`::`→`:` typo in turbofish doesn't have a good error

[estebank]

The following code is using a single : instead of a mod-sep :::

fn main() -> Result<(), ()> {
    vec![Ok(2)].into_iter().collect:<Result<Vec<_>,_>>()?;
    Ok(())
}

The compiler doesn't point at the problem:

error: expected `::`, found `(`
 --> src/main.rs:2:55
  |
2 |     vec![Ok(2)].into_iter().collect:<Result<Vec<_>,_>>()?;
  |                                                       ^ expected `::`

It should point at the : after collect.