Rust is eagerly evaluating range causing inconsistencies

[eloraiby]

Following this small example:

#[cfg(test)]
mod tests {
    #[test]
    fn test_for() {
        let mut v = Vec::new();
        for i in 0..10 {
            v.push(i);
        }

        for i in 0..v.len() {
            if i < 100 {
                v.push(i);
            }
        }

        assert!(v.len() == 110); // Crash! expected 110, count is 20
    }

    #[test]
    fn test_loop() {
        let mut v = Vec::new();
        for i in 0..10 {
            v.push(i);
        }

        let mut i = 0;
        loop {
            if i >= v.len() {
                break;
            }

            if i < 100 {
                v.push(i);
            }

            i += 1;
        }

        assert!(v.len() == 110);
    }

}

If that's the normal behaviour, it should be documented in https://doc.rust-lang.org/std/ops/struct.Range.html, in the language references and tutorials.

The behaviour as it stands is in contrast of what other languages implements and is confusing.

[jonas-schievink]

Nothing in Rust is lazily evaluated. Evaluation order of language constructs like for and the range syntax is in scope of https://github.com/rust-lang/reference/.

[eloraiby]

@jonas-schievink you failed to provide where in that reference! - if it's in there -

[jonas-schievink]

It isn't documented yet – but this is being tracked in rust-lang/reference#248

[eloraiby]

thanks @jonas-schievink