Tracking Issue for LLVM-style inline assembly (`llvm_asm`)

[Amanieu]

This is a tracking issue for the RFC "2843" (rust-lang/rfcs#2843).
The feature gate for the issue is #![feature(llvm_asm)].

This issue tracks the legacy inline assembly macro llvm_asm! which more-or-less maps directly to LLVM's internal representation of inline assembly.

There is currently no plan for stabilizing this macro, but there is a proposal to replace it with the new inline assembly macro specified in RFC 2873 (rust-lang/rfcs#2873).

Implementation PR: #68404

[mark-i-m]

cc #68404

[RalfJung]

The fact that this is a Statement in MIR, and not a Terminator, breaks a bunch of semantic assumptions that one could otherwise make about MIR basic blocks. Even if this feature does not get stabilized, it doesn't seem great to have optimizations that entirely break in the presence of llvm_asm. (I would be very surprised if there isn't a soundness issue here, but don't know about about the area to produce a concrete example.)

Are there plans to either fix that, or else entirely remove legacy inline assembly support?