Fix UB in retain_with_order()

[djc]

Fixes #42.

[Copilot]

Pull request overview

Fixes UB in LinkedHashMap::retain_with_order() when keys can mutate their hash/equality via interior mutability during the user callback (issue #42), preventing the hash table from ending up pointing at a freed/moved-out node.

Changes:

• In retain_with_order(), compute the node’s hash before invoking the callback and remove the hash-table entry by pointer identity (not key equality) to ensure the removed table entry matches the freed list node.
• Add a regression test that triggers the reported scenario (key mutated to compare equal to another key inside the callback) and exercises a subsequent lookup under miri.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/linked_hash_map.rs	Makes retain_with_order() remove the exact node being filtered (pre-hash + pointer-identity match) to avoid stale table pointers / UB.
tests/linked_hash_map.rs	Adds a regression test covering the interior-mutability key mutation case from issue #42.

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