rustc --explain E0744 says "while" is forbidden but accepts it nontheless

[karolherbst]

I have to use some sort of loop construct within a const function. rustc --explain E0744 does explain that loop constructs are at the moment not supported, but as I was trying out for first and the compiler rejected it, it would have been helpful if the explanation would hint me towards while working.

I think the doc just needs a small update and I think the file to update is https://github.com/rust-lang/rust/blob/master/compiler/rustc_error_codes/src/error_codes/E0744.md

I tried this code:

const fn foo() -> [u8; 10] {
    let mut a = [0; 10];
    let b = b"hi";
    let mut i = 0;
    while i < b.len() {
        a[i] = b[i];
        i += 1;
    }
    
    a
}

pub const BAR: [u8; 10] = foo();

Meta

rustc --version --verbose:

rustc 1.47.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.47.0
LLVM version: 11.0

[ThePuzzlemaker]

@rustbot claim

[ThePuzzlemaker]

if, match, and loop are also allowed, but for is not.

[jyn514]

@karolherbst if it helps, the reason for is forbidden but the others work is that for uses IntoIterator and traits in const functions are still unstable (search for 'for loops' on https://rust-lang.github.io/const-eval/).

[karolherbst]

@karolherbst if it helps, the reason for is forbidden but the others work is that for uses IntoIterator and traits in const functions are still unstable (search for 'for loops' on https://rust-lang.github.io/const-eval/).

yeah, that's understandable. The confusion on my side was simply that I didn't know I was allowed to user other control flow constructs until somebody mentioned it on IRC I actually could.