branch prediction hint intrinsics in stable rust

lib.rs

/// std::intrinsics::{likely, unlikely, assume} in stable Rust
mod predict {
  #[inline(never)]
  #[cold]
  fn unlikely_inner() {}

  pub fn unlikely<B: Into<bool>>(b: B) -> bool {
    let b = b.into();
    if b { unlikely_inner(); }
    b
  }

  pub fn likely<B: Into<bool>>(b: B) -> bool {
    let b = b.into();
    if !b { unlikely_inner(); }
    b
  }
  
  pub unsafe fn assume<B: Into<bool>>(b: B) {
    if !b.into() { std::hint::unreachable_unchecked(); }
  }
}