Remove FormatOrString and remaining uses of format()

40 files changed, 102 insertions, 125 deletions

diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 174435e7c..63e3e3fa9 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -219,7 +219,7 @@ static int main_build_remote(int argc, char * * argv)
                                 % concatStringsSep<StringSet>(", ", m.supportedFeatures)
                                 % concatStringsSep<StringSet>(", ", m.mandatoryFeatures);
 
-                        printMsg(couldBuildLocally ? lvlChatty : lvlWarn, error);
+                        printMsg(couldBuildLocally ? lvlChatty : lvlWarn, error.str());
 
                         std::cerr << "# decline\n";
                     }
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 21fc4d0fe..2721b6733 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -368,7 +368,7 @@ void initGC()
             size = (pageSize * pages) / 4; // 25% of RAM
         if (size > maxSize) size = maxSize;
 #endif
-        debug(format("setting initial heap size to %1% bytes") % size);
+        debug("setting initial heap size to %1% bytes", size);
         GC_expand_hp(size);
     }
 
@@ -609,7 +609,7 @@ Path EvalState::checkSourcePath(const Path & path_)
     }
 
     /* Resolve symlinks. */
-    debug(format("checking access to '%s'") % abspath);
+    debug("checking access to '%s'", abspath);
     Path path = canonPath(abspath, true);
 
     for (auto & i : *allowedPaths) {
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index dec5818fc..0f75ed9a0 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -732,7 +732,7 @@ Expr * EvalState::parseExprFromString(std::string s, const Path & basePath)
 
 Expr * EvalState::parseStdin()
 {
-    //Activity act(*logger, lvlTalkative, format("parsing standard input"));
+    //Activity act(*logger, lvlTalkative, "parsing standard input");
     auto buffer = drainFD(0);
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
@@ -835,7 +835,7 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
         }
     }
 
-    debug(format("resolved search path element '%s' to '%s'") % elem.second % res.second);
+    debug("resolved search path element '%s' to '%s'", elem.second, res.second);
 
     searchPathResolved[elem.second] = res;
     return res;
diff --git a/src/libexpr/tests/primops.cc b/src/libexpr/tests/primops.cc
index e1d3ac503..ce3b5d11f 100644
--- a/src/libexpr/tests/primops.cc
+++ b/src/libexpr/tests/primops.cc
@@ -15,8 +15,8 @@ namespace nix {
                 return oss.str();
             }
 
-            void log(Verbosity lvl, const FormatOrString & fs) override {
-                oss << fs.s << std::endl;
+            void log(Verbosity lvl, std::string_view s) override {
+                oss << s << std::endl;
             }
 
             void logEI(const ErrorInfo & ei) override {
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index 3f6222768..341c8922f 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -26,8 +26,8 @@ static void posToXML(EvalState & state, XMLAttrs & xmlAttrs, const Pos & pos)
 {
     if (auto path = std::get_if<Path>(&pos.origin))
         xmlAttrs["path"] = *path;
-    xmlAttrs["line"] = (format("%1%") % pos.line).str();
-    xmlAttrs["column"] = (format("%1%") % pos.column).str();
+    xmlAttrs["line"] = fmt("%1%", pos.line);
+    xmlAttrs["column"] = fmt("%1%", pos.column);
 }
 
 
@@ -64,7 +64,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
     switch (v.type()) {
 
         case nInt:
-            doc.writeEmptyElement("int", singletonAttrs("value", (format("%1%") % v.integer).str()));
+            doc.writeEmptyElement("int", singletonAttrs("value", fmt("%1%", v.integer)));
             break;
 
         case nBool:
@@ -156,7 +156,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             break;
 
         case nFloat:
-            doc.writeEmptyElement("float", singletonAttrs("value", (format("%1%") % v.fpoint).str()));
+            doc.writeEmptyElement("float", singletonAttrs("value", fmt("%1%", v.fpoint)));
             break;
 
         case nThunk:
diff --git a/src/libmain/progress-bar.cc b/src/libmain/progress-bar.cc
index e9205a5e5..024259584 100644
--- a/src/libmain/progress-bar.cc
+++ b/src/libmain/progress-bar.cc
@@ -125,11 +125,11 @@ public:
         return printBuildLogs;
     }
 
-    void log(Verbosity lvl, const FormatOrString & fs) override
+    void log(Verbosity lvl, std::string_view s) override
     {
         if (lvl > verbosity) return;
         auto state(state_.lock());
-        log(*state, lvl, fs.s);
+        log(*state, lvl, s);
     }
 
     void logEI(const ErrorInfo & ei) override
@@ -142,7 +142,7 @@ public:
         log(*state, ei.level, oss.str());
     }
 
