For loop iteration allows mutating container.

[krdln]

Following code passes borrow checking leading to serious memory unsafety.

fn main() {
    let mut v = Vec::new();
    for _ in v.iter() {
        v.push('j');
    }
}

Code compiles with &c instead of _ too. Loop fails to compile only in for c in v.iter() case where there's an explicit variable with reference or when the loop is desugared manually.

[huonw]

cc @pcwalton, nominating.

[ghost]

Duplicate of #16820.

[huonw]

Thanks (both @jakub- and @krdln).