default_boxed_derive/
lib.rs

1extern crate proc_macro;
2
3use proc_macro2::{Span, TokenStream};
4use quote::{quote, quote_spanned, ToTokens};
5use syn::punctuated::Punctuated;
6use syn::spanned::Spanned;
7use syn::token::Comma;
8use syn::{parse_quote, Data, DeriveInput, Fields, Ident, Index, Type};
9
10#[proc_macro_derive(DefaultBoxed)]
11pub fn derive_default_boxed(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
12    derive(syn::parse_macro_input!(input)).into()
13}
14
15fn derive(mut input: DeriveInput) -> TokenStream {
16    let name = &input.ident;
17    let data = match input.data {
18        Data::Struct(data) => data,
19        _ => {
20            return quote_spanned! { input.span() =>
21                compile_error!("only structs are supported");
22            };
23        }
24    };
25
26    // Generate code to write default value into the position pointed by the ptr.
27    let fields = match data.fields {
28        Fields::Named(fields) => fields.named.into_iter(),
29        Fields::Unnamed(fields) => fields.unnamed.into_iter(),
30        Fields::Unit => Punctuated::<_, Comma>::new().into_iter(),
31    };
32    let write_default: TokenStream = fields
33        .enumerate()
34        .map(|(i, field)| {
35            let name = field.ident.map_or_else(
36                || Index::from(i).to_token_stream(),
37                |ident| ident.to_token_stream(),
38            );
39            write_to_uninit(&quote!((*ptr).#name), &field.ty, 0)
40        })
41        .collect();
42
43    if !input.generics.params.is_empty() {
44        let mut where_clause = input.generics.where_clause.take();
45        let predicates = &mut where_clause.get_or_insert(parse_quote!(where)).predicates;
46        for param in input.generics.type_params() {
47            let ident = &param.ident;
48            predicates.push(parse_quote!(#ident: ::default_boxed::DefaultBoxed));
49        }
50        input.generics.where_clause = where_clause;
51    }
52
53    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
54    quote! {
55        unsafe impl#impl_generics default_boxed::DefaultBoxed for #name#ty_generics
56        #where_clause
57        {
58            unsafe fn default_in_place(ptr: *mut Self) {
59                #write_default
60            }
61        }
62    }
63}
64
65fn write_to_uninit(var: &TokenStream, ty: &Type, array_depth: usize) -> TokenStream {
66    match ty {
67        Type::Array(arr) => {
68            let array_depth = array_depth + 1;
69            let i = format!("i{array_depth}");
70            let i = Ident::new(&i, Span::call_site());
71            let len = &arr.len;
72            let inner = write_to_uninit(&quote!(#var[#i]), &arr.elem, array_depth);
73            quote! {
74                for #i in 0..#len {
75                    #inner
76                }
77            }
78        }
79        Type::Tuple(tuple) => tuple
80            .elems
81            .iter()
82            .enumerate()
83            .map(|(i, elem)| {
84                let idx = Index::from(i);
85                write_to_uninit(&quote!(#var.#idx), elem, array_depth)
86            })
87            .collect(),
88        ty => {
89            let call = quote_spanned! { ty.span() =>
90                <#ty as ::default_boxed::DefaultBoxed>::default_in_place
91            };
92            quote! { #call(::core::ptr::addr_of_mut!(#var)); }
93        }
94    }
95}
default_boxed_derive/lib.rs

default_boxed_derive/
lib.rs
