futures_macro_async/

lib.rs

//! Procedural macro for the `#[async]` attribute.
//!
//! This crate is an implementation of the `#[async]` attribute as a procedural
//! macro. This is nightly-only for now as it's using the unstable features of
//! procedural macros. Furthermore it's generating code that's using a new
//! keyword, `yield`, and a new construct, generators, both of which are also
//! unstable.
//!
//! Currently this crate depends on `syn` and `quote` to do all the heavy
//! lifting, this is just a very small shim around creating a closure/future out
//! of a generator.
#![cfg_attr(feature = "nightly", feature(proc_macro))]
#![recursion_limit = "128"]


macro_rules! if_nightly {
    ($($i:item)*) => ($(
        #[cfg(feature = "nightly")]
        $i
    )*)
}

if_nightly! {
    extern crate proc_macro2;
    extern crate proc_macro;
    #[macro_use]
    extern crate quote;
    #[macro_use]
    extern crate syn;

    use proc_macro2::{Ident, Span};
    use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
    use quote::ToTokens;
    use syn::*;
    use syn::punctuated::Punctuated;
    use syn::fold::Fold;

    mod attribute;
    mod elision;

    use attribute::Attribute;

    macro_rules! quote_cs {
        ($($t:tt)*) => (quote_spanned!(Span::call_site() => $($t)*))
    }

    fn async_inner<F>(
        attribute: Attribute,
        function: TokenStream,
        gen_function: proc_macro2::TokenStream,
        return_ty: F,
    ) -> TokenStream
    where F: FnOnce(&Type, &[&Lifetime]) -> proc_macro2::TokenStream
    {
        // Parse our item, expecting a function. This function may be an actual
        // top-level function or it could be a method (typically dictated by the
        // arguments). We then extract everything we'd like to use.
        let ItemFn {
            ident,
            vis,
            unsafety,
            constness,
            abi,
            block,
            decl,
            attrs,
            ..
        } = match syn::parse(function).expect("failed to parse tokens as a function") {
            Item::Fn(item) => item,
            _ => panic!("#[async] can only be applied to functions"),
        };
        let FnDecl {
            inputs,
            output,
            variadic,
            mut generics,
            fn_token,
            ..
        } = { *decl };
        let where_clause = &generics.where_clause;
        assert!(variadic.is_none(), "variadic functions cannot be async");
        let (output, rarrow_token) = match output {
            ReturnType::Type(rarrow_token, t) => (*t, rarrow_token),
            ReturnType::Default => {
                (TypeTuple {
                    elems: Default::default(),
                    paren_token: Default::default(),
                }.into(), Default::default())
            }
        };

        // We've got to get a bit creative with our handling of arguments. For a
        // number of reasons we translate this:
        //
        //      fn foo(ref a: u32) -> Result<u32, u32> {
        //          // ...
        //      }
        //
        // into roughly:
        //
        //      fn foo(__arg_0: u32) -> impl Future<...> {
        //          gen_move(move || {
        //              let ref a = __arg0;
        //
        //              // ...
        //          })
        //      }
        //
        // The intention here is to ensure that all local function variables get
        // moved into the generator we're creating, and they're also all then bound
        // appropriately according to their patterns and whatnot.
        //
        // We notably skip everything related to `self` which typically doesn't have
        // many patterns with it and just gets captured naturally.
        let mut inputs_no_patterns = Vec::new();
        let mut patterns = Vec::new();
        let mut temp_bindings = Vec::new();
        for (i, input) in inputs.into_iter().enumerate() {
            // `self: Box<Self>` will get captured naturally
            let mut is_input_no_pattern = false;
            if let FnArg::Captured(ref arg) = input {
                if let Pat::Ident(PatIdent { ref ident, ..}) = arg.pat {
                    if ident == "self" {
                        is_input_no_pattern = true;
                    }
                }
            }
            if is_input_no_pattern {
                inputs_no_patterns.push(input);
                continue
            }

            match input {
                FnArg::Captured(ArgCaptured {
                    pat: syn::Pat::Ident(syn::PatIdent {
                        by_ref: None,
                        ..
                    }),
                    ..
                }) => {
                    inputs_no_patterns.push(input);
                }

                // `ref a: B` (or some similar pattern)
                FnArg::Captured(ArgCaptured { pat, ty, colon_token }) => {
                    patterns.push(pat);
                    let ident = Ident::new(&format!("__arg_{}", i), Span::call_site());
                    temp_bindings.push(ident.clone());
                    let pat = PatIdent {
                        by_ref: None,
                        mutability: None,
                        ident: ident,
                        subpat: None,
                    };
                    inputs_no_patterns.push(ArgCaptured {
                        pat: pat.into(),
                        ty,
                        colon_token,
                    }.into());
                }