-    void log(State & state, Verbosity lvl, const std::string & s)
+    void log(State & state, Verbosity lvl, std::string_view s)
     {
         if (state.active) {
             writeToStderr("\r\e[K" + filterANSIEscapes(s, !isTTY) + ANSI_NORMAL "\n");
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index d4871a8e2..27552d5bf 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -347,7 +347,7 @@ void parseCmdLine(const std::string & programName, const Strings & args,
 
 void printVersion(const std::string & programName)
 {
-    std::cout << format("%1% (Nix) %2%") % programName % nixVersion << std::endl;
+    std::cout << fmt("%1% (Nix) %2%", programName, nixVersion) << std::endl;
     if (verbosity > lvlInfo) {
         Strings cfg;
 #if HAVE_BOEHMGC
diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 2021d0023..38b73d531 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -732,7 +732,7 @@ void replaceValidPath(const Path & storePath, const Path & tmpPath)
        tmpPath (the replacement), so we have to move it out of the
        way first.  We'd better not be interrupted here, because if
        we're repairing (say) Glibc, we end up with a broken system. */
-    Path oldPath = (format("%1%.old-%2%-%3%") % storePath % getpid() % random()).str();
+    Path oldPath = fmt("%1%.old-%2%-%3%", storePath, getpid(), random());
     if (pathExists(storePath))
         movePath(storePath, oldPath);
 
diff --git a/src/libstore/build/goal.cc b/src/libstore/build/goal.cc
index 58e805f55..d59b94797 100644
--- a/src/libstore/build/goal.cc
+++ b/src/libstore/build/goal.cc
@@ -78,9 +78,9 @@ void Goal::amDone(ExitCode result, std::optional<Error> ex)
 }
 
 
-void Goal::trace(const FormatOrString & fs)
+void Goal::trace(std::string_view s)
 {
-    debug("%1%: %2%", name, fs.s);
+    debug("%1%: %2%", name, s);
 }
 
 }
diff --git a/src/libstore/build/goal.hh b/src/libstore/build/goal.hh
index 35121c5d9..776eb86bc 100644
--- a/src/libstore/build/goal.hh
+++ b/src/libstore/build/goal.hh
@@ -88,7 +88,7 @@ struct Goal : public std::enable_shared_from_this<Goal>
         abort();
     }
 
-    void trace(const FormatOrString & fs);
+    void trace(std::string_view s);
 
     std::string getName()
     {
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index a961d8eed..d17715efd 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -650,7 +650,7 @@ void LocalDerivationGoal::startBuilder()
         /* Clean up the chroot directory automatically. */
         autoDelChroot = std::make_shared<AutoDelete>(chrootRootDir);
 
-        printMsg(lvlChatty, format("setting up chroot environment in '%1%'") % chrootRootDir);
+        printMsg(lvlChatty, "setting up chroot environment in '%1%'", chrootRootDir);
 
         // FIXME: make this 0700
         if (mkdir(chrootRootDir.c_str(), buildUser && buildUser->getUIDCount() != 1 ? 0755 : 0750) == -1)
@@ -753,8 +753,7 @@ void LocalDerivationGoal::startBuilder()
         throw Error("home directory '%1%' exists; please remove it to assure purity of builds without sandboxing", homeDir);
 
     if (useChroot && settings.preBuildHook != "" && dynamic_cast<Derivation *>(drv.get())) {
-        printMsg(lvlChatty, format("executing pre-build hook '%1%'")
-            % settings.preBuildHook);
+        printMsg(lvlChatty, "executing pre-build hook '%1%'", settings.preBuildHook);
         auto args = useChroot ? Strings({worker.store.printStorePath(drvPath), chrootRootDir}) :
             Strings({ worker.store.printStorePath(drvPath) });
         enum BuildHookState {
@@ -1104,7 +1103,7 @@ void LocalDerivationGoal::initEnv()
     env["NIX_STORE"] = worker.store.storeDir;
 
     /* The maximum number of cores to utilize for parallel building. */
-    env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();
+    env["NIX_BUILD_CORES"] = fmt("%d", settings.buildCores);
 
     initTmpDir();
 
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 5e6fd011f..7f8b0f905 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -67,12 +67,12 @@ struct TunnelLogger : public Logger
             state->pendingMsgs.push_back(s);
     }
 
-    void log(Verbosity lvl, const FormatOrString & fs) override
+    void log(Verbosity lvl, std::string_view s) override
     {
         if (lvl > verbosity) return;
 
         StringSink buf;
-        buf << STDERR_NEXT << (fs.s + "\n");
+        buf << STDERR_NEXT << (s + "\n");
         enqueueMsg(buf.s);
     }
 
diff --git a/src/libstore/export-import.cc b/src/libstore/export-import.cc
index 9875da909..4eb838b68 100644
--- a/src/libstore/export-import.cc
+++ b/src/libstore/export-import.cc
@@ -16,7 +16,7 @@ void Store::exportPaths(const StorePathSet & paths, Sink & sink)
     //logger->incExpected(doneLabel, sorted.size());
 
     for (auto & path : sorted) {
-        //Activity act(*logger, lvlInfo, format("exporting path '%s'") % path);
+        //Activity act(*logger, lvlInfo, "exporting path '%s'", path);
         sink << 1;
         exportPath(path, sink);
         //logger->incProgress(doneLabel);
@@ -71,7 +71,7 @@ StorePaths Store::importPaths(Source & source, CheckSigsFlag checkSigs)
 
         auto path = parseStorePath(readString(source));
 
