More support for std::filepath in libnixutil

We're not replacing `Path` in exposed definitions in many cases, but
just adding alternatives. This will allow us to "top down" change `Path`
to `std::fileysystem::path`, and then we can remove the `Path`-using
utilities which will become unused.

Also add some test files which we forgot to include in the libutil unit
tests `meson.build`.

Co-Authored-By: siddhantCodes <siddhantk232@gmail.com>

src/libcmd/common-eval-args.cc

@@ -18,6 +18,8 @@
 
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 fetchers::Settings fetchSettings;
 
 static GlobalConfig::Register rFetchSettings(&fetchSettings);
@@ -119,8 +121,8 @@ MixEvalArgs::MixEvalArgs()
         .category = category,
         .labels = {"original-ref", "resolved-ref"},
         .handler = {[&](std::string _from, std::string _to) {
-            auto from = parseFlakeRef(fetchSettings, _from, absPath("."));
-            auto to = parseFlakeRef(fetchSettings, _to, absPath("."));
+            auto from = parseFlakeRef(fetchSettings, _from, fs::current_path().string());
+            auto to = parseFlakeRef(fetchSettings, _to, fs::current_path().string());
             fetchers::Attrs extraAttrs;
             if (to.subdir != "") extraAttrs["dir"] = to.subdir;
             fetchers::overrideRegistry(from.input, to.input, extraAttrs);

src/libcmd/installables.cc

@@ -31,6 +31,8 @@
 
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 void completeFlakeInputPath(
     AddCompletions & completions,
     ref<EvalState> evalState,
@@ -341,7 +343,7 @@ void completeFlakeRefWithFragment(
             auto flakeRefS = std::string(prefix.substr(0, hash));
 
             // TODO: ideally this would use the command base directory instead of assuming ".".
-            auto flakeRef = parseFlakeRef(fetchSettings, expandTilde(flakeRefS), absPath("."));
+            auto flakeRef = parseFlakeRef(fetchSettings, expandTilde(flakeRefS), fs::current_path().string());
 
             auto evalCache = openEvalCache(*evalState,
                 std::make_shared<flake::LockedFlake>(lockFlake(

src/libcmd/repl.cc

@@ -622,7 +622,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
                 // When missing, trigger the normal exception
                 // e.g. :doc builtins.foo
                 // behaves like
-                // nix-repl> builtins.foo      
+                // nix-repl> builtins.foo<tab>
                 // error: attribute 'foo' missing
                 evalString(arg, v);
                 assert(false);
@@ -720,7 +720,7 @@ void NixRepl::loadFlake(const std::string & flakeRefS)
     if (flakeRefS.empty())
         throw Error("cannot use ':load-flake' without a path specified. (Use '.' for the current working directory.)");
 
-    auto flakeRef = parseFlakeRef(fetchSettings, flakeRefS, absPath("."), true);
+    auto flakeRef = parseFlakeRef(fetchSettings, flakeRefS, std::filesystem::current_path().string(), true);
     if (evalSettings.pureEval && !flakeRef.input.isLocked())
         throw Error("cannot use ':load-flake' on locked flake reference '%s' (use --impure to override)", flakeRefS);
 

src/libexpr/parser.y

@@ -350,7 +350,7 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, state->basePath.path.abs()));
+    Path path(absPath(std::string_view{$1.p, $1.l}, state->basePath.path.abs()));
     /* add back in the trailing '/' to the first segment */
     if ($1.p[$1.l-1] == '/' && $1.l > 1)
       path += "/";

src/libstore/gc.cc

@@ -333,7 +333,7 @@ static std::string quoteRegexChars(const std::string & raw)
 }
 
 #if __linux__
-static void readFileRoots(const char * path, UncheckedRoots & roots)
+static void readFileRoots(const std::filesystem::path & path, UncheckedRoots & roots)
 {
     try {
         roots[readFile(path)].emplace(path);

src/libstore/local-overlay-store.cc

@@ -31,7 +31,7 @@ LocalOverlayStore::LocalOverlayStore(std::string_view scheme, PathView path, con
     if (checkMount.get()) {
         std::smatch match;
         std::string mountInfo;
-        auto mounts = readFile("/proc/self/mounts");
+        auto mounts = readFile(std::filesystem::path{"/proc/self/mounts"});
         auto regex = std::regex(R"((^|\n)overlay )" + realStoreDir.get() + R"( .*(\n|$))");
 
         // Mount points can be stacked, so there might be multiple matching entries.

src/libutil/args.hh

@@ -113,6 +113,16 @@ protected:
             , arity(1)
         { }
 
+        Handler(std::filesystem::path * dest)
+            : fun([dest](std::vector<std::string> ss) { *dest = ss[0]; })
+            , arity(1)
+        { }
+
+        Handler(std::optional<std::filesystem::path> * dest)
+            : fun([dest](std::vector<std::string> ss) { *dest = ss[0]; })
+            , arity(1)
+        { }
+
         template<class T>
         Handler(T * dest, const T & val)
             : fun([dest, val](std::vector<std::string> ss) { *dest = val; })
@@ -283,6 +293,18 @@ public:
         });
     }
 
