1 files changed, 170 insertions, 0 deletions
diff --git a/src/join.rs b/src/join.rs
new file mode 100644
index 0000000..2a7b11c
--- /dev/null
+++ b/src/join.rs
@@ -0,0 +1,170 @@
+//! 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::{parenthesized, parse_quote, Expr, Ident, Token};
+
+mod kw {
+    syn::custom_keyword!(futures_crate_path);
+}
+
+#[derive(Default)]
+struct Join {
+    futures_crate_path: Option<syn::Path>,
+    fut_exprs: Vec<Expr>,
+}
+
+impl Parse for Join {
+    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
+        let mut join = Join::default();
+
+        // When `futures_crate_path(::path::to::futures::lib)` is provided,
+        // it sets the path through which futures library functions will be
+        // accessed.
+        if input.peek(kw::futures_crate_path) {
+            input.parse::<kw::futures_crate_path>()?;
+            let content;
+            parenthesized!(content in input);
+            join.futures_crate_path = Some(content.parse()?);
+        }
+
+        while !input.is_empty() {
+            join.fut_exprs.push(input.parse::<Expr>()?);
+
+            if !input.is_empty() {
+                input.parse::<Token![,]>()?;
+            }
+        }
+
+        Ok(join)
+    }
+}
+
+fn bind_futures(
+    futures_crate: &syn::Path,
+    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);
+            });
+            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);
+
+    let futures_crate = parsed
+        .futures_crate_path
+        .unwrap_or_else(|| parse_quote!(::futures_util));
+
+    // should be def_site, but that's unstable
+    let span = Span::call_site();
+
+    let (future_let_bindings, future_names) = bind_futures(&futures_crate, parsed.fut_exprs, span);
+
+    let poll_futures = future_names.iter().map(|fut| {
+        quote! {
+            __all_done &= #futures_crate::core_reexport::future::Future::poll(
+                unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }, __cx).is_ready();
+        }
+    });
+    let take_outputs = future_names.iter().map(|fut| {
+        quote! {
+            unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.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::core_reexport::task::Poll::Ready((
+                    #( #take_outputs )*
+                ))
+            } else {
+                #futures_crate::core_reexport::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);
+
+    let futures_crate = parsed
+        .futures_crate_path
+        .unwrap_or_else(|| parse_quote!(::futures_util));
+
+    // should be def_site, but that's unstable
+    let span = Span::call_site();
+
+    let (future_let_bindings, future_names) = bind_futures(&futures_crate, parsed.fut_exprs, span);
+
+    let poll_futures = future_names.iter().map(|fut| {
+        quote! {
+            if #futures_crate::core_reexport::future::Future::poll(
+                unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }, __cx).is_pending()
+            {
+                __all_done = false;
+            } else if unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.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::core_reexport::task::Poll::Ready(
+                    #futures_crate::core_reexport::result::Result::Err(
+                        unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.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)]
+            unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.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::core_reexport::task::Poll::Ready(
+                    #futures_crate::core_reexport::result::Result::Ok((
+                        #( #take_outputs )*
+                    ))
+                )
+            } else {
+                #futures_crate::core_reexport::task::Poll::Pending
+            }
+        }).await
+    } })
+}