Write custom lints

[kmcallister]

First, implement loadable lints for Rust (rust-lang/rust#14067) or wait for someone else to do it. Then we can statically enforce safety properties beyond Rust's type system.

For example, JS<T> has this currently unchecked requirement:

A rooted, JS-owned value. Must only be used as a field in other JS-owned types.

Each new lint should have some compile-fail tests (#2385).

[jdm]

It would be really useful to annotate various DOM helper traits as only usable by script or layout code (such as LayoutNodeHelpers). This could be done with an annotation like #[layout_only] and ensuring that it's only imported by code under src/components/main/layout.

[kmcallister]

@jdm: I'll start working on the JS<T> lint, since we're blogging about it in a month :) Did you write any code for that?

[jdm]

No code for that yet.

[jdm]

Well, I have a branch for compile-fail tests that I could push.

[kmcallister]

Work in progress: https://github.com/kmcallister/servo/tree/lint

[kmcallister]

We have this now! Thanks @Manishearth for finishing this up.