Rust+GNOME Hackfest #4

Less than 2 weeks ago, I was working on improving the integration of Rust with GNOME libraries at the fourth Hackfest, which happened this time in Thessaloniki.

Improving ergonomics of gnome-class

As I said in my previous blog post, there is a issue when inheriting from a gtk::Widget:

As a reminder, we need to specify both the wrapper widget and its ffi counterpart.

glib_wrapper!(pub struct MyWidget(Object<MyWidgetFFI>): [Widget => GtkWidget];)

What I did not specify last time is that we also need to specify all parents. So, if we were to inherit from a gtk::Button, we would need something like:

glib_wrapper!(pub struct MyWidget(Object<MyWidgetFFI>): [Button => GtkButton, Widget => GtkWidget];)

But, we do not want the user to have to specify all of this extra information; it would be more ergonomic if the user has to write only Button and the proc-macro would figure the rest. During this Hackfest, I worked on this issue: thanks to the work of Guillaume Gomez during the previous Hackfest, I was able to use gir as a library and with it, I was able to query the needed info. Here is the pull request for this ergonomic improvement.

Creating GNOME class for Servo Web View

As I said in my previous blog article, it would be awesome to have a GNOME class for the Servo web view for these reasons:

  • We will be able to use Servo from another language.

  • The users won’t have to compile Servo themselves as we will be able to create a shared library with a C API (which is a huge benefit).

Since I fixed the issues we had in gnome-class that were blocking us from creating such a GNOME class, I also worked on creating a GNOME class for the Servo web view. My work is in this branch and you can see the almost working example here. For some reason, Servo broke after this change, perhaps because I had to update a dependency in order to make it work with gnome-class: the code compiles but the widget remains black. So, we’re nearly there and when this branch and the one in gnome-class will be merged, we will have the opportunity to have this cool servo GNOME class!

Relm improvements

I’ve also worked on relm during the Hackfest.

One of the biggest improvement is the switch to syn 0.15 which dramatically improved the error messages. Before this update, a syntax error in relm looked this way:

error: custom attribute panicked
  --> examples/
58 | #[widget]
   | ^^^^^^^^^
   = help: message: parse() Widget: ParseError(None)

Now, it looks like this:

error: expected `,`
  --> examples/
93 |                     text &self.model.counter.to_string(),
   |                          ^

(the message could be better, but at least we know where the error is)

I also fixed a long standing issue in relm which was that the only way to distinguish a gtk widget from a relm widget in the view! macro was whether its name was a path or an identifier. This was limiting because you needed to import every relm widget you created in order to use them in your views. Now, you can use the $ symbol in order to specify that it is a relm widget.

There was also a bug with EventStream that could panic because of a BorrowMut error if shared between multiple threads. Remember that it was relm was created to have no error of this kind, so it was a bit of a shame to still have that kind of issues. But, it was a very easy fix because it was caused by the Rust compiler to wrongly auto-generate a Send impl for this type that should not be shared with other threads. Indeed, when you want to send a message from another thread in relm, you should not use an EventStream but a Channel. So the fix was simply to make this type non-Send.


A big thank to Sebastian Dröge and Vivia for organizing the Hackfest and a huge thank to the GNOME foundation for sponsoring my flight and accommodation to allow me to participate to this Hackfest.

GNOME Foundation