[steveklabnik]

Square brackets would be semantically dependent if they appeared in the same position of angle brackets. There's nothing magical about [] that makes the problems with <> disappear.

[GoblinSlayer]

It disappears the problem that angle brackets are sometimes not brackets. I.e. a<b>c is parsed as (a<b)>c or as (a(<b>))c.

[xscott]

It also comes up when you want compile time expressions as parameters to your generics:

    // nice and clean
    let a = Generic[T, A > B]::new(); 

    // gross curlies needed because of poor choices
    let a = Generic::<T, {A > B}>::new();