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//! //! # `#[real_async_trait]` //! [![travis]](https://travis-ci.org/4lDO2/real-async-trait-rs) //! [![cratesio]](https://crates.io/crates/real-async-trait) //! [![docsrs]](https://docs.rs/real-async-trait/) //! //! [travis]: https://travis-ci.org/4lDO2/real-async-trait-rs.svg?branch=master //! [cratesio]: https://img.shields.io/crates/v/real-async-trait.svg //! [docsrs]: https://docs.rs/real-async-trait/badge.svg //! //! This crate provides a producedural macro that works around the current limitation of not being //! able to put `async fn`s in a trait, _without type erasure_, by using experimental //! nightly-features, namely [generic associated types //! (GATs)](https://github.com/rust-lang/rfcs/blob/master/text/1598-generic_associated_types.md) //! and [existential //! types](https://github.com/rust-lang/rfcs/blob/master/text/2515-type_alias_impl_trait.md). //! //! ## Caveats //! //! While this proc macro will allow you to write non-type-erased allocation-free async fns within //! traits, there are a few caveats to this (non-exhaustive): //! //! * at the moment, all references used in the async fn, must have their lifetimes be explicitly //! specified, either from the top-level of the trait, or in the function declaration; //! * there can only be a single lifetime in use simultaneously. I have no idea why, but it could //! be due to buggy interaction between existential types and generic associated types; //! * since GATs are an "incomplete" feature in rust, it may not be sound or just not compile //! correctly or at all. __Don't use this in production code!__ //! //! ## Example //! ```ignore //! #[async_std::main] //! # async fn main() { //! /// An error code, similar to `errno` in C. //! pub type Errno = usize; //! //! /// A UNIX-like file descriptor. //! pub type FileDescriptor = usize; //! //! /// "No such file or directory" //! pub const ENOENT: usize = 1; //! //! /// "Bad file descriptor" //! pub const EBADF: usize = 2; //! //! /// A filesystem-like primitive, used in the Redox Operating System. //! #[real_async_trait] //! pub trait RedoxScheme { //! async fn open<'a>(&'a self, path: &'a [u8], flags: usize) -> Result<FileDescriptor, Errno>; //! async fn read<'a>(&'a self, fd: FileDescriptor, buffer: &'a mut [u8]) -> Result<usize, Errno>; //! async fn write<'a>(&'a self, fd: FileDescriptor, buffer: &'a [u8]) -> Result<usize, Errno>; //! async fn close<'a>(&'a self, fd: FileDescriptor) -> Result<(), Errno>; //! } //! //! /// A scheme that does absolutely nothing. //! struct MyNothingScheme; //! //! #[real_async_trait] //! impl RedoxScheme for MyNothingScheme { //! async fn open<'a>(&'a self, path: &'a [u8], flags: usize) -> Result<FileDescriptor, Errno> { //! // I can write async code in here! //! Err(ENOENT) //! } //! async fn read<'a>(&'a self, buffer: &'a mut [u8]) -> Result<usize, Errno> { //! Err(EBADF) //! } //! async fn write<'a>(&'a self, path: &'a [u8]) -> Result<usize, Errno> { //! Err(EBADF) //! } //! async fn close<'a>(&'a self, path: &'a [u8]) -> Result<(), Errno> { //! Err(EBADF) //! } //! } //! //! let my_nothing_scheme = MyNothingScheme; //! //! assert_eq!(my_nothing_scheme.open(b"nothing exists here", 0).await, Err(ENOENT), "why would anything exist here?"); //! assert_eq!(my_nothing_scheme.read(1337, &mut []).await, Err(EBADF)); //! assert_eq!(my_nothing_scheme.write(1337, &[]).await, Err(EBADF)); //! assert_eq!