lib.rs

#[macro_use]
extern crate syn;
#[macro_use]
extern crate quote;
extern crate approx;
extern crate proc_macro;

use proc_macro::TokenStream;

#[proc_macro_derive(AbsDiffEq)]
pub fn derive_relative_eq(tokens: TokenStream) -> TokenStream {
    let input = parse_macro_input!(tokens as syn::DeriveInput);
    let generic_params = &input.generics;
    let where_clause = &generic_params.where_clause;
    let ident = &input.ident;

    if let syn::Data::Struct(ref obj) = input.data {
        let fields = &obj.fields;
        let epsilon_ty = get_epsilon_type(fields.iter());

        let mut statements = syn::punctuated::Punctuated::<_, Token![&&]>::new();

        for field in fields {
            let ident = field.ident.as_ref().unwrap();
            let field_ty = &field.ty;
            let field_statement: syn::Expr = parse_quote! {
                approx::AbsDiffEq::abs_diff_eq(&self.#ident, &other.#ident, epsilon.clone() as <#field_ty as approx::AbsDiffEq>::Epsilon)
            };
            statements.push(field_statement);
        }

        let out = quote! {
            #[automatically_derived]
            #[allow(unused_qualifications)]
            impl<#generic_params> approx::AbsDiffEq for #ident
            #where_clause
            {
                type Epsilon = <#epsilon_ty as approx::AbsDiffEq>::Epsilon;

                #[inline]
                fn default_epsilon() -> Self::Epsilon {
                    <#epsilon_ty as approx::AbsDiffEq>::default_epsilon()
                }

                fn abs_diff_eq(&self, other: &Self, epsilon: Self::Epsilon) -> bool {
                    #statements
                }
            }
        };
        out.into()
    } else {
        let out = quote! {};
        out.into()
    }
}

fn get_epsilon_type(fields: syn::punctuated::Iter<syn::Field>) -> syn::Type {
    let f32_ty: syn::Type = parse_quote! {f32};
    let f64_ty: syn::Type = parse_quote! {f64};
    let mut out_type: syn::Type = f32_ty.clone();
    for field in fields {
        let ty = &field.ty;
        if *ty == f64_ty && out_type == f32_ty {
            out_type = ty.clone();
        }
    }
    out_type
}