=
 
 libutil-tests_SOURCES := $(wildcard $(d)/*.cc)
 
-libutil-tests_CXXFLAGS += -I src/libutil
+libutil-tests_CXXFLAGS += -I src/libutil -I src/libexpr
 
 libutil-tests_LIBS = libutil
 
diff --git a/src/libutil/tests/logging.cc b/src/libutil/tests/logging.cc
new file mode 100644
index 000000000..2ffcbc41d
--- /dev/null
+++ b/src/libutil/tests/logging.cc
@@ -0,0 +1,255 @@
+#include "logging.hh"
+#include "nixexpr.hh"
+#include "util.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    /* ----------------------------------------------------------------------------
+     * logEI
+     * --------------------------------------------------------------------------*/
+
+    TEST(logEI, catpuresBasicProperties) {
+
+        MakeError(TestError, Error);
+        ErrorInfo::programName = std::optional("error-unit-test");
+
+        try {
+            throw TestError("an error for testing purposes");
+        } catch (Error &e) {
+            testing::internal::CaptureStderr();
+            logger->logEI(e.info());
+            auto str = testing::internal::GetCapturedStderr();
+
+            ASSERT_STREQ(str.c_str(),"\x1B[31;1merror:\x1B[0m\x1B[34;1m --- TestError --- error-unit-test\x1B[0m\nan error for testing purposes\n");
+        }
+    }
+
+    TEST(logEI, appendingHintsToPreviousError) {
+
+        MakeError(TestError, Error);
+        ErrorInfo::programName = std::optional("error-unit-test");
+
+        try {
+            auto e = Error("initial error");
+            throw TestError(e.info());
+        } catch (Error &e) {
+            ErrorInfo ei = e.info();
+            ei.hint = hintfmt("%s; subsequent error message.", normaltxt(e.info().hint ? e.info().hint->str() : ""));
+
+            testing::internal::CaptureStderr();
+            logger->logEI(ei);
+            auto str = testing::internal::GetCapturedStderr();
+
+            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- TestError --- error-unit-test\x1B[0m\n\x1B[33;1m\x1B[0minitial error\x1B[0m; subsequent error message.\n");
+        }
+
+    }
+
+    TEST(logEI, picksUpSysErrorExitCode) {
+
+        MakeError(TestError, Error);
+        ErrorInfo::programName = std::optional("error-unit-test");
+
+        try {
+            auto x = readFile(-1);
+        }
+        catch (SysError &e) {
+            testing::internal::CaptureStderr();
+            logError(e.info());
+            auto str = testing::internal::GetCapturedStderr();
+
+            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- SysError --- error-unit-test\x1B[0m\n\x1B[33;1m\x1B[0mstatting file\x1B[0m: \x1B[33;1mBad file descriptor\x1B[0m\n");
+
+        }
+    }
+
+    TEST(logEI, loggingErrorOnInfoLevel) {
+        testing::internal::CaptureStderr();
+
+        logger->logEI({ .level = Verbosity::Info,
+                .name = "Info name",
+                .description = "Info description",
+                });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[32;1minfo:\x1B[0m\x1B[34;1m --- Info name --- error-unit-test\x1B[0m\nInfo description\n");
+    }
+
+    TEST(logEI, loggingErrorOnTalkativeLevel) {
+        verbosity = Verbosity::Talkative;
+
+        testing::internal::CaptureStderr();
+
+        logger->logEI({ .level = Verbosity::Talkative,
+                        .name = "Talkative name",
+                        .description = "Talkative description",
+                        });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[32;1mtalk:\x1B[0m\x1B[34;1m --- Talkative name --- error-unit-test\x1B[0m\nTalkative description\n");
+    }
+
+    TEST(logEI, loggingErrorOnChattyLevel) {
+        verbosity = Verbosity::Chatty;
+
+        testing::internal::CaptureStderr();
+
+        logger->logEI({ .level = Verbosity::Chatty,
+                        .name = "Chatty name",
+                        .description = "Talkative description",
+                        });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[32;1mchat:\x1B[0m\x1B[34;1m --- Chatty name --- error-unit-test\x1B[0m\nTalkative description\n");
+    }
+
+    TEST(logEI, loggingErrorOnDebugLevel) {
+        verbosity = Verbosity::Debug;
+
+        testing::internal::CaptureStderr();
+
+        logger->logEI({ .level = Verbosity::Debug,
+                        .name = "Debug name",
+                        .description = "Debug description",
+                        });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[33;1mdebug:\x1B[0m\x1B[34;1m --- Debug name --- error-unit-test\x1B[0m\nDebug description\n");
+    }
+