(my_nothing_scheme.close(1337).await, Err(EBADF)); //! //! # } //! //! ``` //! ## How it works //! //! Under the hood, this proc macro will insert generic associated types (GATs) for the the futures //! that are the return types of the async fns in the trait definition. The macro will generate the //! following for the `RedoxScheme` trait (simplified generated names): //! //! ```ignore //! pub trait RedoxScheme { //! // Downgraded functions, from async fn to fn. Their types have changed into a generic //! // associated type. //! fn open<'a>(&'a self, path: &'a [u8], flags: usize) -> Self::OpenFuture<'a>; //! fn read<'a>(&'a self, fd: usize, buf: &'a mut [u8]) -> Self::ReadFuture<'a>; //! fn write<'a>(&'a self, fd: usize, buf: &'a [u8]) -> Self::WriteFuture<'a>; //! fn close<'a>(&'a self, fd: usize) -> Self::CloseFuture<'a>; //! //! // Generic associated types, the return values are moved to here. //! type OpenFuture<'a>: ::core::future::Future<Output = Result<FileDescriptor, Errno>> + 'a; //! type ReadFuture<'a>: ::core::future::Future<Output = Result<usize, Errno>> + 'a; //! type WriteFuture<'a>: ::core::future::Future<Output = Result<usize, Errno>> + 'a; //! type CloseFuture<'a>: ::core::future::Future<Output = Result<(), Errno>> + 'a; //! } //! ``` //! //! Meanwhile, the impls will get the following generated code (simplified here as well): //! //! ```ignore //! //! // Wrap everything in a private module to prevent the existential types from leaking. //! mod __private { //! impl RedoxScheme for MyNothingScheme { //! // Async fns are downgraded here as well, and the same thing goes with the return //! // values. //! fn open<'a>(&'a self, path: &'a [u8], flags: usize) -> Self::OpenFuture<'a> { //! // All expressions in async fns are wrapped in async closures. The compiler will //! // automagically figure out the actual types of the existential type aliases, even //! // though they are anonymous. //! async move { Err(ENOENT) } //! } //! fn read<'a>(&'a self, fd: usize, buf: &'a mut [u8]) -> Self::ReadFuture<'a> { //! async move { Err(EBADF) } //! } //! fn write<'a>(&'a self, fd: usize, buf: &'a [u8]) -> Self::WriteFuture<'a> { //! async move { Err(EBADF) } //! } //! fn close<'a>(&'a self, fd: usize) -> Self::CloseFuture<'a> { //! async move { Err(EBADF) } //! } //! //! // This is the part where the existential types come in. Currently, there is no //! // possible way to use types within type aliases within traits, that aren't publicly //! // accessible. This we need async closures to avoid having to redefine our futures with //! // custom state machines, or use type erased pointers, we'll use existential types. //! type OpenFuture<'a> = OpenFutureExistentialType<'a>; //! type ReadFuture<'a> = ReadFutureExistentialType<'a>; //! type WriteFuture<'a> = WriteFutureExistentialType<'a>; //! type CloseFuture<'a> = CloseFutureExistentialType<'a>; //! } //! // This is where the return values actually are defined. At the moment these type alises //! // with impl trait can only occur outside of the trait itself, unfortunately. There can //! // only be one type that this type alias refers to, which the compiler will keep track of. //! type OpenFutureExistentialType<'a> = impl Future<Output = Result<FileDescriptor, Errno>> + //! 