+    /**
+     * Expect a path argument.
+     */
+    void expectArg(const std::string & label, std::filesystem::path * dest, bool optional = false)
+    {
+        expectArgs({
+            .label = label,
+            .optional = optional,
+            .handler = {dest}
+        });
+    }
+
     /**
      * Expect 0 or more arguments.
      */

src/libutil/exec.hh

@@ -10,9 +10,6 @@ namespace nix {
  *
  * We use our own implementation unconditionally for consistency.
  */
-int execvpe(
-    const OsString::value_type * file0,
-    const OsString::value_type * const argv[],
-    const OsString::value_type * const envp[]);
+int execvpe(const OsChar * file0, const OsChar * const argv[], const OsChar * const envp[]);
 
 }

src/libutil/executable-path.cc

@@ -6,7 +6,9 @@
 
 namespace nix {
 
-namespace fs = std::filesystem;
+namespace fs {
+using namespace std::filesystem;
+}
 
 constexpr static const OsStringView path_var_separator{
     &ExecutablePath::separator,
@@ -24,7 +26,7 @@ ExecutablePath ExecutablePath::load()
 ExecutablePath ExecutablePath::parse(const OsString & path)
 {
     auto strings = path.empty() ? (std::list<OsString>{})
-                                : basicSplitString<std::list<OsString>, OsString::value_type>(path, path_var_separator);
+                                : basicSplitString<std::list<OsString>, OsChar>(path, path_var_separator);
 
     std::vector<fs::path> ret;
     ret.reserve(strings.size());

src/libutil/executable-path.hh

@@ -7,11 +7,15 @@ namespace nix {
 
 MakeError(ExecutableLookupError, Error);
 
+/**
+ * @todo rename, it is not just good for execuatable paths, but also
+ * other lists of paths.
+ */
 struct ExecutablePath
 {
     std::vector<std::filesystem::path> directories;
 
-    constexpr static const OsString::value_type separator =
+    constexpr static const OsChar separator =
 #ifdef WIN32
         L';'
 #else

src/libutil/file-system.cc

@@ -26,10 +26,10 @@
 
 #include "strings-inline.hh"
 
-namespace fs = std::filesystem;
-
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 /**
  * Treat the string as possibly an absolute path, by inspecting the
  * start of it. Return whether it was probably intended to be
@@ -73,6 +73,10 @@ Path absPath(PathView path, std::optional<PathView> dir, bool resolveSymlinks)
     return canonPath(path, resolveSymlinks);
 }
 
+std::filesystem::path absPath(const std::filesystem::path & path, bool resolveSymlinks)
+{
+    return absPath(path.string(), std::nullopt, resolveSymlinks);
+}
 
 Path canonPath(PathView path, bool resolveSymlinks)
 {
@@ -206,10 +210,10 @@ bool pathExists(const Path & path)
     return maybeLstat(path).has_value();
 }
 
-bool pathAccessible(const Path & path)
+bool pathAccessible(const std::filesystem::path & path)
 {
     try {
-        return pathExists(path);
+        return pathExists(path.string());
     } catch (SysError & e) {
         // swallow EPERM
         if (e.errNo == EPERM) return false;
@@ -238,6 +242,11 @@ std::string readFile(const Path & path)
     return readFile(fd.get());
 }
 
+std::string readFile(const std::filesystem::path & path)
+{
+    return readFile(os_string_to_string(PathViewNG { path }));
+}
+
 
 void readFile(const Path & path, Sink & sink)
 {
@@ -324,7 +333,7 @@ void recursiveSync(const Path & path)
     /* If it's a file, just fsync and return. */
     auto st = lstat(path);
     if (S_ISREG(st.st_mode)) {
-        AutoCloseFD fd = open(path.c_str(), O_RDONLY, 0);
+        AutoCloseFD fd = toDescriptor(open(path.c_str(), O_RDONLY, 0));
         if (!fd)
             throw SysError("opening file '%1%'", path);
         fd.fsync();
@@ -344,7 +353,7 @@ void recursiveSync(const Path & path)
             if (fs::is_directory(st)) {
                 dirsToEnumerate.emplace_back(entry.path());
             } else if (fs::is_regular_file(st)) {
-                AutoCloseFD fd = open(entry.path().c_str(), O_RDONLY, 0);
+                AutoCloseFD fd = toDescriptor(open(entry.path().string().c_str(), O_RDONLY, 0));
                 if (!fd)
                     throw SysError("opening file '%1%'", entry.path());
                 fd.fsync();
@@ -355,7 +364,7 @@ void recursiveSync(const Path & path)
 
     /* Fsync all the directories. */
     for (auto dir = dirsToFsync.rbegin(); dir != dirsToFsync.rend(); ++dir) {
-        AutoCloseFD fd = open(dir->c_str(), O_RDONLY, 0);
+        AutoCloseFD fd = toDescriptor(open(dir->string().c_str(), O_RDONLY, 0));
         if (!fd)
             throw SysError("opening directory '%1%'", *dir);
         fd.fsync();
@@ -595,19 +604,20 @@ void createSymlink(const Path & target, const Path & link)
     fs::create_symlink(target, link);
 }
 
