Isn't the initial capacity insufficient whenever `align_of::<H> >= align_of::<T>`? about header-vec HOT 2 CLOSED

    /// Gives the offset in units of T (as if the pointer started at an array of T) that the slice actually starts at.
    #[inline(always)]
    fn offset() -> usize {
        // We need to first compute the first location we can start in align units.
        // Then we go from align units to offset units using mem::align_of::<T>() / mem::size_of::<T>().
        (mem::size_of::<HeaderVecHeader<H>>() + mem::size_of::<T>() - 1) / mem::size_of::<T>()
    }

    /// Compute the number of elements (in units of T) to allocate for a given capacity.
    #[inline(always)]
    fn elems_to_mem_elems(capacity: usize) -> usize {
        Self::offset() + capacity
    }

    /// Compute the number of elements (in units of T) to allocate for a given capacity.
    #[inline(always)]
    fn elems_to_mem_bytes(capacity: usize) -> usize {
        Self::elems_to_mem_elems(capacity) * mem::size_of::<T>()
    }

    /// Compute the number of elements (in units of T) to allocate for a given capacity.
    #[inline(always)]
    fn layout(capacity: usize) -> alloc::alloc::Layout {
        alloc::alloc::Layout::from_size_align(
            Self::elems_to_mem_bytes(capacity),
            cmp::max(mem::align_of::<H>(), mem::align_of::<T>()),
        )
        .expect("unable to produce memory layout with Hrc key type (is it a zero sized type? they are not permitted)")
    }

alloc::alloc::Layout::from_size_align(
    5,
    4,
)