Fixes#1321
A better fix would be to change `MessageBody` to take a `Pin<&mut
Self>`, rather than a `Pin<&mut Self>`. This will avoid requiring the
use of `Box` for all consumers by allowing the caller to determine how
to pin the `MessageBody` implementation (e.g. via stack pinning).
However, doing so is a breaking change that will affect every user of
`MessageBody`. By pinning the inner stream ourselves, we can fix the
undefined behavior without breaking the API.
I've included @sebzim4500's reproduction case as a new test case.
However, due to the nature of undefined behavior, this could pass (and
not segfault) even if underlying issue were to regress.
Unfortunately, until rust-lang/unsafe-code-guidelines#148 is resolved,
it's not even possible to write a Miri test that will pass when the bug
is fixed.
Co-authored-by: Yuki Okushi <huyuumi.dev@gmail.com>
