#AnimationFrame issues

We believe that the problem(s) are:

• WebIDL callbacks (JS closures) need to be properly rooted. Currently not.

  • Solution: Implement darn rooting API

• FnBox trait objects are opaque to rooting. Can't be put in JS

  • Solution: Don't do that.

• In order to do rooting properly we need two types for each rootable thingy. One for stack, one for heap. Code explosion.

  • Solution: Generics? JS<T, Rooted>.

MIR pass! Just check list of all local variables.

#Generics

We can't lint-check their bodies. We don't know if T is rooted or now.

`fn foo<#[must_root] T>()? Function MUST have bound of T: Rootable or #[must_root] T if you wish to pass a rootable T to it.

mem::replace isn't a problem because it gets checked at the call site.

#Action items:

• Land nox's thing
• Do the same thing we do for promises for callbacks
  • jdm (https://github.com/servo/servo/issues/14447)
• Try a MIR lint which lints local variables
  • manish (filed https://github.com/servo/servo/issues/14902)
• JS<T, Rooted>?
  • jdm filed https://github.com/servo/servo/issues/14901
• Add cfail tests. Lots.
  • ms2ger (filed https://github.com/servo/servo/issues/14903)