+    TEST(logEI, loggingErrorOnVomitLevel) {
+        verbosity = Verbosity::Vomit;
+
+        testing::internal::CaptureStderr();
+
+        logger->logEI({ .level = Verbosity::Vomit,
+                        .name = "Vomit name",
+                        .description = "Vomit description",
+                        });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[32;1mvomit:\x1B[0m\x1B[34;1m --- Vomit name --- error-unit-test\x1B[0m\nVomit description\n");
+    }
+
+    /* ----------------------------------------------------------------------------
+     * logError
+     * --------------------------------------------------------------------------*/
+
+
+    TEST(logError, logErrorWithoutHintOrCode) {
+        testing::internal::CaptureStderr();
+
+        logError({
+                .name = "name",
+                .description = "error description",
+                });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- name --- error-unit-test\x1B[0m\nerror description\n");
+    }
+
+    TEST(logError, logErrorWithPreviousAndNextLinesOfCode) {
+        SymbolTable testTable;
+        auto problem_file = testTable.create("myfile.nix");
+
+        testing::internal::CaptureStderr();
+
+        logError({
+                .name = "error name",
+                .description = "error with code lines",
+                .hint = hintfmt("this hint has %1% templated %2%!!",
+                        "yellow",
+                        "values"),
+                .nixCode = NixCode {
+                .errPos = Pos(problem_file, 40, 13),
+                .prevLineOfCode = "previous line of code",
+                .errLineOfCode = "this is the problem line of code",
+                .nextLineOfCode = "next line of code",
+                }});
+
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nerror with code lines\n\n    39| previous line of code\n    40| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n    41| next line of code\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+    }
+
+    TEST(logError, logErrorWithoutLinesOfCode) {
+        SymbolTable testTable;
+        auto problem_file = testTable.create("myfile.nix");
+        testing::internal::CaptureStderr();
+
+        logError({
+                .name = "error name",
+                .description = "error without any code lines.",
+                .hint = hintfmt("this hint has %1% templated %2%!!",
+                        "yellow",
+                        "values"),
+                .nixCode = NixCode {
+                .errPos = Pos(problem_file, 40, 13)
+                }});
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nerror without any code lines.\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+    }
+
+    TEST(logError, logErrorWithOnlyHintAndName) {
+        SymbolTable testTable;
+        auto problem_file = testTable.create("myfile.nix");
+        testing::internal::CaptureStderr();
+
+        logError({
+                .name = "error name",
+                .hint = hintfmt("hint %1%", "only"),
+                .nixCode = NixCode {
+                .errPos = Pos(problem_file, 40, 13)
+                }});
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nhint \x1B[33;1monly\x1B[0m\n");
+
+    }
+
+    /* ----------------------------------------------------------------------------
+     * logWarning
+     * --------------------------------------------------------------------------*/
+
+    TEST(logWarning, logWarningWithNameDescriptionAndHint) {
+        testing::internal::CaptureStderr();
+
+        logWarning({
+            .name = "name",
+            .description = "error description",
+            .hint = hintfmt("there was a %1%", "warning"),
+        });
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[33;1mwarning:\x1B[0m\x1B[34;1m --- name --- error-unit-test\x1B[0m\nerror description\n\nthere was a \x1B[33;1mwarning\x1B[0m\n");
+    }
+
+    TEST(logWarning, logWarningWithFileLineNumAndCode) {
+
+        SymbolTable testTable;
+        auto problem_file = testTable.create("myfile.nix");
+
+        testing::internal::CaptureStderr();
+
+        logWarning({
+                .name = "warning name",
+                .description = "warning description",
+                .hint = hintfmt("this hint has %1% templated %2%!!",
+                        "yellow",
+                        "values"),
+                .nixCode = NixCode {
+                .errPos = Pos(problem_file, 40, 13),
+                .prevLineOfCode = std::nullopt,
+                .errLineOfCode = "this is the problem line of code",