'a; //! type ReadFutureExistentialType<'a> = impl Future<Output = Result<usize, Errno>> + 'a; //! type WriteFutureExistentialType<'a> = impl Future<Output = Result<usize, Errno>> + 'a; //! type CloseFutureExistentialType<'a> = impl Future<Output = Result<(), Errno>> + 'a; //! } //! ``` //! extern crate proc_macro; use std::str::FromStr; use std::{iter, mem}; use proc_macro2::{Span, TokenStream}; use quote::quote; use syn::punctuated::Punctuated; use syn::token; use syn::{ AngleBracketedGenericArguments, Binding, Block, Expr, ExprAsync, FnArg, GenericArgument, GenericParam, Generics, Ident, ImplItem, ImplItemType, ItemImpl, ItemTrait, ItemType, Lifetime, LifetimeDef, PatType, Path, PathArguments, PathSegment, ReturnType, Signature, Stmt, Token, TraitBound, TraitBoundModifier, TraitItem, TraitItemType, Type, TypeImplTrait, TypeParamBound, TypePath, TypeReference, TypeTuple, Visibility, }; mod tests; struct LifetimeVisitor; impl<'ast> syn::visit::Visit<'ast> for LifetimeVisitor { fn visit_type_reference(&mut self, i: &'ast TypeReference) { if i.lifetime.is_none() { panic!("Reference at {:?} lacked an explicit lifetime, which is required by this proc macro", i.and_token.span); } } } fn handle_item_impl(mut item: ItemImpl) -> TokenStream { let mut existential_type_defs = Vec::new(); let mut gat_defs = Vec::new(); for method in item .items .iter_mut() .filter_map(|item| { if let ImplItem::Method(method) = item { Some(method) } else { None } }) .filter(|method| method.sig.asyncness.is_some()) { method.sig.asyncness = None; validate_that_function_always_has_lifetimes(&method.sig); let (toplevel_lifetimes, function_lifetimes) = already_defined_lifetimes(&item.generics, &method.sig.generics); let existential_type_name = format!( "__real_async_trait_impl_ExistentialTypeFor_{}", method.sig.ident ); let existential_type_ident = Ident::new(&existential_type_name, Span::call_site()); existential_type_defs.push(ItemType { attrs: Vec::new(), eq_token: Token!(=)(Span::call_site()), generics: Generics { gt_token: Some(Token!(>)(Span::call_site())), lt_token: Some(Token!(<)(Span::call_site())), params: toplevel_lifetimes .iter() .cloned() .map(GenericParam::Lifetime) .collect(), where_clause: None, }, ident: existential_type_ident, semi_token: Token!(;)(Span::call_site()), vis: Visibility::Inherited, ty: Box::new(Type::ImplTrait(TypeImplTrait { bounds: iter::once(TypeParamBound::Trait(future_trait_bound(return_type( method.sig.output.clone(), )))) .chain( toplevel_lifetimes .iter() .cloned() .map(|lifetime_def| TypeParamBound::Lifetime(lifetime_def.lifetime)), ) .collect(), impl_token: Token!(impl)(Span::call_site()), })), type_token: Token!(type)(Span::call_site()), }); let existential_type_path_for_impl = Path { // self::__real_async_trait_impl_ExistentialTypeFor_FUNCTIONNAME leading_colon: None, segments: vec![ PathSegment { arguments: PathArguments::None, ident: Ident::new("self", Span::call_site()), }, PathSegment { arguments: PathArguments::AngleBracketed(lifetime_angle_bracketed_bounds( toplevel_lifetimes .into_iter() .map(|lifetime_def| lifetime_def.lifetime), )), ident: Ident::new(&existential_type_name, Span::call_site()), }, ] .into_iter() .collect(), }; let existential_path_type = Type::Path(TypePath { path: existential_type_path_for_impl, qself: None, }); let gat_ident = gat_ident_for_sig(&method.sig); gat_defs.push(ImplItemType { attrs: Vec::new(), defaultness: None, eq_token: Token!(=)(Span::call_site()), generics: Generics { lt_token: Some(Token!(<)(Span::call_site())), gt_token: Some(Token!(>)(Span::call_site())), where_clause: None, params: function_lifetimes .iter() .cloned() .map(GenericParam::Lifetime) .collect(), }, ident: gat_ident.clone(), semi_token: Token!(;)(Span::call_site()), ty: existential_path_type.clone(), type_token: Token!