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extern crate proc_macro;

use syn::{Data, DeriveInput, Error, parse_macro_input};
use quote::quote;

#[proc_macro_derive(Columns)]
pub fn columns_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let ast: syn::DeriveInput = parse_macro_input!(input as DeriveInput);
    let ident = &ast.ident;
    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
    let data = match ast.data {
        Data::Struct(ref data) => data,
        _ => {
            let e = Error::new_spanned(&ast, "trait `Columns` can only be implemented for structs");
            return proc_macro::TokenStream::from(e.to_compile_error());
        }
    };

    let pointers = data.fields.iter().count();
    let dangling = data.fields.iter().map(|field| &field.ty);

    let expanded = quote! {
        unsafe impl #impl_generics ::soak::Columns for #ident #ty_generics #where_clause {
            type Pointers = [::core::ptr::NonNull<u8>; #pointers];

            fn dangling() -> Self::Pointers {
                [ #(unsafe { ::core::ptr::NonNull::new_unchecked(
                    ::core::mem::align_of::<#dangling>() as *mut u8
                ) },)* ]
            }
        }
    };

    proc_macro::TokenStream::from(expanded)
}