Tracking issue for the define_in_every_cgu_used feature

[jbatez]

The feature gate for the issue is #![feature(define_in_every_cgu_used)].

#[unsafe(define_in_every_cgu_used)] is an attribute that can be applied to static item definitions. It causes codegen to generate a full definition for the item instead of just an external declaration in every codegen unit where the symbol is referenced. It can be used to implement several Objective-C ABIs and reduce the overhead of interoperating with Objective-C code.

Original issue: #53929
Zulip:

• https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Low-overhead.20Objective-C.20interop
• https://rust-lang.zulipchat.com/#narrow/channel/427678-t-lang.2Finterop/topic/Attribute.20Proposal.20for.20Low-Overhead.20Objective-C.20Interop

Steps

• Implement in the LLVM backend
• Implement in the Cranelift backend
• Implement in the GCC backend
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should we disallow access to these items from inline assembly to help simplify the Cranelift backend implementation?
• Should the linkage of these symbols or any other symbol metadata be adjusted as well (e.g. should they be marked as private or linkonce_odr)?
• Given that this attribute is opt-in, where would it be unacceptable to have multiple copies of the same static?
• Should this be an internal rustc_ attribute used only as an implementation detail for objc macros in the standard library without the goal of ever stabilizing the attribute?

Implementation history

Experimental branch: master...jbatez:rust:define_in_every_cgu_used

[jbatez]

@rustbot label +A-attributes +A-codegen +A-linkage +T-compiler +T-lang