src/librustc_macros/src/hash_stable.rs

use synstructure;
use syn::{self, Meta, NestedMeta, parse_quote};
use proc_macro2::{self, Ident};
use quote::quote;

struct Attributes {
    ignore: bool,
    project: Option<Ident>,
}

fn parse_attributes(field: &syn::Field) -> Attributes {
    let mut attrs = Attributes {
        ignore: false,
        project: None,
    };
    for attr in &field.attrs {
        if let Ok(meta) = attr.parse_meta() {
            if &meta.name().to_string() != "stable_hasher" {
                continue;
            }
            let mut any_attr = false;
            if let Meta::List(list) = meta {
                for nested in list.nested.iter() {
                    if let NestedMeta::Meta(meta) = nested {
                        if &meta.name().to_string() == "ignore" {
                            attrs.ignore = true;
                            any_attr = true;
                        }
                        if &meta.name().to_string() == "project" {
                            if let Meta::List(list) = meta {
                                if let Some(nested) = list.nested.iter().next() {
                                    if let NestedMeta::Meta(meta) = nested {
                                        attrs.project = Some(meta.name());
                                        any_attr = true;
                                    }
                                }
                            }
                        }
                    }
                }
            }
            if !any_attr {
                panic!("error parsing stable_hasher");
            }
        }
    }
    attrs
}

pub fn hash_stable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
    let generic: syn::GenericParam = parse_quote!('__ctx);
    s.add_bounds(synstructure::AddBounds::Generics);
    s.add_impl_generic(generic);
    let body = s.each(|bi| {
        let attrs = parse_attributes(bi.ast());
        if attrs.ignore {
             quote!{}
        } else if let Some(project) = attrs.project {
            quote!{
                &#bi.#project.hash_stable(__hcx, __hasher);
            }
        } else {
            quote!{
                #bi.hash_stable(__hcx, __hasher);
            }
        }
    });

    let discriminant = match s.ast().data {
        syn::Data::Enum(_) => quote! {
            ::std::mem::discriminant(self).hash_stable(__hcx, __hasher);
        },
        syn::Data::Struct(_) => quote! {},
        syn::Data::Union(_) => panic!("cannot derive on union"),
    };

    s.bound_impl(quote!(::rustc_data_structures::stable_hasher::HashStable
                        <::rustc::ich::StableHashingContext<'__ctx>>), quote!{
        fn hash_stable<__W: ::rustc_data_structures::stable_hasher::StableHasherResult>(
            &self,
            __hcx: &mut ::rustc::ich::StableHashingContext<'__ctx>,
            __hasher: &mut ::rustc_data_structures::stable_hasher::StableHasher<__W>) {
            #discriminant
            match *self { #body }
        }
    })
}