What makes the Go code so verbose isn't so much that you have to type "func()", but rather that you need to list out all the types, as well as the need for an explicit return.

  squares := numbers.
      filter(func(x) { x >= 0 }).
      map(func(x) { x * x })

  let squares: Vec<_> = numbers.iter()
      .filter(fn(x) -> { x >= 0 })
      .map(fn(x) -> { x * x })
      .collect();

You don't necessarily need new syntax for this, although it may help with clarity.

The Rust => code is still a bit shorter, but not much and I don't think it matters much.