+                .nextLineOfCode = std::nullopt
+                }});
+
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[33;1mwarning:\x1B[0m\x1B[34;1m --- warning name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nwarning description\n\n    40| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+    }
+
+}
diff --git a/src/libutil/tests/pool.cc b/src/libutil/tests/pool.cc
new file mode 100644
index 000000000..127e42dda
--- /dev/null
+++ b/src/libutil/tests/pool.cc
@@ -0,0 +1,127 @@
+#include "pool.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    struct TestResource
+    {
+
+        TestResource() {
+            static int counter = 0;
+            num = counter++;
+        }
+
+        int dummyValue = 1;
+        bool good = true;
+        int num;
+    };
+
+    /* ----------------------------------------------------------------------------
+     * Pool
+     * --------------------------------------------------------------------------*/
+
+    TEST(Pool, freshPoolHasZeroCountAndSpecifiedCapacity) {
+        auto isGood = [](const ref<TestResource> & r) { return r->good; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+
+        ASSERT_EQ(pool.count(), 0);
+        ASSERT_EQ(pool.capacity(), 1);
+    }
+
+    TEST(Pool, freshPoolCanGetAResource) {
+        auto isGood = [](const ref<TestResource> & r) { return r->good; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+        ASSERT_EQ(pool.count(), 0);
+
+        TestResource r = *(pool.get());
+
+        ASSERT_EQ(pool.count(), 1);
+        ASSERT_EQ(pool.capacity(), 1);
+        ASSERT_EQ(r.dummyValue, 1);
+        ASSERT_EQ(r.good, true);
+    }
+
+    TEST(Pool, capacityCanBeIncremented) {
+        auto isGood = [](const ref<TestResource> & r) { return r->good; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+        ASSERT_EQ(pool.capacity(), 1);
+        pool.incCapacity();
+        ASSERT_EQ(pool.capacity(), 2);
+    }
+
+    TEST(Pool, capacityCanBeDecremented) {
+        auto isGood = [](const ref<TestResource> & r) { return r->good; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+        ASSERT_EQ(pool.capacity(), 1);
+        pool.decCapacity();
+        ASSERT_EQ(pool.capacity(), 0);
+    }
+
+    TEST(Pool, flushBadDropsOutOfScopeResources) {
+        auto isGood = [](const ref<TestResource> & r) { return false; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+
+        {
+            auto _r = pool.get();
+            ASSERT_EQ(pool.count(), 1);
+        }
+
+        pool.flushBad();
+        ASSERT_EQ(pool.count(), 0);
+    }
+
+    // Test that the resources we allocate are being reused when they are still good.
+    TEST(Pool, reuseResource) {
+        auto isGood = [](const ref<TestResource> & r) { return true; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+
+        // Compare the instance counter between the two handles. We expect them to be equal
+        // as the pool should hand out the same (still) good one again.
+        int counter = -1;
+        {
+            Pool<TestResource>::Handle h = pool.get();
+            counter = h->num;
+        } // the first handle goes out of scope
+
+        { // the second handle should contain the same resource (with the same counter value)
+            Pool<TestResource>::Handle h = pool.get();
+            ASSERT_EQ(h->num, counter);
+        }
+    }
+
+    // Test that the resources we allocate are being thrown away when they are no longer good.
+    TEST(Pool, badResourceIsNotReused) {
+        auto isGood = [](const ref<TestResource> & r) { return false; };
+        auto createResource = []() { return make_ref<TestResource>(); };
+
+        Pool<TestResource> pool = Pool<TestResource>((size_t)1, createResource, isGood);
+
+        // Compare the instance counter between the two handles. We expect them
+        // to *not* be equal as the pool should hand out a new instance after
+        // the first one was returned.
+        int counter = -1;
+        {
+            Pool<TestResource>::Handle h = pool.get();
+            counter = h->num;
+        } // the first handle goes out of scope
+
+        {
+          // the second handle should contain a different resource (with a
+          //different counter value)
+            Pool<TestResource>::Handle h = pool.get();
+            ASSERT_NE(h->num, counter);
+        }
+    }
+}
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 250c9581d..3af485fa0 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -1,164 +1,29 @@
 #pragma once
 
