Add isqrt to NonZero<uN>

[ivan-shrimp]

Implements #70887 (comment), with the following signature:

impl NonZero<uN> {
    const fn isqrt(self) -> Self;
}

Unintended benefits include one fewer panicking branch in ilog2 for LLVM to optimize away, and one fewer assume_unchecked as NonZero already does that.

The fast path for self == 1 is dropped, but the current implementation is very slow anyways compared to hardware. Performance improvements can always come later.

(I didn't add the function to NonZero<iN>, since every existing NonZero method is non-panicking, and it might be nice to leave it that way.)

[rustbot]

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[Noratrieb]

r? libs-api

[Noratrieb]

I recommend you open an API Change Proposal as described here, it's a more efficient way to get the libs-api team to decide on whether this should be accepted or not: https://std-dev-guide.rust-lang.org/development/feature-lifecycle.html
Use @rustbot ready when your ACP has been accepted.
@rustbot author
@rustbot label S-waiting-on-ACP

[ivan-shrimp]

The original isqrt PR landed without an ACP, and this is probably just a straightforward extension. Nonetheless, I started an ACP and I'll wait for it to be accepted.

[tgross35]

ACP accepted rust-lang/libs-team#391 (comment)

@rustbot ready

[ivan-shrimp]

@rustbot label -S-waiting-on-ACP

[tgross35]

Maybe change these to match rather than if let, to be consistent with the new isqrt? Or even .map(NonZero::<Self>::ilog10).

[ivan-shrimp]

Option::map is not const (not even unstably). match does look a bit nicer though.

[tgross35]

Suggested change

	unsafe {
	hint::assert_unchecked(res < 1 << (Self::BITS / 2));
	}
	unsafe { hint::assert_unchecked(res < 1 << (Self::BITS / 2)) };

Extremely minor nit but semi usually looks nicer outside single-line braced blocks

[ivan-shrimp]

@rustbot author

[ivan-shrimp]

@rustbot ready

[bors]

☔ The latest upstream changes (presumably #126914) made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

libs-api already discussed this and it only modifies unstable API, so I think this is okay to merge.

Thanks for the addition!

r? @tgross35
@bors r+ rollup

[bors]

📌 Commit 90bba8b has been approved by tgross35

It is now in the queue for this repository.