-        //Activity act(*logger, lvlInfo, format("importing path '%s'") % info.path);
+        //Activity act(*logger, lvlInfo, "importing path '%s'", info.path);
 
         auto references = worker_proto::read(*this, source, Phantom<StorePathSet> {});
         auto deriver = readString(source);
diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index 1c8676a59..404f980b6 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -183,7 +183,7 @@ struct curlFileTransfer : public FileTransfer
         {
             size_t realSize = size * nmemb;
             std::string line((char *) contents, realSize);
-            printMsg(lvlVomit, format("got header for '%s': %s") % request.uri % trim(line));
+            printMsg(lvlVomit, "got header for '%s': %s", request.uri, trim(line));
             static std::regex statusLine("HTTP/[^ ]+ +[0-9]+(.*)", std::regex::extended | std::regex::icase);
             std::smatch match;
             if (std::regex_match(line, match, statusLine)) {
@@ -207,7 +207,7 @@ struct curlFileTransfer : public FileTransfer
                         long httpStatus = 0;
                         curl_easy_getinfo(req, CURLINFO_RESPONSE_CODE, &httpStatus);
                         if (result.etag == request.expectedETag && httpStatus == 200) {
-                            debug(format("shutting down on 200 HTTP response with expected ETag"));
+                            debug("shutting down on 200 HTTP response with expected ETag");
                             return 0;
                         }
                     } else if (name == "content-encoding")
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 996f26a95..0aecc2d3b 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -34,8 +34,7 @@ static void makeSymlink(const Path & link, const Path & target)
     createDirs(dirOf(link));
 
     /* Create the new symlink. */
-    Path tempLink = (format("%1%.tmp-%2%-%3%")
-        % link % getpid() % random()).str();
+    Path tempLink = fmt("%1%.tmp-%2%-%3%", link, getpid(), random());
     createSymlink(target, tempLink);
 
     /* Atomically replace the old one. */
@@ -197,7 +196,7 @@ void LocalStore::findTempRoots(Roots & tempRoots, bool censor)
 
         pid_t pid = std::stoi(i.name);
 
-        debug(format("reading temporary root file '%1%'") % path);
+        debug("reading temporary root file '%1%'", path);
         AutoCloseFD fd(open(path.c_str(), O_CLOEXEC | O_RDWR, 0666));
         if (!fd) {
             /* It's okay if the file has disappeared. */
@@ -263,7 +262,7 @@ void LocalStore::findRoots(const Path & path, unsigned char type, Roots & roots)
                 target = absPath(target, dirOf(path));
                 if (!pathExists(target)) {
                     if (isInDir(path, stateDir + "/" + gcRootsDir + "/auto")) {
-                        printInfo(format("removing stale link from '%1%' to '%2%'") % path % target);
+                        printInfo("removing stale link from '%1%' to '%2%'", path, target);
                         unlink(path.c_str());
                     }
                 } else {
@@ -863,7 +862,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                 continue;
             }
 
-            printMsg(lvlTalkative, format("deleting unused link '%1%'") % path);
+            printMsg(lvlTalkative, "deleting unused link '%1%'", path);
 
             if (unlink(path.c_str()) == -1)
                 throw SysError("deleting '%1%'", path);
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 82edaa9bf..c9a466ee8 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -280,7 +280,7 @@ LocalStore::LocalStore(const Params & params)
     else if (curSchema == 0) { /* new store */
         curSchema = nixSchemaVersion;
         openDB(*state, true);
-        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str(), 0666, true);
+        writeFile(schemaPath, fmt("%1%", nixSchemaVersion), 0666, true);
     }
 
     else if (curSchema < nixSchemaVersion) {
@@ -329,7 +329,7 @@ LocalStore::LocalStore(const Params & params)
             txn.commit();
         }
 
-        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str(), 0666, true);
+        writeFile(schemaPath, fmt("%1%", nixSchemaVersion), 0666, true);
 
         lockFile(globalLock.get(), ltRead, true);
     }
@@ -1560,7 +1560,7 @@ void LocalStore::invalidatePathChecked(const StorePath & path)
 
 bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 {
-    printInfo(format("reading the Nix store..."));
+    printInfo("reading the Nix store...");
 
     bool errors = false;
 
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index 4d2781180..4a79cf4a1 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -55,7 +55,7 @@ LocalStore::InodeHash LocalStore::loadInodeHash()
     }
     if (errno) throw SysError("reading directory '%1%'", linksDir);
 
-    printMsg(lvlTalkative, format("loaded %1% hash inodes") % inodeHash.size());
+    printMsg(lvlTalkative, "loaded %1% hash inodes", inodeHash.size());
 
     return inodeHash;
 }
@@ -73,7 +73,7 @@ Strings LocalStore::readDirectoryIgnoringInodes(const Path & path, const InodeHa
         checkInterrupt();
 
         if (inodeHash.count(dirent->d_ino)) {
-            debug(format("'%1%' is already linked") % dirent->d_name);
+            debug("'%1%' is already linked", dirent->d_name);
             continue;
         }
 
