BTreeMap: relax the explicit borrow rule to make code shorter and safer

[ssomers]

Expressions like .reborrow_mut().into_len_mut() are annoyingly long, and kind of dangerous for the reason reborrow_mut() is unsafe. By relaxing the single rule, we no longer have to make an exception for functions with a borrow name and functions like as_leaf_mut. This is largely restoring the declaration style of the btree::node API about a year ago, but with more explanation and consistency.

r? @Mark-Simulacrum

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 22d06052a76860c4ff84198fbb9c068ac4cb0909 has been approved by Mark-Simulacrum

[Mark-Simulacrum]

@bors rollup=never

[bors]

🔒 Merge conflict

This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository, you can resolve the conflict following these steps:

1. git checkout btree_cleanup_2 (switch to your branch)
2. git fetch upstream master (retrieve the latest master)
3. git rebase upstream/master -p (rebase on top of it)
4. Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
5. git push self btree_cleanup_2 --force-with-lease (update this PR)

You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub. It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Error message

Auto-merging library/alloc/src/collections/btree/node.rs
CONFLICT (content): Merge conflict in library/alloc/src/collections/btree/node.rs
Automatic merge failed; fix conflicts and then commit the result.

[ssomers]

The conflict in clear_parent_link, and the fact that the previous code no longer compiles with:

error[E0716]: temporary value dropped while borrowed
   --> library\alloc\src\collections\btree\node.rs:632:46
    |
632 |         let leaf = NodeRef::as_leaf_mut(&mut self.borrow_mut());
    |                                              ^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
    |                                              |
    |                                              creates a temporary which is freed while still in use
633 |         leaf.parent = None;
    |         ------------------ borrow later used here
    |

reminds me of an issue I tried in vain to list here in an earlier version of the description: the old definition of fn as_leaf(this: &Self) -> &'a LeafNode<K, V> was more dangerous than it is after this PR: it extended the return value's lifetime beyond the &Self borrow.

[ssomers]

Actually, these lifetime rules must be right because they mimic the public API of OccupiedEntry.

[bors]

☔ The latest upstream changes (presumably #80114) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[ssomers]

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit 29114ff has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 29114ff with merge 3d10d3e...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 3d10d3e to master...