`BitSlice` or a sound way to implement one

For anyone not already aware of roughly what the problem is: Rust basically has a few stable ways of supporting “higher-kinded types” (HKTs), which are families of types, like Vec (without a <T>).

Here are the three stable mechanisms for HKTs with one lifetime parameter:

• You can denote the HKT family &'varying T with by just T, and feed the HKT a 'varying parameter to get &'varying T.
• You can use a generic associated type (GAT) like trait GatHKT { type Apply<'varying>; }, perhaps to make a GatDeref equivalent of std’s Deref.
• You can use higher-kinded trait bounds (like for<'varying> Trait<'varying>) to do something insane like my variance-family crate — which is thousands of lines of code long — to try to get much more thorough support for lifetime-parameterized HKTs.

Each of these three options has problems. The first isn’t fully general. The second interacts poorly with higher-kinded trait bounds. The third involves several unsafe traits and presumably isn’t fit for std.

std uses the first option, which means its traits aren’t as general as possible.