Deprecate the asm! macro in favor of llvm_asm!

[Amanieu]

Since we will be changing the syntax of asm! soon, deprecate it and encourage people to use llvm_asm! instead (which preserves the old syntax). This will avoid breakage when asm! is changed.

RFC: rust-lang/rfcs#2843

[rust-highfive]

r? @Mark-Simulacrum

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[rust-highfive]

⚠️ Warning ⚠️

• These commits modify submodules.

[Mark-Simulacrum]

I have not reviewed the stdarch submodule bump, but this otherwise looks good. I think we should hold off on landing this until we have a replacement in nightly for some time (say, 2 weeks) just to give people time to migrate without generating lots of warnings in their code.

(Have we landed llvm_asm yet? I think no, but I don't remember. If we have, my concern is moot of course.)

[Amanieu]

llvm_asm! landed in #68404 about 2 weeks ago.

[Amanieu]

The stdarch submodule bump also includes additional ARM NEON intrinsics, which are all unstable.

[Mark-Simulacrum]

I think the current note is slightly better, as it directly suggests why we're doing it and gives the user a chance to try to find out why the asm! syntax will change soon. I suspect that ideally we'd include a link to some tracking issue or whatever, but that can happen separately.

@Amanieu -- does the "suggestion" in the deprecation make cargo fix work?

[Amanieu]

It should work with cargo fix but I haven't tried it yet.

[Mark-Simulacrum]

Okay, that's good enough for me (and Centril has separately indicated to me that he's okay with the existing comment), so @bors r+

[bors]

📌 Commit 43ee31f has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 43ee31f with merge c9f88ad146aa20172d3a0e13098c707238fa0be5...

[bors]

💔 Test failed - checks-azure

[Amanieu]

Seems spurious @bors retry

[bors]

⌛ Testing commit 43ee31f with merge 71bccf0e8e6d51e37fda2d367be79597df0413f2...

[bors]

💔 Test failed - checks-azure

[Amanieu]

@bors r=Mark-Simulacrum

[bors]

📌 Commit ce83b49 has been approved by Mark-Simulacrum

[bors]

🌲 The tree is currently closed for pull requests below priority 1000, this pull request will be tested once the tree is reopened

[bors]

⌛ Testing commit ce83b49 with merge 9b2f8db...

[bors]

☀️ Test successful - checks-azure
Approved by: Mark-Simulacrum
Pushing 9b2f8db to master...

[berkus]

Is #![feature(asm)] also going to be renamed?

[Amanieu]

llvm_asm! is gated behind the llvm_asm feature.

[samuela]

The error message when switching from asm! to llvm_asm! is confusing:

error[E0658]: use of unstable library feature 'llvm_asm': LLVM-style inline assembly will never be stabilized, prefer using asm! instead
   --> util_linux/volume_id/sysv.rs:253:17
    |
253 |                 llvm_asm!("bswap $0" : "=r" (fresh4) : "0"
    |                 ^^^^^^^^
    |
    = note: see issue #70173 <https://github.com/rust-lang/rust/issues/70173> for more information
    = help: add `#![feature(llvm_asm)]` to the crate attributes to enable

even though I just got an error message telling me to use llvm_asm! instead of asm!. This is resolved with #![feature(llvm_asm)] as expected, but confusing nonetheless.

[mark-i-m]

@samuela Haha, yes, could you please open an issue for that? The asm macro's syntax changed. What the error means is to use the new asm style, rather than llvm/gcc style inline asm.

[Amanieu]

I opened #72825 to clarify the messages.