-void replaceSymlink(const Path & target, const Path & link)
+void replaceSymlink(const fs::path & target, const fs::path & link)
 {
     for (unsigned int n = 0; true; n++) {
-        Path tmp = canonPath(fmt("%s/.%d_%s", dirOf(link), n, baseNameOf(link)));
+        auto tmp = link.parent_path() / fs::path{fmt(".%d_%s", n, link.filename().string())};
+        tmp = tmp.lexically_normal();
 
         try {
-            createSymlink(target, tmp);
+            fs::create_symlink(target, tmp);
         } catch (fs::filesystem_error & e) {
             if (e.code() == std::errc::file_exists) continue;
             throw;
         }
 
-        std::filesystem::rename(tmp, link);
+        fs::rename(tmp, link);
 
         break;
     }

src/libutil/file-system.hh

@@ -46,16 +46,33 @@ struct Source;
  * @return An absolutized path, resolving paths relative to the
  * specified directory, or the current directory otherwise.  The path
  * is also canonicalised.
+ *
+ * In the process of being deprecated for `std::filesystem::absolute`.
  */
 Path absPath(PathView path,
     std::optional<PathView> dir = {},
     bool resolveSymlinks = false);
 
+inline Path absPath(const Path & path,
+    std::optional<PathView> dir = {},
+    bool resolveSymlinks = false)
+{
+    return absPath(PathView{path}, dir, resolveSymlinks);
+}
+
+std::filesystem::path absPath(const std::filesystem::path & path,
+    bool resolveSymlinks = false);
+
 /**
  * Canonicalise a path by removing all `.` or `..` components and
  * double or trailing slashes.  Optionally resolves all symlink
  * components such that each component of the resulting path is *not*
  * a symbolic link.
+ *
+ * In the process of being deprecated for
+ * `std::filesystem::path::lexically_normal` (for the `resolveSymlinks =
+ * false` case), and `std::filesystem::weakly_canonical` (for the
+ * `resolveSymlinks = true` case).
  */
 Path canonPath(PathView path, bool resolveSymlinks = false);
 
@@ -64,12 +81,18 @@ Path canonPath(PathView path, bool resolveSymlinks = false);
  * everything before the final `/`.  If the path is the root or an
  * immediate child thereof (e.g., `/foo`), this means `/`
  * is returned.
+ *
+ * In the process of being deprecated for
+ * `std::filesystem::path::parent_path`.
  */
 Path dirOf(const PathView path);
 
 /**
  * @return the base name of the given canonical path, i.e., everything
  * following the final `/` (trailing slashes are removed).
+ *
+ * In the process of being deprecated for
+ * `std::filesystem::path::filename`.
  */
 std::string_view baseNameOf(std::string_view path);
 
@@ -98,20 +121,42 @@ std::optional<struct stat> maybeLstat(const Path & path);
 
 /**
  * @return true iff the given path exists.
+ *
+ * In the process of being deprecated for `fs::symlink_exists`.
  */
 bool pathExists(const Path & path);
 
+namespace fs {
+
+/**
+ *  ```
+ *  symlink_exists(p) = std::filesystem::exists(std::filesystem::symlink_status(p))
+ *  ```
+ *  Missing convenience analogous to
+ *  ```
+ *  std::filesystem::exists(p) = std::filesystem::exists(std::filesystem::status(p))
+ *  ```
+ */
+inline bool symlink_exists(const std::filesystem::path & path) {
+    return std::filesystem::exists(std::filesystem::symlink_status(path));
+}
+
+} // namespace fs
+
 /**
  * A version of pathExists that returns false on a permission error.
  * Useful for inferring default paths across directories that might not
  * be readable.
  * @return true iff the given path can be accessed and exists
  */
-bool pathAccessible(const Path & path);
+bool pathAccessible(const std::filesystem::path & path);
 
 /**
  * Read the contents (target) of a symbolic link.  The result is not
  * in any way canonicalised.
+ *
+ * In the process of being deprecated for
+ * `std::filesystem::read_symlink`.
  */
 Path readLink(const Path & path);
 
@@ -124,14 +169,23 @@ Descriptor openDirectory(const std::filesystem::path & path);
  * Read the contents of a file into a string.
  */
 std::string readFile(const Path & path);
+std::string readFile(const std::filesystem::path & path);
 void readFile(const Path & path, Sink & sink);
 
 /**
  * Write a string to a file.
  */
 void writeFile(const Path & path, std::string_view s, mode_t mode = 0666, bool sync = false);
+static inline void writeFile(const std::filesystem::path & path, std::string_view s, mode_t mode = 0666, bool sync = false)
+{
+    return writeFile(path.string(), s, mode, sync);
+}
 
 void writeFile(const Path & path, Source & source, mode_t mode = 0666, bool sync = false);
+static inline void writeFile(const std::filesystem::path & path, Source & source, mode_t mode = 0666, bool sync = false)
+{
+    return writeFile(path.string(), source, mode, sync);
+}
 
 /**
  * Flush a path's parent directory to disk.
@@ -154,6 +208,9 @@ void deletePath(const std::filesystem::path & path, uint64_t & bytesFreed);
 
 /**
  * Create a directory and all its parents, if necessary.
+ *
+ * In the process of being deprecated for
+ * `std::filesystem::create_directories`.
  */
 void createDirs(const Path & path);
 inline void createDirs(PathView path)
@@ -192,13 +249,21 @@ void setWriteTime(const std::filesystem::path & path, const struct stat & st);
 
