Update documentation for cold_path, likely, and unlikely

[tgross35]

• Add a note recommending benchmarks to cold_path, as other hints have
• Note that cold_path can be used to implement likely and unlikely
• Update the tracking issue for the likely_unlikely feature

Tracking issue: #136873
Tracking issue: #151619

[rustbot]

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[jhpratt]

@bors r+ rollup

[rust-bors]

📌 Commit bdf5790 has been approved by jhpratt

It is now in the queue for this repository.

[traviscross]

Suggested change

	/// #[inline(always)]
	/// pub const fn likely(b: bool) -> bool {
	/// if b {
	/// true
	/// } else {
	/// cold_path();
	/// false
	/// }
	/// }
	///
	/// #[inline(always)]
	/// pub const fn unlikely(b: bool) -> bool {
	/// if b {
	/// cold_path();
	/// true
	/// } else {
	/// false
	/// }
	/// }
	/// #[inline(always)]
	/// pub const fn likely(b: bool) -> bool {
	/// if !b {
	/// cold_path();
	/// }
	/// b
	/// }
	///
	/// #[inline(always)]
	/// pub const fn unlikely(b: bool) -> bool {
	/// if b {
	/// cold_path();
	/// }
	/// b
	/// }

Or is there a reason to prefer the less-tight factoring?

[tgross35]

Nope, this is just our implementation in intrinsics. I’ll apply this.

[tgross35]

@bors r-

For a chance to apply TC’s suggestion

[rust-bors]

Commit bdf5790 has been unapproved.

[scottmcm]

Doc update makes sense to me, and linking to #151619 is the right one

@bors r+ rollup

[rust-bors]

📌 Commit 6c0ae93 has been approved by scottmcm

It is now in the queue for this repository.