Unify the task source and task canceller API

I moved away from the `Window` struct all the logic to handle task
sources, into a new struct called `TaskManager`. In a happy world, I'd
be able to just have there two functions, of the types:

```rust
fn task_source<T: TaskSource>(&self, name: TaskSourceName) -> Box<T>
fn task_source_with_canceller<T: TaskSource>(&self, name: TaskSourceName)
  -> (Box<T>, TaskSourceCanceller)
```

And not so much duplicated code. However, because TaskSource can't be a
trait object (because it has generic type parameters), that's not
possible. Instead, I decided to reduce duplicated logic through macros.

For reasons[1], I have to pass both the name of the function with
canceller and the name of the function without, as I'm not able to
concatenate them in the macro itself. I could probably use
`concat_idents` to create both types already defined and reduce the
amount of arguments by one, but that macro is nightly only. At the same
time, not being able to declare macros inside `impl` forces me to pass
`self` as an argument.

All this makes this solution more verbose than it would be ideally. It
does reduce duplication, but it doesn't reduce the size of the file.

[1](https://github.com/rust-lang/rust/issues/29599)

1 files changed, 10 insertions, 7 deletions

diff --git a/components/script/dom/htmltextareaelement.rs b/components/script/dom/htmltextareaelement.rs
index 89ffff3e030..cb4929f4cab 100755
--- a/components/script/dom/htmltextareaelement.rs
+++ b/components/script/dom/htmltextareaelement.rs
@@ -565,13 +565,16 @@ impl VirtualMethods for HTMLTextAreaElement {
         } else if event.type_() == atom!("keypress") && !event.DefaultPrevented() {
             if event.IsTrusted() {
                 let window = window_from_node(self);
-                let _ = window.user_interaction_task_source().queue_event(
-                    &self.upcast(),
-                    atom!("input"),
-                    EventBubbles::Bubbles,
-                    EventCancelable::NotCancelable,
-                    &window,
-                );
+                let _ = window
+                    .task_manager()
+                    .user_interaction_task_source()
+                    .queue_event(
+                        &self.upcast(),
+                        atom!("input"),
+                        EventBubbles::Bubbles,
+                        EventCancelable::NotCancelable,
+                        &window,
+                    );
             }
         }
     }