extern crate proc_macro;
use std::iter::FromIterator;
use proc_macro::TokenStream;
use proc_macro2::TokenStream as TokenStream2;
use quote::quote;
use syn::{
parse::{Error, Parse, ParseStream},
parse_macro_input,
punctuated::Punctuated,
spanned::Spanned,
GenericParam, Generics, Ident, Result, Token, TypeTraitObject, Visibility,
};
struct TraitSet {
visibility: Visibility,
_trait_token: Token![trait],
alias_name: Ident,
generics: Generics,
_eq_token: Token![=],
traits: TypeTraitObject,
}
impl TraitSet {
fn render(self) -> TokenStream2 {
if self.generics.params.is_empty() {
self.render_non_generic()
} else {
self.render_generic()
}
}
fn render_non_generic(self) -> TokenStream2 {
let visibility = self.visibility;
let alias_name = self.alias_name;
let bounds = self.traits.bounds;
quote! {
#visibility trait #alias_name: #bounds {}
impl<_INNER> #alias_name for _INNER where _INNER: #bounds {}
}
}
fn render_generic(self) -> TokenStream2 {
let visibility = self.visibility;
let alias_name = self.alias_name;
let bounds = self.traits.bounds;
let mut unbound_generics = self.generics.clone();
for param in unbound_generics.params.iter_mut() {
if let GenericParam::Type(ty) = param {
if !ty.bounds.is_empty() {
ty.bounds.clear();
}
}
}
let unbound_generics = unbound_generics.params;
let bound_generics = self.generics.params;
quote! {
#visibility trait #alias_name<#bound_generics>: #bounds {}
impl<#bound_generics, _INNER> #alias_name<#unbound_generics> for _INNER where _INNER: #bounds {}
}
}
}
impl Parse for TraitSet {
fn parse(input: ParseStream) -> Result<Self> {
let result = TraitSet {
visibility: input.parse()?,
_trait_token: input.parse()?,
alias_name: input.parse()?,
generics: input.parse()?,
_eq_token: input.parse()?,
traits: input.parse()?,
};
if let Some(where_clause) = result.generics.where_clause {
return Err(Error::new(
where_clause.span(),
"Where clause is not allowed for trait alias",
));
}
Ok(result)
}
}
struct ManyTraitSet {
entries: Punctuated<TraitSet, Token![;]>,
}
impl Parse for ManyTraitSet {
fn parse(input: ParseStream) -> Result<Self> {
Ok(ManyTraitSet {
entries: input.parse_terminated(TraitSet::parse)?,
})
}
}
impl ManyTraitSet {
fn render(self) -> TokenStream2 {
TokenStream2::from_iter(self.entries.into_iter().map(|entry| entry.render()))
}
}
#[proc_macro]
pub fn trait_set(tokens: TokenStream) -> TokenStream {
let input = parse_macro_input!(tokens as ManyTraitSet);
input.render().into()
}