 /**
  * Create a symlink.
+ *
+ * In the process of being deprecated for
+ * `std::filesystem::create_symlink`.
  */
 void createSymlink(const Path & target, const Path & link);
 
 /**
  * Atomically create or replace a symlink.
  */
-void replaceSymlink(const Path & target, const Path & link);
+void replaceSymlink(const std::filesystem::path & target, const std::filesystem::path & link);
+
+inline void replaceSymlink(const Path & target, const Path & link)
+{
+    return replaceSymlink(std::filesystem::path{target}, std::filesystem::path{link});
+}
 
 /**
  * Similar to 'renameFile', but fallback to a copy+remove if `src` and `dst`

src/libutil/linux/namespaces.cc

@@ -118,7 +118,7 @@ void saveMountNamespace()
 void restoreMountNamespace()
 {
     try {
-        auto savedCwd = absPath(".");
+        auto savedCwd = std::filesystem::current_path();
 
         if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
             throw SysError("restoring parent mount namespace");

src/libutil/os-string.hh

@@ -11,21 +11,30 @@ namespace nix {
  * Named because it is similar to the Rust type, except it is in the
  * native encoding not WTF-8.
  *
- * Same as `std::filesystem::path::string_type`, but manually defined to
+ * Same as `std::filesystem::path::value_type`, but manually defined to
  * avoid including a much more complex header.
  */
-using OsString = std::basic_string<
+using OsChar =
 #if defined(_WIN32) && !defined(__CYGWIN__)
     wchar_t
 #else
     char
 #endif
-    >;
+    ;
+
+/**
+ * Named because it is similar to the Rust type, except it is in the
+ * native encoding not WTF-8.
+ *
+ * Same as `std::filesystem::path::string_type`, but manually defined
+ * for the same reason as `OsChar`.
+ */
+using OsString = std::basic_string<OsChar>;
 
 /**
  * `std::string_view` counterpart for `OsString`.
  */
-using OsStringView = std::basic_string_view<OsString::value_type>;
+using OsStringView = std::basic_string_view<OsChar>;
 
 std::string os_string_to_string(OsStringView path);
 

src/libutil/posix-source-accessor.cc

@@ -20,7 +20,7 @@ PosixSourceAccessor::PosixSourceAccessor()
 
 SourcePath PosixSourceAccessor::createAtRoot(const std::filesystem::path & path)
 {
-    std::filesystem::path path2 = absPath(path.string());
+    std::filesystem::path path2 = absPath(path);
     return {
         make_ref<PosixSourceAccessor>(path2.root_path()),
         CanonPath { path2.relative_path().string() },

src/libutil/serialise.cc

@@ -9,6 +9,7 @@
 
 #ifdef _WIN32
 # include <fileapi.h>
+# include <winsock2.h>
 # include "windows-error.hh"
 #else
 # include <poll.h>
@@ -167,13 +168,14 @@ bool FdSource::hasData()
     while (true) {
         fd_set fds;
         FD_ZERO(&fds);
-        FD_SET(fd, &fds);
+        int fd_ = fromDescriptorReadOnly(fd);
+        FD_SET(fd_, &fds);
 
         struct timeval timeout;
         timeout.tv_sec = 0;
         timeout.tv_usec = 0;
 
-        auto n = select(fd + 1, &fds, nullptr, nullptr, &timeout);
+        auto n = select(fd_ + 1, &fds, nullptr, nullptr, &timeout);
         if (n < 0) {
             if (errno == EINTR) continue;
             throw SysError("polling file descriptor");

src/libutil/strings.cc

@@ -14,8 +14,8 @@ template std::list<std::string> splitString(std::string_view s, std::string_view
 template std::set<std::string> splitString(std::string_view s, std::string_view separators);
 template std::vector<std::string> splitString(std::string_view s, std::string_view separators);
 
-template std::list<OsString> basicSplitString(
-    std::basic_string_view<OsString::value_type> s, std::basic_string_view<OsString::value_type> separators);
+template std::list<OsString>
+basicSplitString(std::basic_string_view<OsChar> s, std::basic_string_view<OsChar> separators);
 
 template std::string concatStringsSep(std::string_view, const std::list<std::string> &);
 template std::string concatStringsSep(std::string_view, const std::set<std::string> &);

src/libutil/unix/users.cc

@@ -9,6 +9,8 @@
 
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 std::string getUserName()
 {
     auto pw = getpwuid(geteuid());

src/nix/bundle.cc

@@ -6,6 +6,8 @@
 #include "local-fs-store.hh"
 #include "eval-inline.hh"
 
+namespace nix::fs { using namespace std::filesystem; }
+
 using namespace nix;
 
 struct CmdBundle : InstallableValueCommand
@@ -78,7 +80,7 @@ struct CmdBundle : InstallableValueCommand
 
         auto [bundlerFlakeRef, bundlerName, extendedOutputsSpec] =
             parseFlakeRefWithFragmentAndExtendedOutputsSpec(
-                fetchSettings, bundler, absPath("."));
+                fetchSettings, bundler, fs::current_path().string());
         const flake::LockFlags lockFlags{ .writeLockFile = false };
         InstallableFlake bundler{this,
             evalState, std::move(bundlerFlakeRef), bundlerName, std::move(extendedOutputsSpec),

src/nix/config-check.cc

@@ -10,6 +10,8 @@
 #include "worker-protocol.hh"
 #include "executable-path.hh"
 