-
 #include "ref.hh"
 
-#include <string>
 #include <list>
 #include <set>
-#include <memory>
 #include <map>
-
-#include <boost/format.hpp>
-
-/* Before 4.7, gcc's std::exception uses empty throw() specifiers for
- * its (virtual) destructor and what() in c++11 mode, in violation of spec
- */
-#ifdef __GNUC__
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
-#define EXCEPTION_NEEDS_THROW_SPEC
-#endif
-#endif
-
+#include <vector>
 
 namespace nix {
 
-
-/* Inherit some names from other namespaces for convenience. */
-using std::string;
 using std::list;
 using std::set;
 using std::vector;
-using boost::format;
-
-
-/* A variadic template that does nothing. Useful to call a function
-   for all variadic arguments but ignoring the result. */
-struct nop { template<typename... T> nop(T...) {} };
-
-
-struct FormatOrString
-{
-    string s;
-    FormatOrString(const string & s) : s(s) { };
-    template<class F>
-    FormatOrString(const F & f) : s(f.str()) { };
-    FormatOrString(const char * s) : s(s) { };
-};
-
-
-/* A helper for formatting strings. ‘fmt(format, a_0, ..., a_n)’ is
-   equivalent to ‘boost::format(format) % a_0 % ... %
-   ... a_n’. However, ‘fmt(s)’ is equivalent to ‘s’ (so no %-expansion
-   takes place). */
-
-template<class F>
-inline void formatHelper(F & f)
-{
-}
-
-template<class F, typename T, typename... Args>
-inline void formatHelper(F & f, const T & x, const Args & ... args)
-{
-    formatHelper(f % x, args...);
-}
-
-inline std::string fmt(const std::string & s)
-{
-    return s;
-}
-
-inline std::string fmt(const char * s)
-{
-    return s;
-}
-
-inline std::string fmt(const FormatOrString & fs)
-{
-    return fs.s;
-}
-
-template<typename... Args>
-inline std::string fmt(const std::string & fs, const Args & ... args)
-{
-    boost::format f(fs);
-    f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
-    formatHelper(f, args...);
-    return f.str();
-}
-
-
-/* BaseError should generally not be caught, as it has Interrupted as
-   a subclass. Catch Error instead. */
-class BaseError : public std::exception
-{
-protected:
-    string prefix_; // used for location traces etc.
-    string err;
-public:
-    unsigned int status = 1; // exit status
-
-    template<typename... Args>
-    BaseError(unsigned int status, const Args & ... args)
-        : err(fmt(args...))
-        , status(status)
-    {
-    }
-
-    template<typename... Args>
-    BaseError(const Args & ... args)
-        : err(fmt(args...))
-    {
-    }
-
-#ifdef EXCEPTION_NEEDS_THROW_SPEC
-    ~BaseError() throw () { };
-    const char * what() const throw () { return err.c_str(); }
-#else
-    const char * what() const noexcept { return err.c_str(); }
-#endif
-
-    const string & msg() const { return err; }
-    const string & prefix() const { return prefix_; }
-    BaseError & addPrefix(const FormatOrString & fs);
-};
-
-#define MakeError(newClass, superClass) \
-    class newClass : public superClass                  \
-    {                                                   \
-    public:                                             \
-        using superClass::superClass;                   \
-    }
-
-MakeError(Error, BaseError);
-
-class SysError : public Error
-{
-public:
-    int errNo;
-
-    template<typename... Args>
-    SysError(const Args & ... args)
-        : Error(addErrno(fmt(args...)))
-    { }
-
-private:
-
-    std::string addErrno(const std::string & s);
-};
-
+using std::string;
 
