You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: derive_buffer_contents.rs
    path of file: ./repos/vulkano/vulkano-macros/src
    
    the code of the file until where you have to start completion: use crate::bail;
use proc_macro2::TokenStream;
use quote::{quote, quote_spanned};
use syn::{
    parse_quote, spanned::Spanned, Data, DeriveInput, Fields, FieldsNamed, FieldsUnnamed, Ident,
    Result, Type, TypeArray, TypeSlice, WherePredicate,
};

pub fn derive_buffer_contents(crate_ident: &Ident, mut ast: DeriveInput) -> Result<TokenStream> {
    let struct_ident = &ast.ident;

    let is_repr_rust = ast
        .attrs
        .iter()
        .filter(|&attr| attr.path().is_ident("repr"))
        .map(|attr| {
            let mut is_repr_rust = true;

            let _ = attr.parse_nested_meta(
    
pub fn derive_buffer_contents(crate_ident: &Ident, mut ast: DeriveInput) -> Result<TokenStream> {
    let struct_ident = &ast.ident;

    let is_repr_rust = ast
        .attrs
        .iter()
        .filter(|&attr| attr.path().is_ident("repr"))
        .map(|attr| {
            let mut is_repr_rust = true;

            let _ = attr.parse_nested_meta(|meta| {
                if meta.path.is_ident("C") || meta.path.is_ident("transparent") {
                    is_repr_rust = false;
                }

                Ok(())
            });

            is_repr_rust
        })
        .all(|b| b);

    if is_repr_rust {
        bail!(
            "deriving `BufferContents` is only supported for types that are marked `#[repr(C)]` \
            or `#[repr(transparent)]`",
        );
    }

    let (impl_generics, type_generics, where_clause) = {
        let predicates = ast
            .generics
            .type_params()
            .map(|ty| {
                parse_quote! { #ty: ::#crate_ident::buffer::BufferContents }
            })
            .collect::<Vec<WherePredicate>>();

        ast.generics
            .make_where_clause()
            .predicates
            .extend(predicates);

        ast.generics.split_for_impl()
    };

    let layout = write_layout(crate_ident, &ast)?;

    Ok(quote! {
        #[allow(unsafe_code)]
        unsafe impl #impl_generics ::#crate_ident::buffer::BufferContents
            for #struct_ident #type_generics #where_clause
        {
            const LAYOUT: ::#crate_ident::buffer::BufferContentsLayout = #layout;

            #[inline(always)]
            unsafe fn ptr_from_slice(slice: ::std::ptr::NonNull<[u8]>) -> *mut Self {
                #[repr(C)]
                union PtrRepr<T: ?Sized> {
                    components: PtrComponents,
                    ptr: *mut T,
                }

                #[derive(Clone, Copy)]
                #[repr(C)]
                struct PtrComponents {
                    data: *mut u8,
                    len: usize,
                }

                let data = <*mut [u8]>::cast::<u8>(slice.as_ptr());

                let head_size = <Self as ::#crate_ident::buffer::BufferContents>::LAYOUT
                    .head_size() as usize;
                let element_size = <Self as ::#crate_ident::buffer::BufferContents>::LAYOUT
                    .element_size()
                    .unwrap_or(1) as usize;

                ::std::debug_assert!(slice.len() >= head_size);
                let tail_size = slice.len() - head_size;
                ::std::debug_assert!(tail_size % element_size == 0);
                let len = tail_size / element_size;

                let components = PtrComponents { data, len };

                // SAFETY: All fields must implement `BufferContents`. The last field, if it is
                // unsized, must therefore be a slice or a DST derived from a slice. It cannot be
                // any other kind of DST, unless unsafe code was used to achieve that.
                //
                // That means we can safely rely on knowing what kind of DST the implementing type
                // is, but it doesn't tell us what the correct representation for the pointer of
                // this kind of DST is. For that we have to rely on what the docs tell us, namely
                // that for structs where the last field is a DST, the metadata is the same as the
                // last field's. We also know that the metadata of a slice is its length measured
                // in the number of elements. This tells us that the components of a pointer to the
                // implementing type are the address to the start of the data, and a length. It
                // still does not tell us what the representation of the pointer is though.
                //
                // In fact, there is no way to be certain that this representation is correct.
                // *Theoretically* rustc could decide tomorrow that the metadata comes first and
                // the address comes last, but the chance of that ever happening is zero.
                //
                // But what if the implementing type is actually sized? In that case this
                // conversion will simply discard the length field, and only leave the pointer.
                PtrRepr { components }.ptr
            }
        }
    })
}