Request for an RFC: pimpl for Rust

[CAD97]

See also, wherein I sketch what a minimal "private impl" "stable ABI" could look like:

[Sketch] Minimal pimpl-style "stable ABI"

TL;DR: allow a crate to specify abi = "stable" or similar in Cargo.toml. If it exposes anything that doesn't have a defined ABI (i.e. is repr(Rust)/extern "Rust"), fail compilation. This is a very unbaked sketch based on a few ideas I had in the wake of @matklad's pimpl post. This is not intended to be a short term target nor really a direct suggestion, rather a starting point for discussion on what a hyper minimal stable ABI could look like for Rust. My proposal is that we add a new library…