(type)(Span::call_site()), vis: Visibility::Inherited, }); let gat_self_type = self_gat_type( gat_ident, function_lifetimes .into_iter() .map(|lifetime_def| lifetime_def.lifetime), ); method.sig.output = ReturnType::Type( Token!(->)(Span::call_site()), Box::new(gat_self_type.into()), ); let method_stmts = mem::replace(&mut method.block.stmts, Vec::new()); method.block.stmts = vec![Stmt::Expr(Expr::Async(ExprAsync { async_token: Token!(async)(Span::call_site()), attrs: Vec::new(), block: Block { brace_token: token::Brace { span: Span::call_site(), }, stmts: method_stmts, }, capture: Some(Token!(move)(Span::call_site())), }))]; } item.items.extend(gat_defs.into_iter().map(Into::into)); quote! { mod __real_async_trait_impl { use super::*; #item #(#existential_type_defs)* } } } fn return_type(retval: ReturnType) -> Type { match retval { ReturnType::Default => Type::Tuple(TypeTuple { elems: Punctuated::new(), paren_token: token::Paren { span: Span::call_site(), }, }), ReturnType::Type(_, ty) => *ty, } } fn future_trait_bound(fn_output_ty: Type) -> TraitBound { const FUTURE_TRAIT_PATH_STR: &str = "::core::future::Future"; const FUTURE_TRAIT_OUTPUT_IDENT_STR: &str = "Output"; let mut future_trait_path = syn::parse2::<Path>(TokenStream::from_str(FUTURE_TRAIT_PATH_STR).unwrap()) .expect("failed to parse `::core::future::Future` as a syn `Path`"); let future_angle_bracketed_args = AngleBracketedGenericArguments { colon2_token: None, // FIXME lt_token: Token!(<)(Span::call_site()), gt_token: Token!(>)(Span::call_site()), args: iter::once(GenericArgument::Binding(Binding { ident: Ident::new(FUTURE_TRAIT_OUTPUT_IDENT_STR, Span::call_site()), eq_token: Token!(=)(Span::call_site()), ty: fn_output_ty, })) .collect(), }; future_trait_path .segments .last_mut() .expect("Expected ::core::future::Future to have `Future` as the last segment") .arguments = PathArguments::AngleBracketed(future_angle_bracketed_args); TraitBound { // for TraitBounds, these are HRTBs, which are useless since there are already GATs present lifetimes: None, // This is not ?Sized or something like that modifier: TraitBoundModifier::None, paren_token: None, path: future_trait_path, } } fn validate_that_function_always_has_lifetimes(signature: &Signature) { for input in signature.inputs.iter() { match input { FnArg::Receiver(ref recv) => { if let Some((_ampersand, _lifetime @ None)) = &recv.reference { panic!("{}self parameter lacked an explicit lifetime, which is required by this proc macro", if recv.mutability.is_some() { "&mut " } else { "&" }); } } FnArg::Typed(PatType { ref ty, .. }) => { syn::visit::visit_type(&mut LifetimeVisitor, ty) } } } if let ReturnType::Type(_, ref ty) = signature.output { syn::visit::visit_type(&mut LifetimeVisitor, ty); }; } fn already_defined_lifetimes( toplevel_generics: &Generics, method_generics: &Generics, ) -> (Vec<LifetimeDef>, Vec<LifetimeDef>) { //Global scope //let mut lifetimes = vec! [LifetimeDef::new(Lifetime::new("'static", Span::call_site()))]; let mut lifetimes = Vec::new(); // Trait definition scope lifetimes.extend(toplevel_generics.lifetimes().cloned()); // Function definition scope let function_lifetimes = method_generics.lifetimes().cloned().collect::<Vec<_>>(); lifetimes.extend(function_lifetimes.iter().cloned()); (lifetimes, function_lifetimes) } fn lifetime_angle_bracketed_bounds( lifetimes: impl IntoIterator<Item = Lifetime>, ) -> AngleBracketedGenericArguments { AngleBracketedGenericArguments { colon2_token: None, lt_token: Token!(<)(Span::call_site()), gt_token: Token!