Rollup of 7 pull requests

[frewsxcv]

• Successful merges: rustbuild: Update bootstrap compiler #40524, fixed some clippy warnings in libcollections #40541, Sync all unstable features with Unstable Book; add tidy lint.  #40694, Implement AsRawFd/IntoRawFd for RawFd #40842, rustc: Stabilize the #![windows_subsystem] attribute #40870, Add example to std::process::abort #40904, Add missing link in unstable-book #40925
• Failed merges:

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[frewsxcv]

@bors r+ p=10

[bors]

📌 Commit 9f4b6b8 has been approved by frewsxcv

[bors]

⌛ Testing commit 9f4b6b8 with merge 2057bc4...

[frewsxcv]

@bors r-

[frewsxcv]

@bors retry

[frewsxcv]

@bors r+ p=10

[bors]

📌 Commit 9f4b6b8 has been approved by frewsxcv

[bors]

⌛ Testing commit 9f4b6b8 with merge ebcb2f0...

[bors]

💔 Test failed - status-travis