Tracking Issue for checked bitshifts

Feature gate: #![feature(exact_bitshifts)]

This is a tracking issue for checked bitshifts, i.e. bitshifts that can be losslessly reversed.
ACP: rust-lang/libs-team#570

Public API

impl iX/uX {
    // Shift succeeds if `shift < Self::BITS` and right shifting the result returns the original value.
    fn exact_shl(self, shift: u32) -> Option<Self>;
    unsafe fn unchecked_exact_shl(self, shift: u32) -> Self;

    // Shift succeeds if `shift < Self::BITS` and left shifting the result returns the original value.
    fn exact_shr(self, shift: u32) -> Option<Self>;
    unsafe fn unchecked_exact_shr(self, shift: u32) -> Self;
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation: #144342
• Add new intrinsics rather than building on unchecked_sh{l,r} https://github.com/rust-lang/libs-team/issues/570#issuecomment-2839591488
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Should exact_shl panic instead of return None, then introduce a new checked_exact_shl -> Option? https://github.com/rust-lang/rust/pull/144342#issuecomment-3105660850

• None yet.

Start with the exact_bitshifts feature gate and proposed public API in this issue, then read implementation PR #144342 and the linked libs-team discussion. Pay particular attention to the unresolved exact_shl API question; the feature is done when implementation, intrinsic decisions, FCP, and stabilization are complete.