This came out of a conversation with nox in IRC:
https://mozilla.logbot.info/servo/20171201#c13946454-c13946594
The code I was working on which motivated this change is here:
https://github.com/servo/servo/pull/19461
Previously, InputType::Text was used to represent several different
values of the type attribute on an input element.
If an input element doesn't have a type attribute, or its type attribute
doesn't contain a recognised value, then the input's type defaults to
"text".
Before this change, there were a number of checks in the code which
directly looked at the type attribute. If those checks matched against
the value "text", then they were potentially buggy, since an input with
type=invalid should also behave like an input with type=text.
Rather than have every conditional which cares about the input type also
have to deal with invalid input types, we can convert the type attribute
to an InputType enum once, and then match against the enum.
A secondary benefit is that the compiler can tell us whether we've
missed branches in a match expression. While working on this I
discovered that the HTMLInputElement::value_mode() method misses a case
for inputs with type=hidden (this resulted in a failing WPT test
passing).
I've also implemented the Default trait for InputType, so we now only
have one place in the code which knows that InputType::Text is the
default, where previously there were several.
http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.
This is expected since `Box::new` is defined as:
```rust
impl<T> Box<T> {
#[inline(always)]
pub fn new(x: T) -> Box<T> {
box x
}
}
```
With inlining, it should compile to the same as box syntax.
In a later PR, DomRoot<T> will become a type alias of Root<Dom<T>>,
where Root<T> will be able to handle all the things that need to be
rooted that have a stable traceable address that doesn't move for the
whole lifetime of the root. Stay tuned.
