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//! Attribute macro that automatically implements a symmetric trait //! //! # Motivation //! There is a class of binary operators known as symmetric operators. //! Formally, let `F: D x D -> V` be a binary operator, `F` is symmetric if //! `F(a, b) = F(b, a)` for any `(a, b)`. //! //! Such pattern arises naturally in computational geometry. For example, //! `is_intersected(a: Shape1<Transform>, b: Shape2<Transform>) -> bool` //! decides whether two transformed shapes intersects. Or //! `distance(a: Shape1<Transform>, b: Shape2<Transform>) -> f32` computes //! the distance between two transformed shapes. Both functions could be seen //! as symmetric binary operators. More importantly, the types of the arguments //! can be heterogeneous. We can have `point.distance(circle)` for example. //! //! It is very tempting to represent an operator with a trait: //! ```no_run //! trait Distance<Other> { //! fn distance(&self, other: &Other) -> f64; //! } //! ``` //! And given different shapes: //! ```no_run //! struct Point2D { //! x: f64, //! y: f64, //! } //! //! struct Disk { //! center: Point2D, //! radius: f64 //! } //! ``` //! We can have //! ```no_run //! # trait Distance<Other> { //! # fn distance(&self, other: &Other) -> f64; //! # } //! # struct Point2D { //! # x: f64, //! # y: f64, //! # } //! # struct Disk { //! # center: Point2D, //! # radius: f64 //! # } //! impl Distance<Point2D> for Point2D { //! fn distance(&self, other: &Point2D) -> f64 { //! let dx = self.x - other.x; //! let dy = self.y - other.y; //! (dx * dx + dy * dy).sqrt() //! } //! } //! //! impl Distance<Disk> for Point2D { //! fn distance(&self, other: &Disk) -> f64 { //! let p_diff = self.distance(&other.center); //! if p_diff.le(&other.radius) { //! 0.0_f64 //! } else { //! p_diff - other.radius //! } //! } //! } //! ``` //! It is very helpful to also have `impl Distance<Point2D> for Disk`, but we //! cannot use generic implementation due to conflicting implementation. //! ```compile_fail //! # trait Distance<Other> { //! # fn distance(&self, other: &Other) -> f64; //! # } //! # struct Point2D { //! # x: f64, //! # y: f64, //! # } //! # struct Disk { //! # center: Point2D, //! # radius: f64 //! # } //! # impl Distance<Point2D> for Point2D { //! # fn distance(&self, other: &Point2D) -> f64 { //! # let dx = self.x - other.x; //! # let dy = self.y - other.y; //! # (dx * dx + dy * dy).sqrt() //! # } //! # } //! # impl Distance<Disk> for Point2D { //! # fn distance(&self, other: &Disk) -> f64 { //! # let p_diff = self.distance(&other.center); //! # if p_diff.le(&other.radius) { //! # 0.0_f64 //! # } else { //! # p_diff - other.radius //! # } //! # } //! # } //! // Conflicting implementation because this generic implementation makes //! // Disk: Distance<Point2D>, which in turn implements //! // Distance<Disk> for Point2D again. //! impl<T, U> Distance<U> for T //! where //! U: Distance<T>, { //! fn distance(&self, other: &U) -> f64 { //! other.distance(self) //! } //! } //! ``` //! So one has to manually implement: //! ```no_run //! # trait Distance<Other> { //! # fn distance(&self, other: &Other) -> f64; //! # } //! # struct Point2D { //! # x: f64, //! # y: f64, //! # } //! # struct Disk { //! # center: Point2D, //! # radius: f64 //! # } //! # impl Distance<Point2D> for Point2D { //! # fn distance(&self, other: &Point2D) -> f64 { //! # let dx = self.x - other.x; //! # let dy = self.y - other.y; //! # (dx * dx + dy * dy).sqrt() //! # } //! # } //! # impl Distance<Disk> for Point2D { //! # fn distance(&self, other: &Disk) -> f64 { //! # let p_diff = self.distance(&other.center); //! # if p_diff.le(&other.radius) { //! # 0.0_f64 //! # } else { //! # p_diff - other.radius //! # } //! # } //! # } //! impl Distance<Point2D> for Disk { //! fn distance(&self, other: &Point2D) -> f64 { //! other.distance(self) //! } //! } //! ``` //! This crates tries to address this problem by introducing an attribute //! macro to automatically implement the symmetric case. //! //! # Note //! There are several constraints for a trait to be deemed symmetric: //! * The trait must be generic, with the first non-lifetime parameter being the //! type for the symmetry. //! //! e.g. //! ```no_run //! trait SymmetricTrait<'a, Other, MoreType> { //! fn operator(&self, other: &Other) -> MoreType; //! } //! trait NotSymmetricTrait<'a, MoreType, Other> { //! fn operator(&self, other: &Other) -> MoreType; //! } //! ``` //! * All the methods in the trait must take exactly 2 arguments, where the //! first argument is `self` and the other argument is of the type for the //! symmetry. The two arguments must have the same family in the sense that //! they should both or neither be reference or mutable. //! //! e.g. //! ```no_run //! # type SomeType = i32; //! trait SymmetricTrait<Other> { //! fn operator_1(&self, other: &Other) -> SomeType; //! fn operator_2(self, other: Other) -> SomeType; //! fn operator_3(&mut self, other: &mut Other) -> SomeType; //! } //! trait NotSymmetricTrait<Other> { //! // reference mismatch //! fn operator_1(&self, other: Other) -> SomeType; //! // mutability mismatch //! fn operator_2(&self, other: &mut Other) -> SomeType; //! // incorrect arguments order //! fn operator_3(other: &mut Other, this: &mut Self) -> SomeType; //! // incorrect number of arguments //! fn operator_4(&self, other: &Other, more_other: &Other) -> SomeType; //! } //! ``` //! Associated types in a trait are allowed, and they will be transformed as: //! ```no_run //! # struct A {} //! # struct B {} //! trait TraitWithType<Other> { //! type SomeType; //! } //! impl TraitWithType<B> for A { //! type SomeType = i32; //! } //! // #[symmetric] will expands to //! impl TraitWithType<A> for B { //! type SomeType = <A as TraitWithType<B>>::SomeType; //! } //! ``` //! //! # Example //! ``` //! use symm_impl::symmetric; //! //! trait Distance<Other> { //! fn distance(&self, other: &Other) -> f64; //! } //! struct Point2D { //! x: f64, //! y: f64, //! } //! struct Disk { //! center: Point2D, //! radius: f64 //! } //! impl Distance<Point2D> for Point2D { //! fn distance(&self, other: &Point2D) -> f64 { //! let dx = self.x - other.x; //! let dy = self.y - other.y; //! (dx * dx + dy * dy).sqrt() //! } //! } //! #[symmetric] //! impl Distance<Disk> for Point2D { //! fn distance(&self, other: &Disk) -> f64 { //! let p_diff = self.distance(&other.center); //! if p_diff.le(&other.radius) { //! 0.0_f64 //! } else { //! p_diff - other.radius //! } //! } //! } //! /* Expands to //! impl Distance<Point2D> for Disk { //! #[allow(unused_mut)] //! #[inline] //! fn distance(&self, other: &Point2D) -> f64 { //! <Point2D as Distance<Disk>>::distance(other, self) //! } //! } //! */ //! //! let p = Point2D { x: 5.0, y: 4.0 }; //! let c = Disk { //! center: Point2D { x: 1.0, y: -2.0 }, //! radius: 3.0, //! }; //! assert_eq!(p.distance(&c), c.distance(&p)); //! ``` use std::{iter::FromIterator, mem}; use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree}; use quote::quote; use syn::{ parse::Parser, parse_macro_input, parse_quote, spanned::Spanned, Attribute, Block, FnArg, GenericArgument, ImplItem, ItemImpl, Pat, PatIdent, PathArguments, Type, }; /// See module-level documentation #[proc_macro_attribute] pub fn symmetric( _attr: proc_macro::TokenStream, item: proc_macro::TokenStream, ) -> proc_macro::TokenStream { let ast = parse_macro_input!(item as ItemImpl); let mirrored_ast = mirror(ast.clone()); let expanded = quote! { #ast #mirrored_ast }; proc_macro::TokenStream::from(expanded) } /// Take a syntax tree of impl and generate the mirror implementation for a /// symmetric trait. fn mirror(mut ast: ItemImpl) -> TokenStream { if ast.trait_.is_none() { // not a trait implementation return to_compile_error( "#[symmetric] can only be used on trait implementation".to_string(), Span::call_site(), ); } let trait_ = ast.trait_.as_mut().unwrap(); if let Some(bang) = trait_.0 { // negative marker trait return to_compile_error( "#[symmetric] cannot be used on negative trait bound".to_string(), bang.span.clone(), ); } // it is guaranteed that trait_.1 is a non-empty path sequence since this is a trait impl let original_trait = trait_.1.clone(); let last_segment = trait_.1.segments.last_mut().unwrap(); let trait_generics = match &mut last_segment.arguments { PathArguments::AngleBracketed(generics) => generics, _ => { // no generics arguments return to_compile_error("expected a generic trait".to_string(), trait_.1.span()); } }; // deduce the "other" type for this trait let other_type = trait_generics.args.iter_mut().find_map(|arg| { if let GenericArgument::Type(type_arg) = arg { Some(type_arg) } else { None } }); if other_type.is_none() { // no type arguments return to_compile_error( "symmetric trait must contain at least 1 type argument".to_string(), trait_generics.span(), ); } let other_type = other_type.unwrap(); // deduce the "self" type for this trait let self_type = ast.self_ty.as_mut(); // go through items inside the block // 1. For every associated type, make it // type SomeType = <other_type as Trait>::SomeType // 2. For every method, make sure it is of one of the following // * f(&self, other: &other_type) // * f(&mut self, other: &mut other_type) // * f(self, other: other_type) // * f(mut self, mut other: other_type) // If there are lifetime decorations, they must be the same. // replace other_type with self_type // replace the body with: // Trait::f(other, self) // 3. Leave everything else intact for item in ast .items .iter_mut() .filter(|item| matches!(item, ImplItem::Method(_) | ImplItem::Type(_))) { match item { ImplItem::Method(method) => { if let Some(variadic) = &method.sig.variadic { // variadic method return to_compile_error( "method in a symmetric trait cannot be variadic".to_string(), variadic.span(), ); } // verify the input arguments of the method if method.sig.inputs.len() != 2 { // wrong number of arguments return to_compile_error( "expected 2 arguments".to_string(), method.sig.inputs.span(), ); } let mut iter = method.sig.inputs.iter_mut(); let self_arg = iter.next().unwrap(); let other_arg = iter.next().unwrap(); // self_arg must be one of the 4 form let self_arg = match self_arg { FnArg::Receiver(receiver) => receiver, _ => { return to_compile_error( "expected a receiver".to_string(), self_arg.span(), ); } }; let other_arg = match other_arg { FnArg::Typed(typed_arg) => typed_arg, FnArg::Receiver(_) => unreachable!(), }; let other_ident = if let Some((_, lifetime)) = &self_arg.reference { // both should be reference with the same lifetime match other_arg.ty.as_mut() { Type::Reference(reference) => { if self_arg.mutability != reference.mutability { return to_compile_error( "mismatched mutability".to_string(), other_arg.span(), ); } if lifetime != &reference.lifetime { return to_compile_error( "mismatched lifetime".to_string(), other_arg.span(), ); } // replace the underlying type for other_arg reference.elem = Box::new(self_type.clone()); let pat_ident = PatIdent { attrs: Vec::new(), by_ref: None, mutability: None, ident: Ident::new("other", other_arg.span()), subpat: None, }; other_arg.pat = Box::new(Pat::Ident(pat_ident)); match other_arg.pat.as_ref() { Pat::Ident(ident) => &ident.ident, _ => unreachable!(), } } _ => { return to_compile_error( "expected a reference".to_string(), other_arg.span(), ); } } } else { // replace other_arg by plain pattern let pat_ident = PatIdent { attrs: Vec::new(), by_ref: None, mutability: None, ident: Ident::new("other", other_arg.span()), subpat: None, }; other_arg.pat = Box::new(Pat::Ident(pat_ident)); // replace the type of other_arg other_arg.ty = Box::new(self_type.clone()); match other_arg.pat.as_ref() { Pat::Ident(ident) => &ident.ident, _ => unreachable!(), } }; // replace method body let method_name = &method.sig.ident; let new_block: Block = parse_quote! { { <#self_type as #original_trait>::#method_name(#other_ident, self) } }; method.block = new_block; method.attrs.append( &mut Attribute::parse_outer .parse_str("#[allow(unused_mut)]") .unwrap(), ); method .attrs .append(&mut Attribute::parse_outer.parse_str("#[inline]").unwrap()); } ImplItem::Type(associated_type) => { // replace associated type let type_ident = &associated_type.ident; let delegated_type: Type = parse_quote! { <#self_type as #original_trait>::#type_ident }; associated_type.ty = delegated_type; } // keep as-is _ => (), } } // perform swapping of the types on impl mem::swap(other_type, self_type); quote! { #ast } } fn to_compile_error(message: String, span: Span) -> TokenStream { TokenStream::from_iter(vec![ TokenTree::Ident(Ident::new("compile_error", span)), TokenTree::Punct({ let mut punct = Punct::new('!', Spacing::Alone); punct.set_span(span); punct }), TokenTree::Group({ let mut group = Group::new(Delimiter::Brace, { TokenStream::from_iter(vec![TokenTree::Literal({ let mut string = Literal::string(&message); string.set_span(span); string })]) }); group.set_span(span); group }), ]) }