virtual function elimination: llvm error: `Do not know how to promote this operator`

original:

pub mod kitty {
    use std::fmt;

    pub struct cat {
      meows : usize,
      pub how_hungry : isize,
      pub name : String,
    }

    impl fmt::Display for cat {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "{}", self.name)
        }
    }

    impl cat {
        fn meow(&mut self) {
            println!("Meow");
            self.meows += 1;
            if self.meows % 5 == 0 {
                self.how_hungry += 1;
            }
        }

    }

    impl cat {
        pub fn speak(&mut self) { self.meow(); }

        pub fn eat(&mut self) -> bool {
            if self.how_hungry > 0 {
                println!("OM NOM NOM");
                self.how_hungry -= 2;
                return true;
            }
            else {
                println!("Not hungry!");
                return false;
            }
        }
    }

    pub fn cat(in_x : usize, in_y : isize, in_name: String) -> cat {
        cat {
            meows: in_x,
            how_hungry: in_y,
            name: in_name
        }
    }
}

warning: type `cat` should have an upper camel case name
 --> /tmp/icemaker_global_tempdir.8xGPA271RB07/rustc_testrunner_tmpdir_reporting.GzzcQXIiIZoj/mvce.rs:4:16
  |
4 |     pub struct cat {
  |                ^^^ help: convert the identifier to upper camel case (notice the capitalization): `Cat`
  |
  = note: `#[warn(non_camel_case_types)]` on by default

warning: field `meows` is never read
 --> /tmp/icemaker_global_tempdir.8xGPA271RB07/rustc_testrunner_tmpdir_reporting.GzzcQXIiIZoj/mvce.rs:5:9
  |
4 |     pub struct cat {
  |                --- field in this struct
5 |         meows: usize,
  |         ^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

rustc-LLVM ERROR: Do not know how to promote this operator!