 typedef list<string> Strings;
 typedef set<string> StringSet;
-typedef std::map<std::string, std::string> StringMap;
-
+typedef std::map<string, string> StringMap;
 
 /* Paths are just strings. */
+
 typedef string Path;
 typedef list<Path> Paths;
 typedef set<Path> PathSet;
 
-
 /* Helper class to run code at startup. */
 template<typename T>
 struct OnStartup
@@ -166,5 +31,4 @@ struct OnStartup
     OnStartup(T && t) { t(); }
 };
 
-
 }
diff --git a/src/libutil/url.hh b/src/libutil/url.hh
index 4a0d4071b..2ef88ef2a 100644
--- a/src/libutil/url.hh
+++ b/src/libutil/url.hh
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "types.hh"
+#include "error.hh"
 
 #include <regex>
 
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index d17e952ae..e912d4450 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -35,29 +35,11 @@
 #endif
 
 
-extern char * * environ;
+extern char * * environ __attribute__((weak));
 
 
 namespace nix {
 
-
-const std::string nativeSystem = SYSTEM;
-
-
-BaseError & BaseError::addPrefix(const FormatOrString & fs)
-{
-    prefix_ = fs.s + prefix_;
-    return *this;
-}
-
-
-std::string SysError::addErrno(const std::string & s)
-{
-    errNo = errno;
-    return s + ": " + strerror(errNo);
-}
-
-
 std::optional<std::string> getEnv(const std::string & key)
 {
     char * value = getenv(key.c_str());
@@ -129,7 +111,7 @@ Path canonPath(const Path & path, bool resolveSymlinks)
     string s;
 
     if (path[0] != '/')
-        throw Error(format("not an absolute path: '%1%'") % path);
+        throw Error("not an absolute path: '%1%'", path);
 
     string::const_iterator i = path.begin(), end = path.end();
     string temp;
@@ -165,7 +147,7 @@ Path canonPath(const Path & path, bool resolveSymlinks)
                the symlink target might contain new symlinks). */
             if (resolveSymlinks && isLink(s)) {
                 if (++followCount >= maxFollow)
-                    throw Error(format("infinite symlink recursion in path '%1%'") % path);
+                    throw Error("infinite symlink recursion in path '%1%'", path);
                 temp = absPath(readLink(s), dirOf(s))
                     + string(i, end);
                 i = temp.begin(); /* restart */
@@ -226,7 +208,7 @@ struct stat lstat(const Path & path)
 {
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting status of '%1%'") % path);
+        throw SysError("getting status of '%1%'", path);
     return st;
 }
 
@@ -238,7 +220,7 @@ bool pathExists(const Path & path)
     res = lstat(path.c_str(), &st);
     if (!res) return true;
     if (errno != ENOENT && errno != ENOTDIR)
-        throw SysError(format("getting status of %1%") % path);
+        throw SysError("getting status of %1%", path);
     return false;
 }
 
