nextFilename

    pub fn nextFilename(self: *Diagnostics, name: []const u8) error{OutOfMemory}!void {
        if (!self.saw_first_file) {
            self.saw_first_file = true;
            std.debug.assert(self.root_dir.len == 0);
            const root_len = std.mem.findScalar(u8, name, '/') orelse return;
            std.debug.assert(root_len > 0);
            self.root_dir = try self.allocator.dupe(u8, name[0..root_len]);
        } else if (self.root_dir.len > 0) {
            if (!filenameInRoot(name, self.root_dir)) {
                self.allocator.free(self.root_dir);
                self.root_dir = "";
            }
        }
    }