Set buffer filename if it has none and it is written with an explicit filename

src/core/Buffer.cc

@@ -487,3 +487,8 @@ EncodedString Buffer::get_string()
     m_gap_buffer->compact();
     return EncodedString(m_gap_buffer->address(0u), m_gap_buffer->size(), m_encoding);
 }
+
+void Buffer::set_filename(const std::string & filename)
+{
+    m_filename = std::make_shared<std::string>(filename);
+}

src/core/Buffer.h

@@ -183,6 +183,7 @@ public:
     void erase_code_point(const Buffer::Iterator & position);
     void erase_range(const Buffer::Range & range);
     std::shared_ptr<std::string> filename() const { return m_filename; }
+    void set_filename(const std::string & filename);
     Iterator begin() const { return Iterator(this); }
     std::shared_ptr<Iterator> beginp() const { return std::make_shared<Iterator>(this); }
     Iterator end() const { return *m_eof_iterator; }

src/gui/Window.cc

@@ -551,15 +551,19 @@ void Window::handle_command(const EncodedString & command)
 
 void Window::command_write_file(const CommandParser & cp)
 {
-    std::shared_ptr<Buffer> buffer = m_focused_buffer_pane->buffer();
     std::string path;
+    std::shared_ptr<Buffer> buffer = m_focused_buffer_pane->buffer();
+    auto filename = buffer->filename();
     if (cp.size() >= 2)
     {
         path = cp[1].to_string();
+        if (!filename)
+        {
+            buffer->set_filename(path);
+        }
     }
     else
     {
-        auto filename = buffer->filename();
         if (filename)
         {
             path = *filename;