@@ -286,7 +268,7 @@ DirEntries readDirectory(DIR *dir, const Path & path)
 #endif
         );
     }
-    if (errno) throw SysError(format("reading directory '%1%'") % path);
+    if (errno) throw SysError("reading directory '%1%'", path);
 
     return entries;
 }
@@ -294,7 +276,7 @@ DirEntries readDirectory(DIR *dir, const Path & path)
 DirEntries readDirectory(const Path & path)
 {
     AutoCloseDir dir(opendir(path.c_str()));
-    if (!dir) throw SysError(format("opening directory '%1%'") % path);
+    if (!dir) throw SysError("opening directory '%1%'", path);
 
     return readDirectory(dir.get(), path);
 }
@@ -324,7 +306,7 @@ string readFile(const Path & path)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
     return readFile(fd.get());
 }
 
@@ -332,7 +314,8 @@ string readFile(const Path & path)
 void readFile(const Path & path, Sink & sink)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
-    if (!fd) throw SysError("opening file '%s'", path);
+    if (!fd)
+        throw SysError("opening file '%s'", path);
     drainFD(fd.get(), sink);
 }
 
@@ -341,7 +324,7 @@ void writeFile(const Path & path, const string & s, mode_t mode)
 {
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
     writeFull(fd.get(), s);
 }
 
@@ -350,7 +333,7 @@ void writeFile(const Path & path, Source & source, mode_t mode)
 {
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
 
     std::vector<unsigned char> buf(64 * 1024);
 
@@ -400,7 +383,7 @@ static void _deletePath(int parentfd, const Path & path, unsigned long long & by
     struct stat st;
     if (fstatat(parentfd, name.c_str(), &st, AT_SYMLINK_NOFOLLOW) == -1) {
         if (errno == ENOENT) return;
-        throw SysError(format("getting status of '%1%'") % path);
+        throw SysError("getting status of '%1%'", path);
     }
 
     if (!S_ISDIR(st.st_mode) && st.st_nlink == 1)
@@ -411,15 +394,15 @@ static void _deletePath(int parentfd, const Path & path, unsigned long long & by
         const auto PERM_MASK = S_IRUSR | S_IWUSR | S_IXUSR;
         if ((st.st_mode & PERM_MASK) != PERM_MASK) {
             if (fchmodat(parentfd, name.c_str(), st.st_mode | PERM_MASK, 0) == -1)
-                throw SysError(format("chmod '%1%'") % path);
+                throw SysError("chmod '%1%'", path);
         }
 
         int fd = openat(parentfd, path.c_str(), O_RDONLY);
         if (!fd)
-            throw SysError(format("opening directory '%1%'") % path);
+            throw SysError("opening directory '%1%'", path);
         AutoCloseDir dir(fdopendir(fd));
         if (!dir)
-            throw SysError(format("opening directory '%1%'") % path);
+            throw SysError("opening directory '%1%'", path);
         for (auto & i : readDirectory(dir.get(), path))
             _deletePath(dirfd(dir.get()), path + "/" + i.name, bytesFreed);
     }
@@ -427,7 +410,7 @@ static void _deletePath(int parentfd, const Path & path, unsigned long long & by
     int flags = S_ISDIR(st.st_mode) ? AT_REMOVEDIR : 0;
     if (unlinkat(parentfd, name.c_str(), flags) == -1) {
         if (errno == ENOENT) return;
-        throw SysError(format("cannot unlink '%1%'") % path);
+        throw SysError("cannot unlink '%1%'", path);
     }
 }
 
@@ -443,7 +426,7 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
         // for backwards compatibility.
         if (errno == ENOENT) return;
 
-        throw SysError(format("opening directory '%1%'") % path);
+        throw SysError("opening directory '%1%'", path);
     }
 
     _deletePath(dirfd.get(), path, bytesFreed);
@@ -497,12 +480,12 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
                "wheel", then "tar" will fail to unpack archives that
                have the setgid bit set on directories. */
             if (chown(tmpDir.c_str(), (uid_t) -1, getegid()) != 0)
-                throw SysError(format("setting group of directory '%1%'") % tmpDir);
+                throw SysError("setting group of directory '%1%'", tmpDir);
 #endif
             return tmpDir;
         }
         if (errno != EEXIST)
-            throw SysError(format("creating directory '%1%'") % tmpDir);
+            throw SysError("creating directory '%1%'", tmpDir);
     }
 }
 
