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: lib.rs
    path of file: ./repos/feather/feather/plugin-host/macros/src
    
    the code of the file until where you have to start completion: use quote::{format_ident, quote};
use syn::{FnArg, GenericArgument, PathSegment};

/// Annotates a function so that it implements
/// the NativeHostFunction trait.
#[proc_macro_attribute]
pub fn host_function(
    _args: proc_macro::TokenStream,
    input: proc_macro::To
    
            fn to_function_pointer(self) -> usize {
                // Safety: see the Nomicon: https://rust-lang.github.io/unsafe-code-guidelines/layout/function-pointers.html#representation
                // For all targets that Feather compiles to, function pointers
                // have the same layout as a usize.

                unsafe {
                    std::mem::transmute(#gateway_ident as *const ())
                }
            }