rust: pin-init: internal: pin_data: project self-referential fields

This adds the projection for fields that are shared borrowed or that
borrows other fields but is covariant. Both cases allow a shared reference
to be accessed.

No mutable references can be created for these cases for different reasons:
* For fields that are shared borrowed, aliasing restriction prevents
  creation of mutable reference
* For fields that borrow other fields, their proper type contains field
  lifetimes. These lifetimes cannot be made available in the returned
  `project` struct (because there is no way to represent existential
  lifetime in return position). For covariant types, it is possible to
  shorten these lifetimes to that of `&self`; but doing so requires the
  reference to also be covariant over the pointee type, so we cannot give
  out `&mut` as it is invariant over the pointee.

Due to field-referencing fields being wrapped inside `Erase`, the normal
accessor syntax stop working; create accessor methods for these fields
instead.

Acked-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20261008-dev-selfref-v2-9-e280b3c8fba5@garyguo.net/
Signed-off-by: Gary Guo <gary@garyguo.net>

0 files changed, 0 insertions, 0 deletions