@@ -584,15 +567,15 @@ Paths createDirs(const Path & path)
     if (lstat(path.c_str(), &st) == -1) {
         created = createDirs(dirOf(path));
         if (mkdir(path.c_str(), 0777) == -1 && errno != EEXIST)
-            throw SysError(format("creating directory '%1%'") % path);
+            throw SysError("creating directory '%1%'", path);
         st = lstat(path);
         created.push_back(path);
     }
 
     if (S_ISLNK(st.st_mode) && stat(path.c_str(), &st) == -1)
-        throw SysError(format("statting symlink '%1%'") % path);
+        throw SysError("statting symlink '%1%'", path);
 
-    if (!S_ISDIR(st.st_mode)) throw Error(format("'%1%' is not a directory") % path);
+    if (!S_ISDIR(st.st_mode)) throw Error("'%1%' is not a directory", path);
 
     return created;
 }
@@ -601,7 +584,7 @@ Paths createDirs(const Path & path)
 void createSymlink(const Path & target, const Path & link)
 {
     if (symlink(target.c_str(), link.c_str()))
-        throw SysError(format("creating symlink from '%1%' to '%2%'") % link % target);
+        throw SysError("creating symlink from '%1%' to '%2%'", link, target);
 }
 
 
@@ -618,7 +601,7 @@ void replaceSymlink(const Path & target, const Path & link)
         }
 
         if (rename(tmp.c_str(), link.c_str()) != 0)
-            throw SysError(format("renaming '%1%' to '%2%'") % tmp % link);
+            throw SysError("renaming '%1%' to '%2%'", tmp, link);
 
         break;
     }
@@ -723,7 +706,7 @@ AutoDelete::~AutoDelete()
                 deletePath(path);
             else {
                 if (remove(path.c_str()) == -1)
-                    throw SysError(format("cannot unlink '%1%'") % path);
+                    throw SysError("cannot unlink '%1%'", path);
             }
         }
     } catch (...) {
@@ -789,7 +772,7 @@ void AutoCloseFD::close()
     if (fd != -1) {
         if (::close(fd) == -1)
             /* This should never happen. */
-            throw SysError(format("closing file descriptor %1%") % fd);
+            throw SysError("closing file descriptor %1%", fd);
     }
 }
 
@@ -862,7 +845,7 @@ int Pid::kill()
 {
     assert(pid != -1);
 
-    debug(format("killing process %1%") % pid);
+    debug("killing process %1%", pid);
 
     /* Send the requested signal to the child.  If it has its own
        process group, send the signal to every process in the child
@@ -874,7 +857,7 @@ int Pid::kill()
 #if __FreeBSD__ || __APPLE__
         if (errno != EPERM || ::kill(pid, 0) != 0)
 #endif
-            printError((SysError("killing process %d", pid).msg()));
+            logError(SysError("killing process %d", pid).info());
     }
 
     return wait();
@@ -920,7 +903,7 @@ pid_t Pid::release()
 
 void killUser(uid_t uid)
 {
-    debug(format("killing all processes running under uid '%1%'") % uid);
+    debug("killing all processes running under uid '%1%'", uid);
 
     assert(uid != 0); /* just to be safe... */
 
@@ -949,7 +932,7 @@ void killUser(uid_t uid)
 #endif
             if (errno == ESRCH) break; /* no more processes */
             if (errno != EINTR)
-                throw SysError(format("cannot kill processes for uid '%1%'") % uid);
+                throw SysError("cannot kill processes for uid '%1%'", uid);
         }
 
