Status: Stub
New unstable features can be added and approved without going through a Libs FCP. There should be some buy-in from Libs that a feature is desirable and likely to be stabilized at some point before landing though.
If you're not sure, open an issue against rust-lang/rust first suggesting the feature before developing it.
All public items in the standard library need a #[stable] or #[unstable] attribute on them. When a feature is first added, it gets a #[unstable] attribute.
Before a new feature is merged, those #[unstable] attributes need to be linked to a tracking issue.