Surprising behavior of `'label: for`

[workingjubilee]

Consider the following code:

fn main() {
    let mut s = 0;
    'label: for i in 0..100 {
        println!("s: {s}, i: {i}");
        s += 1;

        if i == 3 {
            continue 'label;
        }
    }
}

The expectation was that it would work like the following C code:

#include <stdio.h>
int main() {
    int s = 0;
    label: for (int i = 0; i < 100; i++) {
        printf("s: %d, i: %d\n", s++, i);
        if (i == 3) {
            goto label;
        }
    }
}

Instead, the continuation at the for just advances the loop counter of the for, running the expression inside it. This is surprising behavior to anyone who is familiar with this C idiom, as while goto is... not exactly equal to continue, for many purposes we make continue and break a "controlled goto" exactly so that we can support these idioms. The code that achieves this idiom is, instead:

fn main() {
    let mut s = 0;
    'label: loop {
        for i in 0..100 {
            println!("{s}, {i}");
            s += 1;
    
            if i == 3 {
                continue 'label;
            }
        }
        break;
    }
}

This "continue 'label but it's equal to continue" pattern could be recognized and linted on as it is almost never what is wanted, much as we warn about other useless syntax.

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rustc --version --verbose:

rustc 1.82.0-nightly (cefe1dcef 2024-07-22)
binary: rustc
commit-hash: cefe1dcef0e21f4d0c8ea856ad61c1936dfb7913
commit-date: 2024-07-22
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 18.1.7