beginner – First Rust program – squares which are sum of squares

I’ve been programming in C++ for over a decade and am thinking of learning Rust. Finding a number whose square is the sum of three different positive squares in at least two disjoint ways, which I used in test code in a C++ program, I wrote a Rust program to find out what other numbers’ squares can be written as the sum of three different positive squares. Here’s the program; how can it be improved?

fn main()
{
  for a in 1..101
  {
    let mut b=1;
    while a*a-b*b>=0
    {
      let mut c=1;
      while a*a-b*b-c*c>=0 && c<b
      {
        let mut d=1;
        while a*a-b*b-c*c-d*d>=0 && d<c
        {
          if a*a-b*b-c*c-d*d==0
          {
            println!("{}²={}²+{}²+{}²",a,b,c,d);
          }
          d+=1;
        }
        c+=1;
      }
      b+=1;
    }
  }
}