@@ -102,7 +102,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
 
     if (std::regex_search(path, std::regex("\\.app/Contents/.+$")))
     {
-        debug(format("'%1%' is not allowed to be linked in macOS") % path);
+        debug("'%1%' is not allowed to be linked in macOS", path);
         return;
     }
 #endif
@@ -146,7 +146,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
        contents of the symlink (i.e. the result of readlink()), not
        the contents of the target (which may not even exist). */
     Hash hash = hashPath(htSHA256, path).first;
-    debug(format("'%1%' has hash '%2%'") % path % hash.to_string(Base32, true));
+    debug("'%1%' has hash '%2%'", path, hash.to_string(Base32, true));
 
     /* Check if this is a known hash. */
     Path linkPath = linksDir + "/" + hash.to_string(Base32, false);
@@ -196,11 +196,11 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
     auto stLink = lstat(linkPath);
 
     if (st.st_ino == stLink.st_ino) {
-        debug(format("'%1%' is already linked to '%2%'") % path % linkPath);
+        debug("'%1%' is already linked to '%2%'", path, linkPath);
         return;
     }
 
-    printMsg(lvlTalkative, format("linking '%1%' to '%2%'") % path % linkPath);
+    printMsg(lvlTalkative, "linking '%1%' to '%2%'", path, linkPath);
 
     /* Make the containing directory writable, but only if it's not
        the store itself (we don't want or need to mess with its
@@ -213,8 +213,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
        its timestamp back to 0. */
     MakeReadOnly makeReadOnly(mustToggle ? dirOfPath : "");
 
-    Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
-        % realStoreDir % getpid() % random()).str();
+    Path tempLink = fmt("%1%/.tmp-link-%2%-%3%", realStoreDir, getpid(), random());
 
     if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
         if (errno == EMLINK) {
@@ -222,7 +221,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
                systems).  This is likely to happen with empty files.
                Just shrug and ignore. */
             if (st.st_size)
-                printInfo(format("'%1%' has maximum number of links") % linkPath);
+                printInfo("'%1%' has maximum number of links", linkPath);
             return;
         }
         throw SysError("cannot link '%1%' to '%2%'", tempLink, linkPath);
diff --git a/src/libstore/pathlocks.cc b/src/libstore/pathlocks.cc
index 42023cd0a..adc763e6a 100644
--- a/src/libstore/pathlocks.cc
+++ b/src/libstore/pathlocks.cc
@@ -96,7 +96,7 @@ bool PathLocks::lockPaths(const PathSet & paths,
         checkInterrupt();
         Path lockPath = path + ".lock";
 
-        debug(format("locking path '%1%'") % path);
+        debug("locking path '%1%'", path);
 
         AutoCloseFD fd;
 
@@ -118,7 +118,7 @@ bool PathLocks::lockPaths(const PathSet & paths,
                 }
             }
 
-            debug(format("lock acquired on '%1%'") % lockPath);
+            debug("lock acquired on '%1%'", lockPath);
 
             /* Check that the lock file hasn't become stale (i.e.,
                hasn't been unlinked). */
@@ -130,7 +130,7 @@ bool PathLocks::lockPaths(const PathSet & paths,
                    a lock on a deleted file.  This means that other
                    processes may create and acquire a lock on
                    `lockPath', and proceed.  So we must retry. */
-                debug(format("open lock file '%1%' has become stale") % lockPath);
+                debug("open lock file '%1%' has become stale", lockPath);
             else
                 break;
         }
@@ -163,7 +163,7 @@ void PathLocks::unlock()
                 "error (ignored): cannot close lock file on '%1%'",
                 i.second);
 
-        debug(format("lock released on '%1%'") % i.second);
+        debug("lock released on '%1%'", i.second);
     }
 
     fds.clear();
diff --git a/src/libstore/profiles.cc b/src/libstore/profiles.cc
index c551c5f3e..179161ff7 100644
--- a/src/libstore/profiles.cc
+++ b/src/libstore/profiles.cc
@@ -64,7 +64,7 @@ std::pair<Generations, std::optional<GenerationNumber>> findGenerations(Path pro
 static void makeName(const Path & profile, GenerationNumber num,
     Path & outLink)
 {
-    Path prefix = (format("%1%-%2%") % profile % num).str();
+    Path prefix = fmt("%1%-%2%", profile, num);
     outLink = prefix + "-link";
 }
 
@@ -269,7 +269,7 @@ void switchGeneration(
 
 void lockProfile(PathLocks & lock, const Path & profile)
 {
-    lock.lockPaths({profile}, (format("waiting for lock on profile '%1%'") % profile).str());
+    lock.lockPaths({profile}, fmt("waiting for lock on profile '%1%'", profile));
     lock.setDeletion(true);
 }
 
