sort: Guard the fast path by length check
The right part must not be empty.
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@ -1066,10 +1066,12 @@ fn merge_sort<T, F>(v: &mut [T], mut compare: F) where F: FnMut(&T, &T) -> Order
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let mut out = buf_tmp.offset(start as isize);
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let out_end = buf_tmp.offset(right_end_idx as isize);
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// if left[last] <= right[0], they are already in order:
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// If left[last] <= right[0], they are already in order:
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// fast-forward the left side (the right side is handled
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// in the loop).
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if compare(&*right.offset(-1), &*right) != Greater {
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// If `right` is not empty then left is not empty, and
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// the offsets are in bounds.
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if right != right_end && compare(&*right.offset(-1), &*right) != Greater {
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let elems = (right_start as usize - left as usize) / mem::size_of::<T>();
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ptr::copy_nonoverlapping(&*left, out, elems);
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out = out.offset(elems as isize);
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