https://x.com/davidtolnay/status/1968356468643741868

lib.rs

#![no_std]
#![feature(likely_unlikely)]

use core::hint::{likely, unlikely};

unsafe extern "Rust" {
    safe fn next() -> isize;
    safe fn process(_: char);
}

pub fn f() {
    let mut id = next();
    let mut first = true;
    'control: loop {
        'end: {
            'd: {
                'c: {
                    'b: {
                        'a: {
                            if first {
                                first = false;
                                if likely(id == 1) {
                                    break 'a;
                                }
                            }
                            match id {
                                1 => break 'a,
                                2 => break 'b,
                                3 => break 'c,
                                4 => break 'd,
                                5 => break 'end,
                                _ => {
                                    id = next();
                                    continue 'control;
                                }
                            }
                        }
                        process('A');
                        id = next();
                        if unlikely(id != 2) {
                            continue 'control;
                        }
                    }
                    process('B');
                    id = next();
                    if unlikely(id != 3) {
                        continue 'control;
                    }
                }
                process('C');
                id = next();
                if unlikely(id != 4) {
                    continue 'control;
                }
            }
            process('D');
            id = next();
            if unlikely(id != 5) {
                continue 'control;
            }
        }
        process('.');
        return;
    }
}

lib.s

example::f::hd332a77f3ea30ba2:
        push    rax
.LBB0_1:
        call    next
.LBB0_2:
        cmp     rax, 1
        jne     .LBB0_3
        mov     edi, 65
        call    process
        call    next
        cmp     rax, 2
        jne     .LBB0_2
.LBB0_6:
        mov     edi, 66
        call    process
        call    next
        cmp     rax, 3
        jne     .LBB0_2
.LBB0_7:
        mov     edi, 67
        call    process
        call    next
        cmp     rax, 4
        jne     .LBB0_2
.LBB0_8:
        mov     edi, 68
        call    process
        call    next
        cmp     rax, 5
        je      .LBB0_9
        jmp     .LBB0_2
.LBB0_3:
        add     rax, -2
        cmp     rax, 3
        ja      .LBB0_1
        jmp     qword ptr [8*rax + .LJTI0_0]
.LBB0_9:
        mov     edi, 46
        pop     rax
        jmp     process
.LJTI0_0:
        .quad   .LBB0_6
        .quad   .LBB0_7
        .quad   .LBB0_8
        .quad   .LBB0_9