Why does println! inline variable syntax look inconsistent?

Question

let a = [10, 20, 30, 40, 50];
let mut index_ = 0;
while index_ < 5 {
    println!("{}", a[index_]); // works
    println!("{a[index_]}");   // does not work
    println!("{index_}");      // works
    println!("{}", index_);    // works
    index_ = index_ + 1;
}

Why does "{a[index_]}" not work? It seems like it should to me.

Answer 1

The documentation says that this syntax is called "named parameters", and it supports names, not arbitrary expressions.

If a named parameter does not appear in the argument list, format! will reference a variable with that name in the current scope.

a[index_] is not a valid name (but is a valid expression), so you get the error because the format! syntax doesn't let you use arbitrary expressions inside {}, like in Python. Note that println! "uses the same syntax as format!", so the same reasoning applies to println! as well.