Rust i64 + (i32 as i64) = i64 + i64(with sign extension) = i64

output

sh-3.2$ rustc test.rs
warning: literal out of range for i32
 --> test.rs:2:40
  |
2 |     println!("{:x}", 0x100000000_i64 + 0xcafebabe_i32 as i64);
  |                                        ^^^^^^^^^^^^^^ help: consider using `u32` instead: `0xcafebabe_u32`
  |
  = note: #[warn(overflowing_literals)] on by default
  = note: the literal `0xcafebabe_i32` (decimal `3405691582`) does not fit into an `i32` and will become `-889275714i32`

sh-3.2$ ./test
cafebabe

test.rs

fn main() {
    println!("{:x}", 0x100000000_i64 + 0xcafebabe_i32 as i64);
}