+namespace nix::fs { using namespace std::filesystem; }
+
 using namespace nix;
 
 namespace {
@@ -40,8 +42,6 @@ void checkInfo(const std::string & msg) {
 
 }
 
-namespace fs = std::filesystem;
-
 struct CmdConfigCheck : StoreCommand
 {
     bool success = true;

src/nix/develop.cc

@@ -21,6 +21,8 @@
 
 #include "strings.hh"
 
+namespace nix::fs { using namespace std::filesystem; }
+
 using namespace nix;
 
 struct DevelopSettings : Config
@@ -341,7 +343,7 @@ struct Common : InstallableCommand, MixProfile
         ref<Store> store,
         const BuildEnvironment & buildEnvironment,
         const std::filesystem::path & tmpDir,
-        const std::filesystem::path & outputsDir = std::filesystem::path { absPath(".") } / "outputs")
+        const std::filesystem::path & outputsDir = fs::path { fs::current_path() } / "outputs")
     {
         // A list of colon-separated environment variables that should be
         // prepended to, rather than overwritten, in order to keep the shell usable.
@@ -450,7 +452,7 @@ struct Common : InstallableCommand, MixProfile
         auto targetFilePath = tmpDir / OS_STR(".attrs.");
         targetFilePath += ext;
 
-        writeFile(targetFilePath.string(), content);
+        writeFile(targetFilePath, content);
 
         auto fileInBuilderEnv = buildEnvironment.vars.find(envVar);
         assert(fileInBuilderEnv != buildEnvironment.vars.end());

src/nix/flake.cc

@@ -25,7 +25,7 @@
 
 #include "strings-inline.hh"
 
-namespace fs = std::filesystem;
+namespace nix::fs { using namespace std::filesystem; }
 
 using namespace nix;
 using namespace nix::flake;
@@ -53,7 +53,7 @@ public:
 
     FlakeRef getFlakeRef()
     {
-        return parseFlakeRef(fetchSettings, flakeUrl, absPath(".")); //FIXME
+        return parseFlakeRef(fetchSettings, flakeUrl, fs::current_path().string()); //FIXME
     }
 
     LockedFlake lockFlake()
@@ -65,7 +65,7 @@ public:
     {
         return {
             // Like getFlakeRef but with expandTilde calld first
-            parseFlakeRef(fetchSettings, expandTilde(flakeUrl), absPath("."))
+            parseFlakeRef(fetchSettings, expandTilde(flakeUrl), fs::current_path().string())
         };
     }
 };
@@ -880,7 +880,7 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
         auto evalState = getEvalState();
 
         auto [templateFlakeRef, templateName] = parseFlakeRefWithFragment(
-            fetchSettings, templateUrl, absPath("."));
+            fetchSettings, templateUrl, fs::current_path().string());
 
         auto installable = InstallableFlake(nullptr,
             evalState, std::move(templateFlakeRef), templateName, ExtendedOutputsSpec::Default(),
@@ -927,7 +927,7 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
                         }
                         continue;
                     } else
-                        writeFile(to2.string(), contents);
+                        writeFile(to2, contents);
                 }
                 else if (fs::is_symlink(st)) {
                     auto target = fs::read_symlink(from2);

src/nix/run.cc

@@ -20,6 +20,8 @@
 
 #include <queue>
 
+namespace nix::fs { using namespace std::filesystem; }
+
 using namespace nix;
 
 std::string chrootHelperName = "__run_in_chroot";
@@ -170,25 +172,25 @@ void chrootHelper(int argc, char * * argv)
     if (!pathExists(storeDir)) {
         // FIXME: Use overlayfs?
 
-        std::filesystem::path tmpDir = createTempDir();
+        fs::path tmpDir = createTempDir();
 
         createDirs(tmpDir + storeDir);
 
         if (mount(realStoreDir.c_str(), (tmpDir + storeDir).c_str(), "", MS_BIND, 0) == -1)
             throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);
 
-        for (auto entry : std::filesystem::directory_iterator{"/"}) {
+        for (auto entry : fs::directory_iterator{"/"}) {
             checkInterrupt();
             auto src = entry.path();
-            Path dst = tmpDir / entry.path().filename();
+            fs::path dst = tmpDir / entry.path().filename();
             if (pathExists(dst)) continue;
             auto st = entry.symlink_status();
-            if (std::filesystem::is_directory(st)) {
+            if (fs::is_directory(st)) {
                 if (mkdir(dst.c_str(), 0700) == -1)
                     throw SysError("creating directory '%s'", dst);
                 if (mount(src.c_str(), dst.c_str(), "", MS_BIND | MS_REC, 0) == -1)
                     throw SysError("mounting '%s' on '%s'", src, dst);
-            } else if (std::filesystem::is_symlink(st))
+            } else if (fs::is_symlink(st))
                 createSymlink(readLink(src), dst);
         }
 
@@ -205,9 +207,9 @@ void chrootHelper(int argc, char * * argv)
         if (mount(realStoreDir.c_str(), storeDir.c_str(), "", MS_BIND, 0) == -1)
             throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);
 
