Rearranging across the interface

As I understand it, the issue is that "interface boundary" can mean either "API boundary" or "ABI boundary". `Vec<T, A>::shrink_to_fit` is an API boundary; it's publicly documented and safe and has stability guarantees. But in a hypothetical Rust ABI, that API couldn't be an ABI boundary, because it depends on generic type parameters that aren't known when the .so is compiled.

`RawVecInner<A>::shrink_to_fit` could be an ABI boundary, because that doesn't depend on `T` (and we'll ignore `A`), but it's currently not an API boundary. It can't be made into a public API because its safety depends on non-trivial preconditions (like being told the correct alignment of `T`) and that'd be terrible API design - preconditions should be as tightly scoped as possible, within a function or module or crate. So you'd have to invent a new category of interface boundary, which is both an internal API and an ABI, with stability guarantees (including for the non-machine-checkable safety preconditions) and with tooling to help you fulfil those guarantees, which sounds really hard.