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index 3bb297fc8..345f4528b 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -39,8 +39,7 @@ static void search(
         if (!match) continue;
         std::string ref(s.substr(i, refLength));
         if (hashes.erase(ref)) {
-            debug(format("found reference to '%1%' at offset '%2%'")
-                  % ref % i);
+            debug("found reference to '%1%' at offset '%2%'", ref, i);
             seen.insert(ref);
         }
         ++i;
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 8d76eee99..8006bd733 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -40,12 +40,12 @@ struct S3Error : public Error
 /* Helper: given an Outcome<R, E>, return R in case of success, or
    throw an exception in case of an error. */
 template<typename R, typename E>
-R && checkAws(const FormatOrString & fs, Aws::Utils::Outcome<R, E> && outcome)
+R && checkAws(std::string_view s, Aws::Utils::Outcome<R, E> && outcome)
 {
     if (!outcome.IsSuccess())
         throw S3Error(
             outcome.GetError().GetErrorType(),
-            fs.s + ": " + outcome.GetError().GetMessage());
+            s + ": " + outcome.GetError().GetMessage());
     return outcome.GetResultWithOwnership();
 }
 
@@ -430,9 +430,9 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStoreConfig, public virtual
         std::string marker;
 
         do {
-            debug(format("listing bucket 's3://%s' from key '%s'...") % bucketName % marker);
+            debug("listing bucket 's3://%s' from key '%s'...", bucketName, marker);
 
-            auto res = checkAws(format("AWS error listing bucket '%s'") % bucketName,
+            auto res = checkAws(fmt("AWS error listing bucket '%s'", bucketName),
                 s3Helper.client->ListObjects(
                     Aws::S3::Model::ListObjectsRequest()
                     .WithBucket(bucketName)
@@ -441,8 +441,8 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStoreConfig, public virtual
 
             auto & contents = res.GetContents();
 
-            debug(format("got %d keys, next marker '%s'")
-                % contents.size() % res.GetNextMarker());
+            debug("got %d keys, next marker '%s'",
+                contents.size(), res.GetNextMarker());
 
             for (auto object : contents) {
                 auto & key = object.GetKey();
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index f32c2d30c..226eb9113 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -790,13 +790,13 @@ std::string Store::makeValidityRegistration(const StorePathSet & paths,
 
         if (showHash) {
             s += info->narHash.to_string(Base16, false) + "\n";
-            s += (format("%1%\n") % info->narSize).str();
+            s += fmt("%1%\n", info->narSize);
         }
 
         auto deriver = showDerivers && info->deriver ? printStorePath(*info->deriver) : "";
         s += deriver + "\n";
 
-        s += (format("%1%\n") % info->references.size()).str();
+        s += fmt("%1%\n", info->references.size());
 
         for (auto & j : info->references)
             s += printStorePath(j) + "\n";
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 0e2b9d12c..268a798d9 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -87,7 +87,7 @@ static time_t dump(const Path & path, Sink & sink, PathFilter & filter)
                 std::string name(i.name);
                 size_t pos = i.name.find(caseHackSuffix);
                 if (pos != std::string::npos) {
-                    debug(format("removing case hack suffix from '%1%'") % (path + "/" + i.name));
+                    debug("removing case hack suffix from '%1%'", path + "/" + i.name);
                     name.erase(pos);
                 }
                 if (!unhacked.emplace(name, i.name).second)
@@ -262,7 +262,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                     if (archiveSettings.useCaseHack) {
                         auto i = names.find(name);
                         if (i != names.end()) {
-                            debug(format("case collision between '%1%' and '%2%'") % i->first % name);
+                            debug("case collision between '%1%' and '%2%'", i->first, name);
                             name += caseHackSuffix;
                             name += std::to_string(++i->second);
                         } else
diff --git a/src/libutil/filesystem.cc b/src/libutil/filesystem.cc
index 3a732cff8..56be76ecc 100644
--- a/src/libutil/filesystem.cc
+++ b/src/libutil/filesystem.cc
@@ -15,9 +15,9 @@ static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
 {
     tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
     if (includePid)
-        return (format("%1%/%2%-%3%-%4%") % tmpRoot % prefix % getpid() % counter++).str();
+        return fmt("%1%/%2%-%3%-%4%", tmpRoot, prefix, getpid(), counter++);
     else
-        return (format("%1%/%2%-%3%") % tmpRoot % prefix % counter++).str();
+        return fmt("%1%/%2%-%3%", tmpRoot, prefix, counter++);
 }
 
 Path createTempDir(const Path & tmpRoot, const Path & prefix,
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index e879fd3b8..e11426b88 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -17,16 +17,6 @@ using boost::format;
 struct nop { template<typename... T> nop(T...) {} };
 
 
-struct FormatOrString
-{
-    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) { };
-};
-
-
 /* 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
@@ -53,11 +43,6 @@ 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)
 {
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 56bdaf87a..7cac75ce1 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -54,7 +54,7 @@ public:
         return printBuildLogs;
     }
 