-    writeFile("/proc/self/setgroups", "deny");
-    writeFile("/proc/self/uid_map", fmt("%d %d %d", uid, uid, 1));
-    writeFile("/proc/self/gid_map", fmt("%d %d %d", gid, gid, 1));
+    writeFile(fs::path{"/proc/self/setgroups"}, "deny");
+    writeFile(fs::path{"/proc/self/uid_map"}, fmt("%d %d %d", uid, uid, 1));
+    writeFile(fs::path{"/proc/self/gid_map"}, fmt("%d %d %d", gid, gid, 1));
 
 #if __linux__
     if (system != "")

src/nix/self-exe.cc

@@ -5,7 +5,9 @@
 
 namespace nix {
 
-namespace fs = std::filesystem;
+namespace fs {
+using namespace std::filesystem;
+}
 
 fs::path getNixBin(std::optional<std::string_view> binaryNameOpt)
 {

tests/unit/libfetchers/public-key.cc

@@ -10,11 +10,11 @@ using nlohmann::json;
 
 class PublicKeyTest : public CharacterizationTest
 {
-    Path unitTestData = getUnitTestData() + "/public-key";
+    std::filesystem::path unitTestData = getUnitTestData() / "public-key";
 
 public:
-    Path goldenMaster(std::string_view testStem) const override {
-        return unitTestData + "/" + testStem;
+    std::filesystem::path goldenMaster(std::string_view testStem) const override {
+        return unitTestData / testStem;
     }
 };
 

tests/unit/libstore-support/tests/nix_api_store.hh

@@ -10,7 +10,7 @@
 #include <filesystem>
 #include <gtest/gtest.h>
 
-namespace fs = std::filesystem;
+namespace fs { using namespace std::filesystem; }
 
 namespace nixC {
 class nix_api_store_test : public nix_api_util_context

tests/unit/libstore-support/tests/protocol.hh

@@ -12,10 +12,10 @@ namespace nix {
 template<class Proto, const char * protocolDir>
 class ProtoTest : public CharacterizationTest, public LibStoreTest
 {
-    Path unitTestData = getUnitTestData() + "/" + protocolDir;
+    std::filesystem::path unitTestData = getUnitTestData() / protocolDir;
 
-    Path goldenMaster(std::string_view testStem) const override {
-        return unitTestData + "/" + testStem + ".bin";
+    std::filesystem::path goldenMaster(std::string_view testStem) const override {
+        return unitTestData / (std::string { testStem + ".bin" });
     }
 };
 

tests/unit/libstore/derivation-advanced-attrs.cc

@@ -16,12 +16,12 @@ using nlohmann::json;
 
 class DerivationAdvancedAttrsTest : public CharacterizationTest, public LibStoreTest
 {
-    Path unitTestData = getUnitTestData() + "/derivation";
+    std::filesystem::path unitTestData = getUnitTestData() / "derivation";
 
 public:
-    Path goldenMaster(std::string_view testStem) const override
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
     {
-        return unitTestData + "/" + testStem;
+        return unitTestData / testStem;
     }
 };
 

tests/unit/libstore/derivation.cc

@@ -13,11 +13,11 @@ using nlohmann::json;
 
 class DerivationTest : public CharacterizationTest, public LibStoreTest
 {
-    Path unitTestData = getUnitTestData() + "/derivation";
+    std::filesystem::path unitTestData = getUnitTestData() / "derivation";
 
 public:
-    Path goldenMaster(std::string_view testStem) const override {
-        return unitTestData + "/" + testStem;
+    std::filesystem::path goldenMaster(std::string_view testStem) const override {
+        return unitTestData / testStem;
     }
 
     /**

tests/unit/libstore/machines.cc

@@ -13,6 +13,8 @@ using testing::Eq;
 using testing::Field;
 using testing::SizeIs;
 
+namespace nix::fs { using namespace std::filesystem; }
+
 using namespace nix;
 
 TEST(machines, getMachinesWithEmptyBuilders) {
@@ -135,10 +137,10 @@ TEST(machines, getMachinesWithIncorrectFormat) {
 }
 
 TEST(machines, getMachinesWithCorrectFileReference) {
-    auto path = absPath(getUnitTestData() + "/machines/valid");
-    ASSERT_TRUE(pathExists(path));
+    auto path = fs::weakly_canonical(getUnitTestData() / "machines/valid");
+    ASSERT_TRUE(fs::exists(path));
 
-    auto actual = Machine::parseConfig({}, "@" + path);
+    auto actual = Machine::parseConfig({}, "@" + path.string());
     ASSERT_THAT(actual, SizeIs(3));
     EXPECT_THAT(actual, Contains(Field(&Machine::storeUri, AuthorityMatches("nix@scratchy.labs.cs.uu.nl"))));
     EXPECT_THAT(actual, Contains(Field(&Machine::storeUri, AuthorityMatches("nix@itchy.labs.cs.uu.nl"))));
@@ -146,20 +148,22 @@ TEST(machines, getMachinesWithCorrectFileReference) {
 }
 
 TEST(machines, getMachinesWithCorrectFileReferenceToEmptyFile) {
-    auto path = "/dev/null";
-    ASSERT_TRUE(pathExists(path));
+    fs::path path = "/dev/null";
+    ASSERT_TRUE(fs::exists(path));
 
-    auto actual = Machine::parseConfig({}, std::string{"@"} + path);
+    auto actual = Machine::parseConfig({}, "@" + path.string());
     ASSERT_THAT(actual, SizeIs(0));
 }
 