(>)(Span::call_site()), args: lifetimes .into_iter() .map(|lifetime_def| GenericArgument::Lifetime(lifetime_def)) .collect(), } } fn gat_ident_for_sig(sig: &Signature) -> Ident { let gat_name = format!("__real_async_trait_impl_TypeFor_{}", sig.ident); Ident::new(&gat_name, Span::call_site()) } fn self_gat_type( gat_ident: Ident, function_lifetimes: impl IntoIterator<Item = Lifetime>, ) -> TypePath { TypePath { path: Path { // represents the pattern Self::GAT_NAME... leading_colon: None, segments: vec![ PathSegment { ident: Ident::new("Self", Span::call_site()), arguments: PathArguments::None, }, PathSegment { ident: gat_ident, arguments: PathArguments::AngleBracketed(lifetime_angle_bracketed_bounds( function_lifetimes, )), }, ] .into_iter() .collect(), }, qself: None, } } fn handle_item_trait(mut item: ItemTrait) -> TokenStream { let mut new_gat_items = Vec::new(); // Loop through every single async fn declared in the trait. for method in item .items .iter_mut() .filter_map(|item| { if let TraitItem::Method(func) = item { Some(func) } else { None } }) .filter(|method| method.sig.asyncness.is_some()) { // For each async fn, remove the async part, replace the return value with a generic // associated type, and add that generic associated type to the trait item. // Check that all types have a lifetime that is either specific to the trait item, or // to the current function (or 'static). Any other lifetime will and must produce a // compiler error. let gat_ident = gat_ident_for_sig(&method.sig); let method_return_ty = return_type(method.sig.output.clone()); validate_that_function_always_has_lifetimes(&method.sig); method.sig.asyncness = None; let (toplevel_lifetimes, function_lifetimes) = already_defined_lifetimes(&item.generics, &method.sig.generics); new_gat_items.push(TraitItemType { attrs: Vec::new(), type_token: Token!(type)(Span::call_site()), bounds: iter::once(TypeParamBound::Trait(future_trait_bound(method_return_ty))) .chain( toplevel_lifetimes .into_iter() .map(|lifetime_def| lifetime_def.lifetime) .map(TypeParamBound::Lifetime), ) .collect(), colon_token: Some(Token!(:)(Span::call_site())), default: None, generics: Generics { lt_token: Some(Token!(<)(Span::call_site())), gt_token: Some(Token!(>)(Span::call_site())), where_clause: None, params: function_lifetimes .iter() .cloned() .map(GenericParam::Lifetime) .collect(), }, ident: gat_ident.clone(), semi_token: Token!(;)(Span::call_site()), }); let self_gat_type = self_gat_type( gat_ident, function_lifetimes .into_iter() .map(|lifetime_def| lifetime_def.lifetime), ); method.sig.output = ReturnType::Type( Token!(->)(Span::call_site()), Box::new(self_gat_type.into()), ); } item.items .extend(new_gat_items.into_iter().map(TraitItem::Type)); quote! { #item } } fn real_async_trait2(_args_stream: TokenStream, token_stream: TokenStream) -> TokenStream { // The #[real_async_trait] attribute macro, is applicable to both trait blocks, and to impl // blocks that operate on that trait. if let Ok(item_trait) = syn::parse2::<ItemTrait>(token_stream.clone()) { handle_item_trait(item_trait) } else if let Ok(item_impl) = syn::parse2::<ItemImpl>(token_stream) { handle_item_impl(item_impl) } else { panic!("expected either a trait or an impl item") } .into() } /// A proc macro that supports using async fn in traits and trait impls. Refer to the top-level /// crate documentation for more information. #[proc_macro_attribute] pub fn real_async_trait( args_stream: proc_macro::TokenStream, token_stream: proc_macro::TokenStream, ) -> proc_macro::TokenStream { real_async_trait2(args_stream.into(), token_stream.into()).into() }