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c3e6c32 Oct 29, 2018
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extern crate proc_macro;
use proc_macro2::TokenStream;
use quote::{quote, quote_spanned};
use syn::spanned::Spanned;
use syn::{parse_macro_input, parse_quote, Data, DeriveInput, Fields, GenericParam, Generics, Index};
#[proc_macro_derive(HeapSize)]
pub fn derive_heap_size(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
// Parse the input tokens into a syntax tree.
let input = parse_macro_input!(input as DeriveInput);
// Used in the quasi-quotation below as `#name`.
let name = input.ident;
// Add a bound `T: HeapSize` to every type parameter T.
let generics = add_trait_bounds(input.generics);
let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
// Generate an expression to sum up the heap size of each field.
let sum = heap_size_sum(&input.data);
let expanded = quote! {
// The generated impl.
impl #impl_generics ::heapsize::HeapSize for #name #ty_generics #where_clause {
fn heap_size_of_children(&self) -> usize {
#sum
}
}
};
// Hand the output tokens back to the compiler.
proc_macro::TokenStream::from(expanded)
}
// Add a bound `T: HeapSize` to every type parameter T.
fn add_trait_bounds(mut generics: Generics) -> Generics {
for param in &mut generics.params {
if let GenericParam::Type(ref mut type_param) = *param {
type_param.bounds.push(parse_quote!(::heapsize::HeapSize));
}
}
generics
}
// Generate an expression to sum up the heap size of each field.
fn heap_size_sum(data: &Data) -> TokenStream {
match *data {
Data::Struct(ref data) => {
match data.fields {
Fields::Named(ref fields) => {
// Expands to an expression like
//
// 0 + self.x.heap_size() + self.y.heap_size() + self.z.heap_size()
//
// but using fully qualified function call syntax.
//
// We take some care to use the span of each `syn::Field` as
// the span of the corresponding `heap_size_of_children`
// call. This way if one of the field types does not
// implement `HeapSize` then the compiler's error message
// underlines which field it is. An example is shown in the
// readme of the parent directory.
let recurse = fields.named.iter().map(|f| {
let name = &f.ident;
quote_spanned! {f.span()=>
::heapsize::HeapSize::heap_size_of_children(&self.#name)
}
});
quote! {
0 #(+ #recurse)*
}
}
Fields::Unnamed(ref fields) => {
// Expands to an expression like
//
// 0 + self.0.heap_size() + self.1.heap_size() + self.2.heap_size()
let recurse = fields.unnamed.iter().enumerate().map(|(i, f)| {
let index = Index::from(i);
quote_spanned! {f.span()=>
::heapsize::HeapSize::heap_size_of_children(&self.#index)
}
});
quote! {
0 #(+ #recurse)*
}
}
Fields::Unit => {
// Unit structs cannot own more than 0 bytes of heap memory.
quote!(0)
}
}
}
Data::Enum(_) | Data::Union(_) => unimplemented!(),
}
}