1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144

//! The futures-rs `join! macro implementation.

use proc_macro::TokenStream;
use proc_macro2::{Span, TokenStream as TokenStream2};
use quote::{format_ident, quote};
use syn::parse::{Parse, ParseStream};
use syn::{Expr, Ident, Token};

#[derive(Default)]
struct Join {
    fut_exprs: Vec<Expr>,
}

impl Parse for Join {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let mut join = Self::default();

        while !input.is_empty() {
            join.fut_exprs.push(input.parse::<Expr>()?);

            if !input.is_empty() {
                input.parse::<Token![,]>()?;
            }
        }

        Ok(join)
    }
}

fn bind_futures(fut_exprs: Vec<Expr>, span: Span) -> (Vec<TokenStream2>, Vec<Ident>) {
    let mut future_let_bindings = Vec::with_capacity(fut_exprs.len());
    let future_names: Vec<_> = fut_exprs
        .into_iter()
        .enumerate()
        .map(|(i, expr)| {
            let name = format_ident!("_fut{}", i, span = span);
            future_let_bindings.push(quote! {
                // Move future into a local so that it is pinned in one place and
                // is no longer accessible by the end user.
                let mut #name = __futures_crate::future::maybe_done(#expr);
                let mut #name = unsafe { __futures_crate::Pin::new_unchecked(&mut #name) };
            });
            name
        })
        .collect();

    (future_let_bindings, future_names)
}

/// The `join!` macro.
pub(crate) fn join(input: TokenStream) -> TokenStream {
    let parsed = syn::parse_macro_input!(input as Join);

    // should be def_site, but that's unstable
    let span = Span::call_site();

    let (future_let_bindings, future_names) = bind_futures(parsed.fut_exprs, span);

    let poll_futures = future_names.iter().map(|fut| {
        quote! {
            __all_done &= __futures_crate::future::Future::poll(
                #fut.as_mut(), __cx).is_ready();
        }
    });
    let take_outputs = future_names.iter().map(|fut| {
        quote! {
            #fut.as_mut().take_output().unwrap(),
        }
    });

    TokenStream::from(quote! { {
        #( #future_let_bindings )*

        __futures_crate::future::poll_fn(move |__cx: &mut __futures_crate::task::Context<'_>| {
            let mut __all_done = true;
            #( #poll_futures )*
            if __all_done {
                __futures_crate::task::Poll::Ready((
                    #( #take_outputs )*
                ))
            } else {
                __futures_crate::task::Poll::Pending
            }
        }).await
    } })
}

/// The `try_join!` macro.
pub(crate) fn try_join(input: TokenStream) -> TokenStream {
    let parsed = syn::parse_macro_input!(input as Join);

    // should be def_site, but that's unstable
    let span = Span::call_site();

    let (future_let_bindings, future_names) = bind_futures(parsed.fut_exprs, span);

    let poll_futures = future_names.iter().map(|fut| {
        quote! {
            if __futures_crate::future::Future::poll(
                #fut.as_mut(), __cx).is_pending()
            {
                __all_done = false;
            } else if #fut.as_mut().output_mut().unwrap().is_err() {
                // `.err().unwrap()` rather than `.unwrap_err()` so that we don't introduce
                // a `T: Debug` bound.
                // Also, for an error type of ! any code after `err().unwrap()` is unreachable.
                #[allow(unreachable_code)]
                return __futures_crate::task::Poll::Ready(
                    __futures_crate::Err(
                        #fut.as_mut().take_output().unwrap().err().unwrap()
                    )
                );
            }
        }
    });
    let take_outputs = future_names.iter().map(|fut| {
        quote! {
            // `.ok().unwrap()` rather than `.unwrap()` so that we don't introduce
            // an `E: Debug` bound.
            // Also, for an ok type of ! any code after `ok().unwrap()` is unreachable.
            #[allow(unreachable_code)]
            #fut.as_mut().take_output().unwrap().ok().unwrap(),
        }
    });

    TokenStream::from(quote! { {
        #( #future_let_bindings )*

        #[allow(clippy::diverging_sub_expression)]
        __futures_crate::future::poll_fn(move |__cx: &mut __futures_crate::task::Context<'_>| {
            let mut __all_done = true;
            #( #poll_futures )*
            if __all_done {
                __futures_crate::task::Poll::Ready(
                    __futures_crate::Ok((
                        #( #take_outputs )*
                    ))
                )
            } else {
                __futures_crate::task::Poll::Pending
            }
        }).await
    } })
}