-    void log(Verbosity lvl, const FormatOrString & fs) override
+    void log(Verbosity lvl, std::string_view s) override
     {
         if (lvl > verbosity) return;
 
@@ -72,7 +72,7 @@ public:
             prefix = std::string("<") + c + ">";
         }
 
-        writeToStderr(prefix + filterANSIEscapes(fs.s, !tty) + "\n");
+        writeToStderr(prefix + filterANSIEscapes(s, !tty) + "\n");
     }
 
     void logEI(const ErrorInfo & ei) override
@@ -174,12 +174,12 @@ struct JSONLogger : Logger {
         prevLogger.log(lvlError, "@nix " + json.dump(-1, ' ', false, nlohmann::json::error_handler_t::replace));
     }
 
-    void log(Verbosity lvl, const FormatOrString & fs) override
+    void log(Verbosity lvl, std::string_view s) override
     {
         nlohmann::json json;
         json["action"] = "msg";
         json["level"] = lvl;
-        json["msg"] = fs.s;
+        json["msg"] = s;
         write(json);
     }
 
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index a68edd15a..59a707eef 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -75,11 +75,11 @@ public:
     // Whether the logger prints the whole build log
     virtual bool isVerbose() { return false; }
 
-    virtual void log(Verbosity lvl, const FormatOrString & fs) = 0;
+    virtual void log(Verbosity lvl, std::string_view s) = 0;
 
-    void log(const FormatOrString & fs)
+    void log(std::string_view s)
     {
-        log(lvlInfo, fs);
+        log(lvlInfo, s);
     }
 
     virtual void logEI(const ErrorInfo & ei) = 0;
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index c653db9d0..7476e6f6c 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -415,7 +415,7 @@ Error readError(Source & source)
     auto msg = readString(source);
     ErrorInfo info {
         .level = level,
-        .msg = hintformat(std::move(format("%s") % msg)),
+        .msg = hintformat(fmt("%s", msg)),
     };
     auto havePos = readNum<size_t>(source);
     assert(havePos == 0);
@@ -424,7 +424,7 @@ Error readError(Source & source)
         havePos = readNum<size_t>(source);
         assert(havePos == 0);
         info.traces.push_back(Trace {
-            .hint = hintformat(std::move(format("%s") % readString(source)))
+            .hint = hintformat(fmt("%s", readString(source)))
         });
     }
     return Error(std::move(info));
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 885bae69c..e4af702a3 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -523,7 +523,7 @@ void deletePath(const Path & path)
 
 void deletePath(const Path & path, uint64_t & bytesFreed)
 {
-    //Activity act(*logger, lvlDebug, format("recursively deleting path '%1%'") % path);
+    //Activity act(*logger, lvlDebug, "recursively deleting path '%1%'", path);
     bytesFreed = 0;
     _deletePath(path, bytesFreed);
 }
@@ -1394,14 +1394,14 @@ std::string statusToString(int status)
 {
     if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
         if (WIFEXITED(status))
-            return (format("failed with exit code %1%") % WEXITSTATUS(status)).str();
+            return fmt("failed with exit code %1%", WEXITSTATUS(status));
         else if (WIFSIGNALED(status)) {
             int sig = WTERMSIG(status);
 #if HAVE_STRSIGNAL
             const char * description = strsignal(sig);
-            return (format("failed due to signal %1% (%2%)") % sig % description).str();
+            return fmt("failed due to signal %1% (%2%)", sig, description);
 #else
-            return (format("failed due to signal %1%") % sig).str();
+            return fmt("failed due to signal %1%", sig);
 #endif
         }
         else
@@ -1470,7 +1470,7 @@ bool shouldANSI()
         && !getEnv("NO_COLOR").has_value();
 }
 
-std::string filterANSIEscapes(const std::string & s, bool filterAll, unsigned int width)
+std::string filterANSIEscapes(std::string_view s, bool filterAll, unsigned int width)
 {
     std::string t, e;
     size_t w = 0;
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index b5625ecef..75959d7e3 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -569,7 +569,7 @@ bool shouldANSI();
    some escape sequences (such as colour setting) are copied but not
    included in the character count. Also, tabs are expanded to
    spaces. */
-std::string filterANSIEscapes(const std::string & s,
+std::string filterANSIEscapes(std::string_view s,
     bool filterAll = false,
     unsigned int width = std::numeric_limits<unsigned int>::max());
 
