Resolve the biggest blockers to Linux building on stable Rust

[nikomatsakis]

Metadata
Owner(s)	@nikomatsakis, @joshtriplett
Team(s)	compiler, lang, libs-api
Goal document	2024h2/rfl_stable

Summary

Stabilize unstable features required by Rust for Linux project including

• Stable support for RFL's customized ARC type
• Labeled goto in inline assembler and extended offset_of! support
• RFL on Rust CI
• Pointers to statics in constants

Tasks and status

• [ ] Overall program management (@nikomatsakis, @joshtriplett)
• [ ] Arbitrary self types v2 (@adetaylor)
  • [x] author RFC
  • [x] RFC decision ()
  • [ ] Implementation
  • [ ] Standard reviews (compiler )
  • [ ] Stabilization decision (lang )
• [ ] Derive smart pointer (@Darksonn)
  • [x] author RFC
  • [x] RFC decision (lang )
  • [ ] Implementation (@dingxiangfei2009)
  • [ ] Author stabilization report (@dingxiangfei2009)
  • [ ] Stabilization decision (lang )
• [ ] asm_goto (@Darksonn)
  • [x] implementation
  • [ ] Real-world usage in Linux kernel
  • [ ] Extend to cover full RFC
  • [ ] Author stabilization report
  • [ ] Stabilization decision (lang )
• [x] Extended offset_of syntax (@dingxiangfei2009)
  • [x] Stabilization report
  • [x] Stabilization decision (libs-api )
  • [x] Stabilization PR https://github.com/rust-lang/rust/pull/128284
• [ ] RFL on Rust CI (@Kobzol)
  • [x] implementation
  • [ ] Policy draft
  • [ ] Policy decision (compiler )
• [ ] Pointers to static in constants (@nikomatsakis)
  • [x] Stabilization report (https://github.com/rust-lang/rust/issues/128183)
  • [x] Stabilization decision (lang )
  • [ ] Stabilization PR .

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[nikomatsakis]

Key developments:

• Implementation work is proceeding in many areas (arbitrary self types v2, derive smart pointer, and sanitizer support.
• Extended offset_of syntax is stable on nightly! 🎉 Credit to @dingxiangfei2009 for driving this.
• RFL on Rust CI is implemented but still waiting on documented policy. The first breakage was detected (and fixed) in #129416. Reading over the PR comments suggests that the policy has not been well communicated to the Rust devs, who weren't clear on who was responsible to fix the breakage etc. Suggested follow-up is to improve the bot message to link to the policy and (as an interim measure) send an email to all@ and post an Inside Rust blog post.
• Stabilization report for Pointers to Statics in Constants is now in FCP. @dingxiangfei2009 to prepare stabilization PR. 💜

Next steps

• Need to decide on a name for derive-smart pointer. @nikomatsakis to follow-up.
• Experimentation with asm-goto has revealed that a weak point in the design. A typical use case for this feature is to design control-flow, where you have some assembly code that decides which block to branch to. The current design looks like inline_asm!(..., exit1 = { ... }, exit2 = { ... }), in which the assembly code can jump to exit1 or exit2. The problem is that, since inline_asm! must be in an unsafe block, this implies the blocks associated with those labels will be allowed to use unsafe code. To circumvent this, the RFL team is having those blocks simply break from an outer block with a false return value, and then branching on that if value outside. That's the same as the C code does, as it happens, but. it's not as nice nor efficient as it could be. Discussion in the meeting suggested that the block bodies should be limited to safe code unless marked as unsafe. @joshtriplett to follow-up.
• For Sanitizer support, we will need to stabilize the -Zfixed-x18 flag as a first step. @Darksonn to start drafting a stabilization report, possibly delegating some questions to @nikomatsakis.