Improve file chooser with buttons reliability

Following the reviews in #602, this commit:
- uses the `with_buttons` function instead of modifying `new` to keep
  backward compatibility.
- saves the button texts in `Stash` objects to avoid invalid pointers
- panics when the number of buttons is too high (instead of silently
  truncating the number of buttons)

src/file_chooser_dialog.rs

@@ -14,23 +14,91 @@ use Widget;
 use Window;
 
 impl FileChooserDialog {
-    pub fn new<T: IsA<Window>>(title: Option<&str>, parent: Option<&T>, action: FileChooserAction, buttons: &[(&str, ResponseType)]) -> FileChooserDialog {
+    // TODO: Keep the other constructor with buttons support as the only constructor (this one was
+    //       left for compatibility) and rename it to `new` for consistency.
+    pub fn new<T: IsA<Window>>(title: Option<&str>, parent: Option<&T>, action: FileChooserAction) -> FileChooserDialog {
+        assert_initialized_main_thread!();
+        unsafe {
+            Widget::from_glib_none(ffi::gtk_file_chooser_dialog_new(
+                title.to_glib_none().0,
+                parent.to_glib_none().0,
+                action.to_glib(),
+                ptr::null::<c_char>()
+            ))
+                .downcast_unchecked()
+        }
+    }
+
+    /// This function creates a `FileChooserDialog` with buttons:
+    /// ```
+    /// let dialog = FileChooserDialog::with_buttons::<ApplicationWindow>(
+    ///   Option::Some("Open File"),
+    ///   Option::None,
+    ///   FileChooserAction::Open,
+    ///   &[("_Cancel", ResponseType::Cancel), ("_Open", ResponseType::Accept)]
+    /// );
+    /// ```
+    pub fn with_buttons<T: IsA<Window>>(title: Option<&str>, parent: Option<&T>, action: FileChooserAction, buttons: &[(&str, ResponseType)]) -> FileChooserDialog {
         assert_initialized_main_thread!();
         unsafe {
             Widget::from_glib_none(match buttons.len() {
-                0 => ffi::gtk_file_chooser_dialog_new(title.to_glib_none().0, parent.to_glib_none().0, action.to_glib(), ptr::null::<c_char>()),
-                1 => ffi::gtk_file_chooser_dialog_new(title.to_glib_none().0, parent.to_glib_none().0, action.to_glib(), buttons[0].0.to_glib_none().0, buttons[0].1.to_glib(), ptr::null::<c_char>()),
+                0 => {
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        ptr::null::<c_char>()
+                    )
+                },
+                1 => {
+                    let first_button_text: Stash<*const c_char, str> = buttons[0].0.to_glib_none();
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        first_button_text.0,
+                        buttons[0].1.to_glib(),
+                        ptr::null::<c_char>(),
+                    )
+                },
                 2 => {
-                    let second_button_text: *const c_char = buttons[1].0.to_glib_none().0;
-                    ffi::gtk_file_chooser_dialog_new(title.to_glib_none().0, parent.to_glib_none().0, action.to_glib(), buttons[0].0.to_glib_none().0, buttons[0].1.to_glib(), second_button_text, buttons[1].1.to_glib(), ptr::null::<c_char>())
+                    let first_button_text: Stash<*const c_char, str> = buttons[0].0.to_glib_none();
+                    let second_button_text: Stash<*const c_char, str> = buttons[1].0.to_glib_none();
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        first_button_text.0,
+                        buttons[0].1.to_glib(),
+                        second_button_text.0,
+                        buttons[1].1.to_glib(),
+                        ptr::null::<c_char>(),
+                    )
                 },
-                _ => {
-                    let second_button_text: *const c_char = buttons[1].0.to_glib_none().0;
-                    let third_button_text: *const c_char = buttons[2].0.to_glib_none().0;
-                    ffi::gtk_file_chooser_dialog_new(title.to_glib_none().0, parent.to_glib_none().0, action.to_glib(), buttons[0].0.to_glib_none().0, buttons[0].1.to_glib(), second_button_text, buttons[1].1.to_glib(), third_button_text, buttons[2].1.to_glib(), ptr::null::<c_char>())
+                3 => {
+                    let first_button_text: Stash<*const c_char, str> = buttons[0].0.to_glib_none();
+                    let second_button_text: Stash<*const c_char, str> = buttons[1].0.to_glib_none();
+                    let third_button_text: Stash<*const c_char, str> = buttons[2].0.to_glib_none();
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        first_button_text.0,
+                        buttons[0].1.to_glib(),
+                        second_button_text.0,
+                        buttons[1].1.to_glib(),
+                        third_button_text.0,
+                        buttons[2].1.to_glib(),
+                        ptr::null::<c_char>(),
+                    )
                 },
+                _ => {
+                    // TODO: Support arbitrary number of buttons once variadic functions are supported.
+                    //       See: https://github.com/rust-lang/rust/issues/44930
+                    panic!(format!("`FileChooserDialog::with_buttons` does not support 4+ buttons, received {}", buttons.len()))
+                }
             })
-                .downcast_unchecked()
+              .downcast_unchecked()
         }
     }
 }