Add fs.path.ComponentIterator and use it in Dir.makePath

lib/std/fs.zig

@@ -1460,32 +1460,22 @@ pub const Dir = struct {
     /// This function is not atomic, and if it returns an error, the file system may
     /// have been modified regardless.
     pub fn makePath(self: Dir, sub_path: []const u8) !void {
-        var end_index: usize = sub_path.len;
+        var it = try path.componentIterator(sub_path);
+        var component = it.last() orelse return;
         while (true) {
-            self.makeDir(sub_path[0..end_index]) catch |err| switch (err) {
+            self.makeDir(component.path) catch |err| switch (err) {
                 error.PathAlreadyExists => {
                     // TODO stat the file and return an error if it's not a directory
                     // this is important because otherwise a dangling symlink
                     // could cause an infinite loop
-                    if (end_index == sub_path.len) return;
                 },
-                error.FileNotFound => {
-                    // march end_index backward until next path component
-                    while (true) {
-                        if (end_index == 0) return err;
-                        end_index -= 1;
-                        if (path.isSep(sub_path[end_index])) break;
-                    }
+                error.FileNotFound => |e| {
+                    component = it.previous() orelse return e;
                     continue;
                 },
-                else => return err,
+                else => |e| return e,
             };
-            if (end_index == sub_path.len) return;
-            // march end_index forward until next path component
-            while (true) {
-                end_index += 1;
-                if (end_index == sub_path.len or path.isSep(sub_path[end_index])) break;
-            }
+            component = it.next() orelse return;
         }
     }