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extern crate proc_macro; use proc_macro2::TokenStream; use quote::quote; use syn::{parse_quote, Data, DeriveInput, Fields, Type}; #[proc_macro_derive(DefaultBoxed)] pub fn derive_default_boxed(input: proc_macro::TokenStream) -> proc_macro::TokenStream { derive(syn::parse_macro_input!(input)).into() } fn derive(mut input: DeriveInput) -> TokenStream { let name = input.ident; let data = match input.data { Data::Struct(data) => data, _ => panic!("only structs are supported"), }; let fields = match data.fields { Fields::Named(fields) => fields, _ => panic!("only structs with named fields are supported"), }; let body: TokenStream = fields.named.into_iter().map(|field| { let name = field.ident.unwrap(); match field.ty { Type::Array(array) => { let ty = array.elem; let len = array.len; quote! { let #name = (*ptr).#name.as_mut_ptr(); for i in 0usize..#len { <#ty as default_boxed::DefaultBoxed>::default_in_place(#name.offset(i as isize)); } } } ty => quote!{ <#ty as default_boxed::DefaultBoxed>::default_in_place(&mut (*ptr).#name); } } }).collect(); if !input.generics.params.is_empty() { let mut where_clause = input.generics.where_clause.take(); let predicates = &mut where_clause.get_or_insert(parse_quote!(where)).predicates; for param in input.generics.type_params() { let ident = ¶m.ident; predicates.push(parse_quote!(#ident: default_boxed::DefaultBoxed)); } input.generics.where_clause = where_clause; } let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl(); quote! { impl#impl_generics default_boxed::DefaultBoxed for #name#ty_generics #where_clause { unsafe fn default_in_place(ptr: *mut Self) { #body } } } }