DOM structs embed their parent type as their first field. This
introduces a `.parent()` method to the DOM struct that returns its first
field, and codegens a type assert that ensures that `.parent()` returns
the parent struct.
This generates:
On `#[dom_struct]`:
```rust
impl HasParent for Type {
type Parent = ParentType;
fn as_parent(&self) -> ParentType {
&self.first_field
}
}
```
In the codegen files:
```rust
impl Type {
fn __assert_parent_type(&self) {
let _: &ParentType = self.as_parent();
}
}
````
Before:
```
error[E0412]: cannot find type `SourceBuffer` in this scope
--> /Users/nox/src/servo/components/script/dom/mediasource.rs:25:1
|
25 | #[dom_struct]
| ^^^^^^^^^^^^^ did you mean `SourceBufferList`?
```
After:
```
error[E0412]: cannot find type `SourceBuffer` in this scope
--> /Users/nox/src/servo/components/script/dom/mediasource.rs:28:39
|
28 | source_buffers: DOMRefCell<Vec<JS<SourceBuffer>>>,
| ^^^^^^^^^^^^ did you mean `SourceBufferList`?
```
