Stabilize asm_goto feature gate

[nbdd0121]

Stabilize asm_goto feature (tracked by #119364). The issue will remain open and be updated to track asm_goto_with_outputs.

Reference PR: rust-lang/reference#1693

Stabilization Report

This feature adds a label <block> operand type to asm!. <block> must be a block expression with type unit or never. The address of the block is substituted and the assembly may jump to the block. When block completes the asm! block returns and continues execution.

The block starts a new safety context and unsafe operations within must have additional unsafes; the effect of unsafe that surrounds asm! block is cancelled. See #119364 (comment) and #131544.

It's currently forbidden to use asm_goto with output operands; that is still unstable under asm_goto_with_outputs.

Example:

unsafe {
    asm!(
        "jmp {}",
        label {
            println!("Jumped from asm!");
        }
    );
}

Tests:

• tests/ui/asm/x86_64/goto.rs
• tests/ui/asm/x86_64/goto-block-safe.stderr
• tests/ui/asm/x86_64/bad-options.rs
• tests/codegen/asm/goto.rs

[rustbot]

r? @ehuss

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[nbdd0121]

@rustbot label: -T-compiler +T-lang +A-inline-assembly +F-asm

Cc @rust-lang/wg-inline-asm

[ehuss]

r? compiler

cc @Amanieu

[nnethercote]

The code changes look fine for stabilizing this feature, so r=me in terms of code. But I am uncertain about the stabilization process. Is any additional T-lang approval needed before this merges? Has such approval already been obtained?

[nbdd0121]

@nnethercote this would need a FCP.

[jieyouxu]

r? lang (for T-lang FCP)

[traviscross]

@rfcbot fcp merge

Thanks @nbdd0121 for putting forward this stabilization. We talked about it in lang triage today. Let's propose this for FCP.

[rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rustbot labels +S-waiting-on-documentation

Note that after the FCP completes, this shouldn't merge until the Reference PR is reviewed and approved.

• Add reference for asm-goto reference#1693

(Thanks @nbdd0121 for creating this PR.)

cc @ehuss

[nikomatsakis]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[Amanieu]

A thought occurs: is label really the right keyword to use here to indicate a block? While it is true is that the address of the given block is represented as a label in the asm code, label doesn't really convey what the label actually points to (a block of Rust code) and the fact that this label is an exit from which the assembly code returns control to Rust code.

With that said I don't really have any better ideas for a keyword (other than goto, but that's not a concept that really exists in Rust).

[tmandry]

@rfcbot reviewed

(Already checked my box, but rfcbot doesn't seem to register this.)

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[traviscross]

Note that this is still pending acceptance of the Reference PR, and it should not be merged until the S-waiting-on-documentation label is removed.

[bors]

☔ The latest upstream changes (presumably #135286) made this pull request unmergeable. Please resolve the merge conflicts.