 TEST(machines, getMachinesWithIncorrectFileReference) {
-    auto actual = Machine::parseConfig({}, "@" + absPath("/not/a/file"));
+    auto path = fs::weakly_canonical("/not/a/file");
+    ASSERT_TRUE(!fs::exists(path));
+    auto actual = Machine::parseConfig({}, "@" + path.string());
     ASSERT_THAT(actual, SizeIs(0));
 }
 
 TEST(machines, getMachinesWithCorrectFileReferenceToIncorrectFile) {
     EXPECT_THROW(
-        Machine::parseConfig({}, "@" + absPath(getUnitTestData() + "/machines/bad_format")),
+        Machine::parseConfig({}, "@" + fs::weakly_canonical(getUnitTestData() / "machines" / "bad_format").string()),
         FormatError);
 }

tests/unit/libstore/nar-info.cc

@@ -13,10 +13,10 @@ using nlohmann::json;
 
 class NarInfoTest : public CharacterizationTest, public LibStoreTest
 {
-    Path unitTestData = getUnitTestData() + "/nar-info";
+    std::filesystem::path unitTestData = getUnitTestData() / "nar-info";
 
-    Path goldenMaster(PathView testStem) const override {
-        return unitTestData + "/" + testStem + ".json";
+    std::filesystem::path goldenMaster(PathView testStem) const override {
+        return unitTestData / (testStem + ".json");
     }
 };
 

tests/unit/libstore/path-info.cc

@@ -12,10 +12,10 @@ using nlohmann::json;
 
 class PathInfoTest : public CharacterizationTest, public LibStoreTest
 {
-    Path unitTestData = getUnitTestData() + "/path-info";
+    std::filesystem::path unitTestData = getUnitTestData() / "path-info";
 
-    Path goldenMaster(PathView testStem) const override {
-        return unitTestData + "/" + testStem + ".json";
+    std::filesystem::path goldenMaster(PathView testStem) const override {
+        return unitTestData / (testStem + ".json");
     }
 };
 

tests/unit/libstore/store-reference.cc

@@ -13,11 +13,11 @@ using nlohmann::json;
 
 class StoreReferenceTest : public CharacterizationTest, public LibStoreTest
 {
-    Path unitTestData = getUnitTestData() + "/store-reference";
+    std::filesystem::path unitTestData = getUnitTestData() / "store-reference";
 
-    Path goldenMaster(PathView testStem) const override
+    std::filesystem::path goldenMaster(PathView testStem) const override
     {
-        return unitTestData + "/" + testStem + ".txt";
+        return unitTestData / (testStem + ".txt");
     }
 };
 

tests/unit/libutil-support/tests/characterization.hh

@@ -13,7 +13,7 @@ namespace nix {
  * The path to the unit test data directory. See the contributing guide
  * in the manual for further details.
  */
-static inline Path getUnitTestData() {
+static inline std::filesystem::path getUnitTestData() {
     return getEnv("_NIX_TEST_UNIT_DATA").value();
 }
 
@@ -36,7 +36,7 @@ protected:
      * While the "golden master" for this characterization test is
      * located. It should not be shared with any other test.
      */
-    virtual Path goldenMaster(PathView testStem) const = 0;
+    virtual std::filesystem::path goldenMaster(PathView testStem) const = 0;
 
 public:
     /**
@@ -77,7 +77,7 @@ public:
 
         if (testAccept())
         {
-            createDirs(dirOf(file));
+            std::filesystem::create_directories(file.parent_path());
             writeFile2(file, got);
             GTEST_SKIP()
                 << "Updating golden master "
@@ -97,10 +97,10 @@ public:
     {
         writeTest(
             testStem, test,
-            [](const Path & f) -> std::string {
+            [](const std::filesystem::path & f) -> std::string {
                 return readFile(f);
             },
-            [](const Path & f, const std::string & c) {
+            [](const std::filesystem::path & f, const std::string & c) {
                 return writeFile(f, c);
             });
     }

tests/unit/libutil/file-system.cc

@@ -12,8 +12,8 @@
 #include <numeric>
 
 #ifdef _WIN32
-#  define FS_SEP "\\"
-#  define FS_ROOT "C:" FS_SEP // Need a mounted one, C drive is likely
+#  define FS_SEP L"\\"
+#  define FS_ROOT L"C:" FS_SEP // Need a mounted one, C drive is likely
 #else
 #  define FS_SEP "/"
 #  define FS_ROOT FS_SEP
@@ -23,6 +23,12 @@
 #  define PATH_MAX 4096
 #endif
 
+#ifdef _WIN32
+#  define GET_CWD _wgetcwd
+#else
+#  define GET_CWD getcwd
+#endif
+
 namespace nix {
 
 /* ----------- tests for file-system.hh -------------------------------------*/
@@ -33,34 +39,34 @@ namespace nix {
 
 TEST(absPath, doesntChangeRoot)
 {
-    auto p = absPath(FS_ROOT);
+    auto p = absPath(std::filesystem::path{FS_ROOT});
 
     ASSERT_EQ(p, FS_ROOT);
 }
 
 TEST(absPath, turnsEmptyPathIntoCWD)
 {
-    char cwd[PATH_MAX + 1];
-    auto p = absPath("");
+    OsChar cwd[PATH_MAX + 1];
+    auto p = absPath(std::filesystem::path{""});
 