                // Other `self`-related arguments get captured naturally
                _ => {
                    inputs_no_patterns.push(input);
                }
            }
        }


        // This is the point where we handle
        //
        //      #[async]
        //      for x in y {
        //      }
        //
        // Basically just take all those expression and expand them.
        let block = ExpandAsyncFor.fold_block(*block);


        let inputs_no_patterns = elision::unelide_lifetimes(&mut generics.params, inputs_no_patterns);
        let lifetimes: Vec<_> = generics.lifetimes().map(|l| &l.lifetime).collect();

        let return_ty = return_ty(&output, &lifetimes);


        let block_inner = quote_cs! {
            #( let #patterns = #temp_bindings; )*
            #block
        };
        let mut result = proc_macro2::TokenStream::new();
        block.brace_token.surround(&mut result, |tokens| {
            block_inner.to_tokens(tokens);
        });
        syn::token::Semi([block.brace_token.0]).to_tokens(&mut result);

        let gen_body_inner = quote_cs! {
            let __e: #output = #result

            // Ensure that this closure is a generator, even if it doesn't
            // have any `yield` statements.
            #[allow(unreachable_code)]
            {
                return __e;
                loop { yield ::futures::__rt::Async::Pending }
            }
        };
        let mut gen_body = proc_macro2::TokenStream::new();
        block.brace_token.surround(&mut gen_body, |tokens| {
            gen_body_inner.to_tokens(tokens);
        });

        // Give the invocation of the `gen` function the same span as the output
        // as currently errors related to it being a result are targeted here. Not
        // sure if more errors will highlight this function call...
        let output_span = first_last(&output);
        let gen_function = respan(gen_function.into(), &output_span);
        let body_inner = quote_cs! {
            #gen_function (static move || -> #output #gen_body)
        };

        let body_inner = if attribute.boxed {
            respan(if attribute.send {
                quote_cs! { (#body_inner).pin() }
            } else {
                quote_cs! { (#body_inner).pin_local() }
            }.into(), &output_span)
        } else {
            body_inner.into()
        };

        let mut body = proc_macro2::TokenStream::new();
        block.brace_token.surround(&mut body, |tokens| {
            body_inner.to_tokens(tokens);
        });

        let output = quote_cs! {
            #(#attrs)*
            #vis #unsafety #abi #constness
            #fn_token #ident #generics(#(#inputs_no_patterns),*)
                #rarrow_token #return_ty
                #where_clause
            #body
        };

        // println!("{}", output);
        output.into()
    }

    #[proc_macro_attribute]
    pub fn async(attribute: TokenStream, function: TokenStream) -> TokenStream {
        let attr = attribute.to_string();
        // Handle arguments to the #[async] attribute, if any
        let args = syn::parse::<AsyncArgs>(attribute)
            .expect(&format!("failed to parse attribute arguments: {}", attr));

        let attribute = Attribute::from(args.0.into_iter().map(|arg| arg.0.to_string()));

        async_inner(attribute, function, quote_cs! { ::futures::__rt::gen_future }, |output, lifetimes| {
            // TODO: can we lift the restriction that `futures` must be at the root of
            //       the crate?
            let output_span = first_last(&output);
            let return_ty = match attribute {
                Attribute::NONE => quote_cs! {
                    impl ::futures::__rt::MyStableFuture<!> + #(#lifetimes +)*
                },
                Attribute::SEND => quote_cs! {
                    impl ::futures::__rt::MyStableFuture<!> + Send + #(#lifetimes +)*
                },
                Attribute::BOXED => quote_cs! {
                    ::futures::__rt::std::boxed::PinBox<::futures::__rt::Future<
                        Item = <! as ::futures::__rt::IsResult>::Ok,
                        Error = <! as ::futures::__rt::IsResult>::Err,
                    > + #(#lifetimes +)*>
                },
                Attribute::BOXED_SEND => quote_cs! {
                    ::futures::__rt::std::boxed::PinBox<::futures::__rt::Future<
                        Item = <! as ::futures::__rt::IsResult>::Ok,
                        Error = <! as ::futures::__rt::IsResult>::Err,
                    > + Send + #(#lifetimes +)*>
                },
            };
            let return_ty = respan(return_ty.into(), &output_span);
            replace_bang(return_ty, &output)
        })
    }