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index da76c2ace..a4b3b1f96 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -219,9 +219,9 @@ static void main_nix_build(int argc, char * * argv)
                 // read the shebang to understand which packages to read from. Since
                 // this is handled via nix-shell -p, we wrap our ruby script execution
                 // in ruby -e 'load' which ignores the shebangs.
-                envCommand = (format("exec %1% %2% -e 'load(ARGV.shift)' -- %3% %4%") % execArgs % interpreter % shellEscape(script) % joined.str()).str();
+                envCommand = fmt("exec %1% %2% -e 'load(ARGV.shift)' -- %3% %4%", execArgs, interpreter, shellEscape(script), joined.str());
             } else {
-                envCommand = (format("exec %1% %2% %3% %4%") % execArgs % interpreter % shellEscape(script) % joined.str()).str();
+                envCommand = fmt("exec %1% %2% %3% %4%", execArgs, interpreter, shellEscape(script), joined.str());
             }
         }
 
diff --git a/src/nix-collect-garbage/nix-collect-garbage.cc b/src/nix-collect-garbage/nix-collect-garbage.cc
index e413faffe..8a4fdcb93 100644
--- a/src/nix-collect-garbage/nix-collect-garbage.cc
+++ b/src/nix-collect-garbage/nix-collect-garbage.cc
@@ -40,7 +40,7 @@ void removeOldGenerations(std::string dir)
                 throw;
             }
             if (link.find("link") != std::string::npos) {
-                printInfo(format("removing old generations of profile %1%") % path);
+                printInfo("removing old generations of profile %s", path);
                 if (deleteOlderThan != "")
                     deleteGenerationsOlderThan(path, deleteOlderThan, dryRun);
                 else
diff --git a/src/nix-copy-closure/nix-copy-closure.cc b/src/nix-copy-closure/nix-copy-closure.cc
index 841d50fd3..7f2bb93b6 100755
--- a/src/nix-copy-closure/nix-copy-closure.cc
+++ b/src/nix-copy-closure/nix-copy-closure.cc
@@ -22,7 +22,7 @@ static int main_nix_copy_closure(int argc, char ** argv)
                 printVersion("nix-copy-closure");
             else if (*arg == "--gzip" || *arg == "--bzip2" || *arg == "--xz") {
                 if (*arg != "--gzip")
-                    printMsg(lvlError, format("Warning: '%1%' is not implemented, falling back to gzip") % *arg);
+                    warn("'%1%' is not implemented, falling back to gzip", *arg);
                 gzip = true;
             } else if (*arg == "--from")
                 toMode = false;
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 0daf374de..3a012638b 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -500,7 +500,7 @@ static bool keep(DrvInfo & drv)
 static void installDerivations(Globals & globals,
     const Strings & args, const Path & profile)
 {
-    debug(format("installing derivations"));
+    debug("installing derivations");
 
     /* Get the set of user environment elements to be installed. */
     DrvInfos newElems, newElemsTmp;
@@ -579,7 +579,7 @@ typedef enum { utLt, utLeq, utEq, utAlways } UpgradeType;
 static void upgradeDerivations(Globals & globals,
     const Strings & args, UpgradeType upgradeType)
 {
-    debug(format("upgrading derivations"));
+    debug("upgrading derivations");
 
     /* Upgrade works as follows: we take all currently installed
        derivations, and for any derivation matching any selector, look
@@ -768,7 +768,7 @@ static void opSet(Globals & globals, Strings opFlags, Strings opArgs)
     if (globals.dryRun) return;
     globals.state->store->buildPaths(paths, globals.state->repair ? bmRepair : bmNormal);
 
-    debug(format("switching to new user environment"));
+    debug("switching to new user environment");
     Path generation = createGeneration(
         ref<LocalFSStore>(store2),
         globals.profile,
@@ -1093,7 +1093,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         try {
             if (i.hasFailed()) continue;
 
-            //Activity act(*logger, lvlDebug, format("outputting query result '%1%'") % i.attrPath);
+            //Activity act(*logger, lvlDebug, "outputting query result '%1%'", i.attrPath);
 
             if (globals.prebuiltOnly &&
                 !validPaths.count(i.queryOutPath()) &&
@@ -1229,11 +1229,11 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
                                 xml.writeEmptyElement("meta", attrs2);
                             } else if (v->type() == nInt) {
                                 attrs2["type"] = "int";
-                                attrs2["value"] = (format("%1%") % v->integer).str();
+                                attrs2["value"] = fmt("%1%", v->integer);
                                 xml.writeEmptyElement("meta", attrs2);
                             } else if (v->type() == nFloat) {
                                 attrs2["type"] = "float";
-                                attrs2["value"] = (format("%1%") % v->fpoint).str();
+                                attrs2["value"] = fmt("%1%", v->fpoint);
                                 xml.writeEmptyElement("meta", attrs2);
                             } else if (v->type() == nBool) {
                                 attrs2["type"] = "bool";
@@ -1337,11 +1337,11 @@ static void opListGenerations(Globals & globals, Strings opFlags, Strings opArgs
     for (auto & i : gens) {
         tm t;
         if (!localtime_r(&i.creationTime, &t)) throw Error("cannot convert time");
-        cout << format("%|4|   %|4|-%|02|-%|02| %|02|:%|02|:%|02|   %||\n")
-            % i.number
-            % (t.tm_year + 1900) % (t.tm_mon + 1) % t.tm_mday
-            % t.tm_hour % t.tm_min % t.tm_sec
-            % (i.number == curGen ? "(current)" : "");
+        logger->cout("%|4|   %|4|-%|02|-%|02| %|02|:%|02|:%|02|   %||",
+            i.number,
+            t.tm_year + 1900, t.tm_mon + 1, t.tm_mday,
+            t.tm_hour, t.tm_min, t.tm_sec,
+            i.number == curGen ? "(current)" : "");
     }
 }
 