-    ASSERT_EQ(p, getcwd((char *) &cwd, PATH_MAX));
+    ASSERT_EQ(p, GET_CWD((OsChar *) &cwd, PATH_MAX));
 }
 
 TEST(absPath, usesOptionalBasePathWhenGiven)
 {
-    char _cwd[PATH_MAX + 1];
-    char * cwd = getcwd((char *) &_cwd, PATH_MAX);
+    OsChar _cwd[PATH_MAX + 1];
+    OsChar * cwd = GET_CWD((OsChar *) &_cwd, PATH_MAX);
 
-    auto p = absPath("", cwd);
+    auto p = absPath(std::filesystem::path{""}.string(), std::filesystem::path{cwd}.string());
 
-    ASSERT_EQ(p, cwd);
+    ASSERT_EQ(p, std::filesystem::path{cwd}.string());
 }
 
 TEST(absPath, isIdempotent)
 {
-    char _cwd[PATH_MAX + 1];
-    char * cwd = getcwd((char *) &_cwd, PATH_MAX);
-    auto p1 = absPath(cwd);
+    OsChar _cwd[PATH_MAX + 1];
+    OsChar * cwd = GET_CWD((OsChar *) &_cwd, PATH_MAX);
+    auto p1 = absPath(std::filesystem::path{cwd});
     auto p2 = absPath(p1);
 
     ASSERT_EQ(p1, p2);
@@ -68,8 +74,8 @@ TEST(absPath, isIdempotent)
 
 TEST(absPath, pathIsCanonicalised)
 {
-    auto path = FS_ROOT "some/path/with/trailing/dot/.";
-    auto p1 = absPath(path);
+    auto path = FS_ROOT OS_STR("some/path/with/trailing/dot/.");
+    auto p1 = absPath(std::filesystem::path{path});
     auto p2 = absPath(p1);
 
     ASSERT_EQ(p1, FS_ROOT "some" FS_SEP "path" FS_SEP "with" FS_SEP "trailing" FS_SEP "dot");
@@ -82,26 +88,26 @@ TEST(absPath, pathIsCanonicalised)
 
 TEST(canonPath, removesTrailingSlashes)
 {
-    auto path = FS_ROOT "this/is/a/path//";
-    auto p = canonPath(path);
+    std::filesystem::path path = FS_ROOT "this/is/a/path//";
+    auto p = canonPath(path.string());
 
-    ASSERT_EQ(p, FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path");
+    ASSERT_EQ(p, std::filesystem::path{FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path"}.string());
 }
 
 TEST(canonPath, removesDots)
 {
-    auto path = FS_ROOT "this/./is/a/path/./";
-    auto p = canonPath(path);
+    std::filesystem::path path = FS_ROOT "this/./is/a/path/./";
+    auto p = canonPath(path.string());
 
-    ASSERT_EQ(p, FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path");
+    ASSERT_EQ(p, std::filesystem::path{FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path"}.string());
 }
 
 TEST(canonPath, removesDots2)
 {
-    auto path = FS_ROOT "this/a/../is/a////path/foo/..";
-    auto p = canonPath(path);
+    std::filesystem::path path = FS_ROOT "this/a/../is/a////path/foo/..";
+    auto p = canonPath(path.string());
 
-    ASSERT_EQ(p, FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path");
+    ASSERT_EQ(p, std::filesystem::path{FS_ROOT "this" FS_SEP "is" FS_SEP "a" FS_SEP "path"}.string());
 }
 
 TEST(canonPath, requiresAbsolutePath)
@@ -243,7 +249,7 @@ TEST(isDirOrInDir, DISABLED_shouldWork)
 
 TEST(pathExists, rootExists)
 {
-    ASSERT_TRUE(pathExists(FS_ROOT));
+    ASSERT_TRUE(pathExists(std::filesystem::path{FS_ROOT}.string()));
 }
 
 TEST(pathExists, cwdExists)

tests/unit/libutil/git.cc

@@ -11,12 +11,12 @@ using namespace git;
 
 class GitTest : public CharacterizationTest
 {
-    Path unitTestData = getUnitTestData() + "/git";
+    std::filesystem::path unitTestData = getUnitTestData() / "git";
 
 public:
 
-    Path goldenMaster(std::string_view testStem) const override {
-        return unitTestData + "/" + testStem;
+    std::filesystem::path goldenMaster(std::string_view testStem) const override {
+        return unitTestData / std::string(testStem);
     }
 
     /**

tests/unit/libutil/meson.build

@@ -48,12 +48,14 @@ subdir('build-utils-meson/diagnostics')
 sources = files(
   'args.cc',
   'canon-path.cc',
+  'checked-arithmetic.cc',
   'chunked-vector.cc',
   'closure.cc',
   'compression.cc',
   'config.cc',
   'executable-path.cc',
   'file-content-address.cc',
+  'file-system.cc',
   'git.cc',
   'hash.cc',
   'hilite.cc',
@@ -62,6 +64,7 @@ sources = files(
   'lru-cache.cc',
   'nix_api_util.cc',
   'pool.cc',
+  'position.cc',
   'processes.cc',
   'references.cc',
   'spawn.cc',