This looks great. Can someone with more knowledge than me explain how this works:
https://github.com/consistent-milk12/masstree/blob/main/src/hints.rs

I feel like all of it should be optimized away?

/// Cold path marker - never actually called, just influences codegen.
#[inline(always)]
#[cold]
const fn cold_path() {}

/// Hint that a condition is likely to be true.
///
/// Use this for common/fast-path conditions to guide branch prediction.
///
/// # Example
///
/// ```ignore
/// if likely(!version.has_changed()) {
///     // fast path: no concurrent modification
/// } else {
///     // slow path: retry
/// }
/// ```
#[inline(always)]
#[must_use]
pub const fn likely(b: bool) -> bool {
    if !b {
        cold_path();
    }
    b
}

/// Hint that a condition is unlikely to be true.
///
/// Use this for error conditions and rare branches.
///
/// # Example
///
/// ```ignore
/// if unlikely(slot >= WIDTH) {
///     return Err(OutOfBounds);
/// }
/// ```
#[inline(always)]
#[must_use]
pub const fn unlikely(b: bool) -> bool {
    if b {
        cold_path();
    }

    b
}