That's is completely false, stop spreading misinformation.

This code will compile (see https://play.rust-lang.org/?version=stable&mode=debug&editio...):

  pub fn foo() -> Result<(), i64> {
      Err(1)
  }
  
  pub fn bar() -> Result<(), ()> {
      foo();
      Ok(())
  }

It will provide a warning, but there's a ton of stuff in c++ that would throw a warning and you wouldn't say that it "will not compile".

A trivial change that still doesn't handle the error would get rid of the warning.

  pub fn foo() -> Result<(), i64> {
      Err(1)
  }
  
  pub fn bar() -> Result<(), ()> {
      println!("{:?}", foo());
      Ok(())
  }