constify VecDeque

[Lars-Schumann]

#79597

[rustbot]

r? @joboet

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The reviewer was selected based on:

• Owners of files modified in this PR: libs
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[joboet]

r? libs-api

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

⚠️ Warning ⚠️

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

  • 52458a5

  You can start a rebase with the following commands:

  $ # rebase
  $ git pull --rebase https://github.com/rust-lang/rust.git main
  $ git push --force-with-lease