lib.rs

extern crate proc_macro;

#[macro_use]
extern crate syn;

#[macro_use]
extern crate quote;

use proc_macro::TokenStream;
use syn::{DeriveInput, Data, Fields, Generics, GenericParam};
use quote::Tokens;

#[proc_macro_derive(HeapSize)]
pub fn derive_heap_size(input: TokenStream) -> TokenStream {
    // Parse the input tokens into a syntax tree.
    let input: DeriveInput = syn::parse(input).unwrap();

    // 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.
    expanded.into()
}

// 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) -> Tokens {
    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.
                    let fnames = fields.named.iter().map(|f| f.ident);
                    quote! {
                        0 #(
                            + ::heapsize::HeapSize::heap_size_of_children(&self.#fnames)
                        )*
                    }
                }
                Fields::Unnamed(ref fields) => {
                    // Expands to an expression like
                    //
                    //     0 + self.0.heap_size() + self.1.heap_size() + self.2.heap_size()
                    let indices = 0..fields.unnamed.len();
                    quote! {
                        0 #(
                            + ::heapsize::HeapSize::heap_size_of_children(&self.#indices)
                        )*
                    }
                }
                Fields::Unit => {
                    // Unit structs cannot own more than 0 bytes of heap memory.
                    quote!(0)
                }
            }
        }
        Data::Enum(_) | Data::Union(_) => unimplemented!(),
    }
}