Tracking issue for RFC 1861: Extern types (original) (raw)

This is a tracking issue for RFC 1861 "Extern types".

Steps:

• Implement the RFC (Implement RFC 1861: Extern types #44295)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

• Rust does not support types that don't have dynamically computed alignment -- we need the alignment to compute the field offset in structs. extern type violates this basic assumption, causing pain, suffering, and ICEs all over the compiler. What is the principled fix for this?
• Should we allow generic lifetime and type parameters on extern types?
  If so, how do they effect the type in terms of variance?
• In std's source, it is mentioned that LLVM expects i8* for C's void*.
  We'd need to continue to hack this for the two c_voids in std and libc.
  But perhaps this should be done across-the-board for all extern types?
  Somebody should check what Clang does. Also see "extern type" should use opaque type in LLVM #59095.
  RESOLVED because all pointer types are ptr now.
• How should this interact with unsized arguments? Currently it ICEs: unsized_fn_params should not accept types that don't have a dynamically fixed size (such as extern types) #115709
• How should we model mem::size_of::<ExternType>()? Tracked in Tracking Issue for Sized Hierarchy #144404.