         _exit(0);
@@ -957,7 +940,7 @@ void killUser(uid_t uid)
 
     int status = pid.wait();
     if (status != 0)
-        throw Error(format("cannot kill processes for uid '%1%': %2%") % uid % statusToString(status));
+        throw Error("cannot kill processes for uid '%1%': %2%", uid, statusToString(status));
 
     /* !!! We should really do some check to make sure that there are
        no processes left running under `uid', but there is no portable
@@ -989,7 +972,7 @@ pid_t startProcess(std::function<void()> fun, const ProcessOptions & options)
 {
     auto wrapper = [&]() {
         if (!options.allowVfork)
-            logger = makeDefaultLogger();
+            logger = makeSimpleLogger();
         try {
 #if __linux__
             if (options.dieWithParent && prctl(PR_SET_PDEATHSIG, SIGKILL) == -1)
@@ -1216,7 +1199,7 @@ void _interrupted()
     /* Block user interrupts while an exception is being handled.
        Throwing an exception while another exception is being handled
        kills the program! */
-    if (!interruptThrown && !std::uncaught_exception()) {
+    if (!interruptThrown && !std::uncaught_exceptions()) {
         interruptThrown = true;
         throw Interrupted("interrupted by the user");
     }
@@ -1314,7 +1297,7 @@ bool statusOk(int status)
 }
 
 
-bool hasPrefix(const string & s, const string & prefix)
+bool hasPrefix(std::string_view s, std::string_view prefix)
 {
     return s.compare(0, prefix.size(), prefix) == 0;
 }
@@ -1351,7 +1334,7 @@ void ignoreException()
     try {
         throw;
     } catch (std::exception & e) {
-        printError(format("error (ignored): %1%") % e.what());
+        printError("error (ignored): %1%", e.what());
     }
 }
 
@@ -1408,7 +1391,7 @@ std::string filterANSIEscapes(const std::string & s, bool filterAll, unsigned in
 static char base64Chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
 
-string base64Encode(const string & s)
+string base64Encode(std::string_view s)
 {
     string res;
     int data = 0, nbits = 0;
@@ -1429,7 +1412,7 @@ string base64Encode(const string & s)
 }
 
 
-string base64Decode(const string & s)
+string base64Decode(std::string_view s)
 {
     bool init = false;
     char decode[256];
@@ -1464,17 +1447,6 @@ string base64Decode(const string & s)
 }
 
 
-void callFailure(const std::function<void(std::exception_ptr exc)> & failure, std::exception_ptr exc)
-{
-    try {
-        failure(exc);
-    } catch (std::exception & e) {
-        printError(format("uncaught exception: %s") % e.what());
-        abort();
-    }
-}
-
-
 static Sync<std::pair<unsigned short, unsigned short>> windowSize{{0, 0}};
 
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 750957d54..3641daaec 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "types.hh"
+#include "error.hh"
 #include "logging.hh"
 #include "ansicolor.hh"
 
@@ -416,7 +417,7 @@ template<class N> bool string2Float(const string & s, N & n)
 
 
 /* Return true iff `s' starts with `prefix'. */
-bool hasPrefix(const string & s, const string & prefix);
+bool hasPrefix(std::string_view s, std::string_view prefix);
 
 
 /* Return true iff `s' ends in `suffix'. */
@@ -454,13 +455,12 @@ std::string filterANSIEscapes(const std::string & s,
     unsigned int width = std::numeric_limits<unsigned int>::max());
 
 
-/* Base::Base64 encoding/decoding. */
-string base64Encode(const string & s);
-string base64Decode(const string & s);
+/* Base64 encoding/decoding. */
+string base64Encode(std::string_view s);
+string base64Decode(std::string_view s);
 
 
-/* Get a value for the specified key from an associate container, or a
-   default value if the key doesn't exist. */
+/* Get a value for the specified key from an associate container. */
 template <class T>
 std::optional<typename T::mapped_type> get(const T & map, const typename T::key_type & key)
 {