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index cad7f9c88..745e9e174 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -41,7 +41,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
         if (auto drvPath = i.queryDrvPath())
             drvsToBuild.push_back({*drvPath});
 
-    debug(format("building user environment dependencies"));
+    debug("building user environment dependencies");
     state.store->buildPaths(
         toDerivedPaths(drvsToBuild),
         state.repair ? bmRepair : bmNormal);
@@ -159,7 +159,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
             return false;
         }
 
-        debug(format("switching to new user environment"));
+        debug("switching to new user environment");
         Path generation = createGeneration(ref<LocalFSStore>(store2), profile, topLevelOut);
         switchLink(profile, generation);
     }
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 3bbefedbe..52211e341 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -457,7 +457,7 @@ static void opPrintEnv(Strings opFlags, Strings opArgs)
     /* Print each environment variable in the derivation in a format
      * that can be sourced by the shell. */
     for (auto & i : drv.env)
-        cout << format("export %1%; %1%=%2%\n") % i.first % shellEscape(i.second);
+        logger->cout("export %1%; %1%=%2%\n", i.first, shellEscape(i.second));
 
     /* Also output the arguments.  This doesn't preserve whitespace in
        arguments. */
diff --git a/src/nix/daemon.cc b/src/nix/daemon.cc
index a22bccba1..7e4a7ba86 100644
--- a/src/nix/daemon.cc
+++ b/src/nix/daemon.cc
@@ -249,9 +249,9 @@ static void daemonLoop()
             if ((!trusted && !matchUser(user, group, allowedUsers)) || group == settings.buildUsersGroup)
                 throw Error("user '%1%' is not allowed to connect to the Nix daemon", user);
 
-            printInfo(format((std::string) "accepted connection from pid %1%, user %2%" + (trusted ? " (trusted)" : ""))
-                % (peer.pidKnown ? std::to_string(peer.pid) : "<unknown>")
-                % (peer.uidKnown ? user : "<unknown>"));
+            printInfo((std::string) "accepted connection from pid %1%, user %2%" + (trusted ? " (trusted)" : ""),
+                peer.pidKnown ? std::to_string(peer.pid) : "<unknown>",
+                peer.uidKnown ? user : "<unknown>");
 
             //  Fork a child to handle the connection.
             ProcessOptions options;
diff --git a/src/nix/doctor.cc b/src/nix/doctor.cc
index ea87e3d87..7da4549a1 100644
--- a/src/nix/doctor.cc
+++ b/src/nix/doctor.cc
@@ -18,7 +18,7 @@ std::string formatProtocol(unsigned int proto)
     if (proto) {
         auto major = GET_PROTOCOL_MAJOR(proto) >> 8;
         auto minor = GET_PROTOCOL_MINOR(proto);
-        return (format("%1%.%2%") % major % minor).str();
+        return fmt("%1%.%2%", major, minor);
     }
     return "unknown";
 }
diff --git a/src/nix/sigs.cc b/src/nix/sigs.cc
index ee27e3725..1431652e0 100644
--- a/src/nix/sigs.cc
+++ b/src/nix/sigs.cc
@@ -45,7 +45,7 @@ struct CmdCopySigs : StorePathsCommand
         //logger->setExpected(doneLabel, storePaths.size());
 
         auto doPath = [&](const Path & storePathS) {
-            //Activity act(*logger, lvlInfo, format("getting signatures for '%s'") % storePath);
+            //Activity act(*logger, lvlInfo, "getting signatures for '%s'", storePath);
 
             checkInterrupt();
 
diff --git a/src/resolve-system-dependencies/resolve-system-dependencies.cc b/src/resolve-system-dependencies/resolve-system-dependencies.cc
index c6023eb03..4ea268d24 100644
--- a/src/resolve-system-dependencies/resolve-system-dependencies.cc
+++ b/src/resolve-system-dependencies/resolve-system-dependencies.cc
@@ -157,13 +157,9 @@ int main(int argc, char ** argv)
 
         uname(&_uname);
 
-        auto cacheParentDir = (format("%1%/dependency-maps") % settings.nixStateDir).str();
+        auto cacheParentDir = fmt("%1%/dependency-maps", settings.nixStateDir);
 
-        cacheDir = (format("%1%/%2%-%3%-%4%")
-                % cacheParentDir
-                % _uname.machine
-                % _uname.sysname
-                % _uname.release).str();
+        cacheDir = fmt("%1%/%2%-%3%-%4%", cacheParentDir, _uname.machine, _uname.sysname, _uname.release);
 
         mkdir(cacheParentDir.c_str(), 0755);
         mkdir(cacheDir.c_str(), 0755);