range notation with for expression for i in 0 .. n {} should be parsed as for i in (0 .. n) {}

[mandel59]

The problem is that for i in 0 .. n {} is ambiguous between for i in (0 .. n) {} and for i in 0 .. (n {}) for consistency.

fn main() {
    let n = 10;

    for i in 0..n {
        println!("{}", i);
    }
}

This code yields:

% rustc ambiguous.rs
ambiguous.rs:5:16: 5:17 error: expected `:`, found `!`
ambiguous.rs:5         println!("{}", i);
                              ^

While these codes pass:

fn main() {
    for i in 0..10 {
        println!("{}", i);
    }
}

fn main() {
    let mut n = 0..10;
    for i in n {
        println!("{}", i);
    }
}

[csouth3]

possibly related to #20811?

[Gankra]

Particularly nasty dupe of #20811