The src pointers in CopyOnDrop and InsertionHole used to be *mut T, and
were derived via automatic conversion from &mut T. According to Stacked
Borrows 2.1, this means that those pointers become invalidated by
interior mutation in the comparison function.
But there's no need for mutability in this code path. Thus, we can
change the drop guards to use *const and derive those from &T.
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