    #[proc_macro_attribute]
    pub fn async_stream(attribute: TokenStream, function: TokenStream) -> TokenStream {
        // Handle arguments to the #[async_stream] attribute, if any
        let args = syn::parse::<AsyncStreamArgs>(attribute)
            .expect("failed to parse attribute arguments");

        let (args, valued_args): (Vec<_>, Vec<_>) = args.0.into_iter().partition(|arg| arg.1.is_none());
        let args = args.into_iter().map(|arg| arg.0);
        let valued_args = valued_args.into_iter().map(|arg| (arg.0, arg.1.unwrap()));

        let mut item_ty = None;

        for (ident, ty) in valued_args {
            match ident.to_string().as_str() {
                "item" => {
                    if item_ty.is_some() {
                        panic!("duplicate 'item' argument");
                    }
                    item_ty = Some(ty);
                }
                _ => {
                    panic!("unexpected macro argument '{}'", quote_cs!(#ident = #ty));
                }
            }
        }

        let item_ty = item_ty.expect("#[async_stream] requires item type to be specified");
        let attribute = Attribute::from(args.map(|arg| arg.to_string()));

        async_inner(attribute, function, quote_cs! { ::futures::__rt::gen_stream }, |output, lifetimes| {
            let return_ty = match attribute {
                Attribute::NONE => quote_cs! {
                    impl ::futures::__rt::MyStableStream<!, !> + #(#lifetimes +)*
                },
                Attribute::SEND => quote_cs! {
                    impl ::futures::__rt::MyStableStream<!, !> + Send + #(#lifetimes +)*
                },
                Attribute::BOXED => quote_cs! {
                    ::futures::__rt::std::boxed::PinBox<::futures::__rt::Stream<
                        Item = !,
                        Error = <! as ::futures::__rt::IsResult>::Err,
                    > + #(#lifetimes +)*>
                },
                Attribute::BOXED_SEND => quote_cs! {
                    ::futures::__rt::std::boxed::PinBox<::futures::__rt::Stream<
                        Item = !,
                        Error = <! as ::futures::__rt::IsResult>::Err,
                    > + Send + #(#lifetimes +)*>
                },
            };
            let output_span = first_last(&output);
            let return_ty = respan(return_ty.into(), &output_span);
            replace_bangs(return_ty, &[&item_ty, &output])
        })
    }

    #[proc_macro]
    pub fn async_block(input: TokenStream) -> TokenStream {
        let input = TokenStream::from(TokenTree::Group(Group::new(Delimiter::Brace, input)));
        let expr = syn::parse(input)
            .expect("failed to parse tokens as an expression");
        let expr = ExpandAsyncFor.fold_expr(expr);

        let mut tokens = quote_cs! {
            ::futures::__rt::gen_future
        };

        // Use some manual token construction here instead of `quote_cs!` to ensure
        // that we get the `call_site` span instead of the default span.
        let span = Span::call_site();
        syn::token::Paren(span).surround(&mut tokens, |tokens| {
            syn::token::Static(span).to_tokens(tokens);
            syn::token::Move(span).to_tokens(tokens);
            syn::token::OrOr([span, span]).to_tokens(tokens);
            syn::token::Brace(span).surround(tokens, |tokens| {
                (quote_cs! {
                    if false { yield ::futures::__rt::Async::Pending }
                }).to_tokens(tokens);
                expr.to_tokens(tokens);
            });
        });

        tokens.into()
    }

    #[proc_macro]
    pub fn async_stream_block(input: TokenStream) -> TokenStream {
        let input = TokenStream::from(TokenTree::Group(Group::new(Delimiter::Brace, input)));
        let expr = syn::parse(input)
            .expect("failed to parse tokens as an expression");
        let expr = ExpandAsyncFor.fold_expr(expr);

        let mut tokens = quote_cs! {
            ::futures::__rt::gen_stream
        };

        // Use some manual token construction here instead of `quote_cs!` to ensure
        // that we get the `call_site` span instead of the default span.
        let span = Span::call_site();
        syn::token::Paren(span).surround(&mut tokens, |tokens| {
            syn::token::Static(span).to_tokens(tokens);
            syn::token::Move(span).to_tokens(tokens);
            syn::token::OrOr([span, span]).to_tokens(tokens);
            syn::token::Brace(span).surround(tokens, |tokens| {
                (quote_cs! {
                    if false { yield ::futures::__rt::Async::Pending }
                }).to_tokens(tokens);
                expr.to_tokens(tokens);
            });
        });

        tokens.into()
    }

    struct ExpandAsyncFor;

    impl Fold for ExpandAsyncFor {
        fn fold_expr(&mut self, expr: Expr) -> Expr {
            let expr = fold::fold_expr(self, expr);
            let mut async = false;
            {
                let attrs = match expr {
                    Expr::ForLoop(syn::ExprForLoop { ref attrs, .. }) => attrs,
                    _ => return expr,
                };
                if attrs.len() == 1 {
                    // TODO: more validation here
                    if attrs[0].path.segments.first().unwrap().value().ident == "async" {
                        async = true;
                    }
                }
            }
            if !async {
                return expr
            }
            let all = match expr {
                Expr::ForLoop(item) => item,
                _ => panic!("only for expressions can have #[async]"),
            };
            let ExprForLoop { pat, expr, body, label, .. } = all;

