Representation of structs

[nikomatsakis]

Discussion topic about how structs are represented in Rust. Some things to work out:

• Do we ever say anything about how a #[repr(rust)] struct is laid out (and/or treated by the ABI)?
  • e.g., what about different structs with same definition
  • across executions of the same program?
• For example, rkruppe writes that we might "want to guarantee (some subset of) newtype unpacking and relegate #[repr(transparent)] to being the way to guarantee to other crates that a type with private fields is and will remain a newtype?"
• When is interop with #[rust(C)] guaranteed and what can we say there?

[nikomatsakis]

Actually, I see some of the latest comments here -- these are great. Perhaps we can redirect them to #35, which is a subissue on this topic? I'll copy over some of those now.