update isolate_highest_one for NonZero<T>

[vrtgs]

Rationale

Let x = self and
m = (((1 as $Int) << (<$Int>::BITS - 1)).wrapping_shr(self.leading_zeros()))
Then the previous code computed NonZero::new_unchecked(x & m).
Since m has exactly one bit set (the most significant 1-bit of x), (x & m) == m.
Therefore, the masking step was redundant.

The shift is safe and does not need wrapping because:

• self.leading_zeros() < $Int::BITS because self is non-zero.
• The result of unchecked_shr is non-zero, satisfying the NonZero invariant. if wrapping happens we would be violating NonZero invariants.

why this micro optimization?
the old code was suboptimal it duplicated $Int’s isolate_highest_one logic instead of delegating to it. Since the type already wraps $Int, either delegation should be used for clarity or, if keeping a custom implementation, it should be optimized as above.

[rustbot]

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[joboet]

Thanks!
r=me after squashing commits
@bors delegate+
r? joboet

[bors]

✌️ @vrtgs, you can now approve this pull request!

If @joboet told you to "r=me" after making some further change, please make that change, then do @bors r=@joboet

[vrtgs]

@bors r=@joboet

[bors]

📌 Commit 057127c has been approved by joboet

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

spellchecking files
building external tool typos from package typos-cli@1.38.1
finished building tool typos
npm WARN deprecated puppeteer@22.15.0: < 24.10.2 is no longer supported
npm ERR! code 127
npm ERR! git dep preparation failed
npm ERR! command /node/bin/node /node/lib/node_modules/npm/bin/npm-cli.js install --force --cache=/home/user/.npm --prefer-offline=false --prefer-online=false --offline=false --no-progress --no-save --no-audit --include=dev --include=peer --include=optional --no-package-lock-only --no-dry-run
npm ERR! npm WARN using --force Recommended protections disabled.
npm ERR! npm WARN deprecated inflight@1.0.6: This module is not supported, and leaks memory. Do not use it. Check out lru-cache if you want a good and tested way to coalesce async requests by a key value, which is much more comprehensive and powerful.
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated readdir-scoped-modules@1.1.0: This functionality has been moved to @npmcli/fs
npm ERR! npm WARN deprecated debuglog@1.0.1: Package no longer supported. Contact Support at https://www.npmjs.com/support for more info.
npm ERR! npm WARN deprecated read-package-json@2.1.2: This package is no longer supported. Please use @npmcli/package-json instead.
npm ERR! npm WARN deprecated read-installed@4.0.3: This package is no longer supported.
npm ERR! npm ERR! code 127
npm ERR! npm ERR! path /home/user/.npm/_cacache/tmp/git-cloneXXXXXXbAD7op/node_modules/rollup
npm ERR! npm ERR! command failed
npm ERR! npm ERR! command sh -c patch-package
npm ERR! npm ERR! sh: 1: patch-package: not found
npm ERR! 
npm ERR! npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T21_29_44_495Z-debug-0.log

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T21_29_36_831Z-debug-0.log
npm install did not exit successfully
tidy [extra_checks]: IO error: npm install returned exit code exit status: 127
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:03:06
  local time: Fri Nov  7 21:30:05 UTC 2025
  network time: Fri, 07 Nov 2025 21:30:05 GMT
##[error]Process completed with exit code 1.