            // Basically just expand to a `poll` loop
            let tokens = quote_cs! {{
                let mut __stream = #expr;
                #label
                loop {
                    let #pat = {
                        let r = {
                            let pin = unsafe {
                                ::futures::__rt::std::mem::PinMut::new_unchecked(&mut __stream)
                            };
                            ::futures::__rt::in_ctx(|ctx| ::futures::__rt::StableStream::poll_next(pin, ctx))
                        };
                        match r? {
                            ::futures::__rt::Async::Ready(e) => {
                                match e {
                                    ::futures::__rt::std::option::Option::Some(e) => e,
                                    ::futures::__rt::std::option::Option::None => break,
                                }
                            }
                            ::futures::__rt::Async::Pending => {
                                yield ::futures::__rt::Async::Pending;
                                continue
                            }
                        }
                    };

                    #body
                }
            }};
            syn::parse(tokens.into()).unwrap()
        }

        // Don't recurse into items
        fn fold_item(&mut self, item: Item) -> Item {
            item
        }
    }

    fn first_last(tokens: &ToTokens) -> (Span, Span) {
        let mut spans = proc_macro2::TokenStream::new();
        tokens.to_tokens(&mut spans);
        let good_tokens = proc_macro2::TokenStream::from(spans).into_iter().collect::<Vec<_>>();
        let first_span = good_tokens.first().map(|t| t.span()).unwrap_or(Span::call_site());
        let last_span = good_tokens.last().map(|t| t.span()).unwrap_or(first_span);
        (first_span, last_span)
    }

    fn respan(input: proc_macro2::TokenStream,
              &(first_span, last_span): &(Span, Span)) -> proc_macro2::TokenStream {
        let mut new_tokens = input.into_iter().collect::<Vec<_>>();
        if let Some(token) = new_tokens.first_mut() {
            token.set_span(first_span);
        }
        for token in new_tokens.iter_mut().skip(1) {
            token.set_span(last_span);
        }
        new_tokens.into_iter().collect()
    }

    fn replace_bang(input: proc_macro2::TokenStream, tokens: &ToTokens)
        -> proc_macro2::TokenStream
    {
        let mut new_tokens = proc_macro2::TokenStream::new();
        for token in input.into_iter() {
            match token {
                proc_macro2::TokenTree::Punct(ref punct) if punct.as_char() == '!' => {
                    tokens.to_tokens(&mut new_tokens)
                },
                _ => token.to_tokens(&mut new_tokens),
            }
        }
        new_tokens.into()
    }

    fn replace_bangs(input: proc_macro2::TokenStream, replacements: &[&ToTokens])
        -> proc_macro2::TokenStream
    {
        let mut replacements = replacements.iter().cycle();
        let mut new_tokens = proc_macro2::TokenStream::new();
        for token in input.into_iter() {
            match token {
                proc_macro2::TokenTree::Punct(ref punct) if punct.as_char() == '!' => {
                    replacements.next().unwrap().to_tokens(&mut new_tokens);
                }
                _ => token.to_tokens(&mut new_tokens),
            }
        }
        new_tokens.into()
    }

    struct AsyncArg(syn::Ident);

    impl synom::Synom for AsyncArg {
        named!(parse -> Self, do_parse!(
            i: syn!(syn::Ident) >>
            (AsyncArg(i))));
    }

    struct AsyncArgs(Vec<AsyncArg>);

    impl synom::Synom for AsyncArgs {
        named!(parse -> Self, map!(
            call!(Punctuated::<AsyncArg, syn::token::Comma>::parse_separated),
            |p| AsyncArgs(p.into_iter().collect())
        ));
    }

    struct AsyncStreamArg(syn::Ident, Option<syn::Type>);

    impl synom::Synom for AsyncStreamArg {
        named!(parse -> Self, do_parse!(
            i: syn!(syn::Ident) >>
            p: option!(do_parse!(
                syn!(syn::token::Eq) >>
                p: syn!(syn::Type) >>
                (p))) >>
            (AsyncStreamArg(i, p))));
    }

    struct AsyncStreamArgs(Vec<AsyncStreamArg>);

    impl synom::Synom for AsyncStreamArgs {
        named!(parse -> Self, map!(
            call!(Punctuated::<AsyncStreamArg, syn::token::Comma>::parse_separated),
            |p| AsyncStreamArgs(p.into_iter().collect())
        ));
    }
}