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`const fn new` and `const fn capacity` sit in an impl with a `B: Behavior` bound, which const fns accept only since 1.61 (`const_fn_trait_bound`), so the crate has not built on 1.59 since 3d9a1fd. Update the README, the crate docs and the CI matrix to 1.61, and add `rust-version` to Cargo.toml so cargo reports the requirement instead of failing mid-build. While an LLM was used to help implement this, I can vouch for every line.
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The previous implementation of `extend_back()` pushed one element at a time, each push recomputing the wrapped index, checking for a full deque and storing `len`. Compute the free capacity up front as its (at most two) contiguous regions and fill them with plain slice loops, counting in a local that an `AddLenOnDrop` guard adds to `len` on exit, so a panicking iterator keeps the elements already written and the loop body is one unconditional store per element, which LLVM vectorizes for primitive `Copy` elements from slice iterators. The storage cast behind `as_uninit_slice_mut()` moves into a function of the `xs` field, so `extend_back()` can borrow the storage and `len` apart without unsafe code of its own. Also add `extend_back_from_slice()` for `T: Copy`: one block copy per free region, for element types the optimizer doesn't automatically vectorize (odd sizes, padded structs) and for older compilers. The copy goes through a private copy of std's `write_copy_of_slice()` (stable since 1.93). The `Wrapping` deque gets neither change. Its `extend_back()` overwrites from the front once full, so filling the free regions is only its first phase, and its `Extend` impl saturates rather than evicts (andylokandy#32); the behavior's contract should be settled before touching the one or adding the other. While an LLM was used to help implement this, I can vouch for every line.
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This was a rendering check of the pull request text before opening it upstream. Nothing to review here: the real pull request is PR 35 on andylokandy/arraydeque, from this same branch.