macros: preserve none-delimiters in function body

When user-generated token streams are returned from proc macros, all
none-delimited groups are flattened. This can cause compilation errors
as shown in the issue below. By instead passing the function body token
stream straight through from the input of the proc macro to its output,
it does not get deconstructed and reconstructed, and so the
none-delimited groups are not lost.

Fixes: #3579

tests-integration/tests/macros_main.rs

@@ -26,3 +26,21 @@ fn shell() {
     assert_eq!(1, basic_main());
     assert_eq!(bool::default(), generic_fun::<bool>())
 }
+
+macro_rules! generate_preserve_none_delimiters_tests {
+    ($e:expr) => {
+        #[test]
+        fn preserve_none_delimiters_in_main() {
+            #[tokio::main]
+            async fn f() -> i32 { $e() }
+
+            assert_eq!(f(), ($e)());
+        }
+
+        #[tokio::test]
+        async fn preserve_none_delimiters_in_test() {
+            assert_eq!($e(), ($e)());
+        }
+    }
+}
+generate_preserve_none_delimiters_tests!(|| 5);

tokio-macros/src/entry.rs

@@ -174,13 +174,29 @@ fn parse_bool(bool: syn::Lit, span: Span, field: &str) -> Result<bool, syn::Erro
 }
 
 fn parse_knobs(
-    mut input: syn::ItemFn,
+    input_tokens: TokenStream,
     args: syn::AttributeArgs,
     is_test: bool,
     rt_multi_thread: bool,
 ) -> Result<TokenStream, syn::Error> {
+    let mut input: syn::ItemFn = syn::parse(input_tokens.clone())?;
+
+    if (is_test || input.sig.ident == "main") && !input.sig.inputs.is_empty() {
+        let function = if is_test { "test" } else { "main" };
+
+        return Err(syn::Error::new_spanned(
+            &input.sig.inputs,
+            format_args!("the {} function cannot accept arguments", function),
+        ));
+    }
+    if is_test {
+        if let Some(attr) = input.attrs.iter().find(|attr| attr.path.is_ident("test")) {
+            let msg = "second test attribute is supplied";
+            return Err(syn::Error::new_spanned(attr, msg));
+        }
+    }
+
     let sig = &mut input.sig;
-    let body = &input.block;
     let attrs = &input.attrs;
     let vis = input.vis;
 
@@ -291,6 +307,12 @@ fn parse_knobs(
         }
     };
 
+    // The last token of a function item is always its body. We use the input token stream directly
+    // instead of printing the parsed function to make sure that Rust preserves None-delimited
+    // groups inside the function body; see <https://github.com/tokio-rs/tokio/issues/3579>.
+    let body =
+        proc_macro2::TokenStream::from(TokenStream::from(input_tokens.into_iter().last().unwrap()));
+
     let result = quote! {
         #header
         #(#attrs)*
@@ -308,38 +330,11 @@ fn parse_knobs(
 
 #[cfg(not(test))] // Work around for rust-lang/rust#62127
 pub(crate) fn main(args: TokenStream, item: TokenStream, rt_multi_thread: bool) -> TokenStream {
-    let input = syn::parse_macro_input!(item as syn::ItemFn);
     let args = syn::parse_macro_input!(args as syn::AttributeArgs);
-
-    if input.sig.ident == "main" && !input.sig.inputs.is_empty() {
-        let msg = "the main function cannot accept arguments";
-        return syn::Error::new_spanned(&input.sig.ident, msg)
-            .to_compile_error()
-            .into();
-    }
-
-    parse_knobs(input, args, false, rt_multi_thread).unwrap_or_else(|e| e.to_compile_error().into())
+    parse_knobs(item, args, false, rt_multi_thread).unwrap_or_else(|e| e.to_compile_error().into())
 }
 
 pub(crate) fn test(args: TokenStream, item: TokenStream, rt_multi_thread: bool) -> TokenStream {
-    let input = syn::parse_macro_input!(item as syn::ItemFn);
     let args = syn::parse_macro_input!(args as syn::AttributeArgs);
-
-    for attr in &input.attrs {
-        if attr.path.is_ident("test") {
-            let msg = "second test attribute is supplied";
-            return syn::Error::new_spanned(&attr, msg)
-                .to_compile_error()
-                .into();
-        }
-    }
-
-    if !input.sig.inputs.is_empty() {
-        let msg = "the test function cannot accept arguments";
-        return syn::Error::new_spanned(&input.sig.inputs, msg)
-            .to_compile_error()
-            .into();
-    }
-
-    parse_knobs(input, args, true, rt_multi_thread).unwrap_or_else(|e| e.to_compile_error().into())
+    parse_knobs(item, args, true, rt_multi_thread).unwrap_or_else(|e| e.to_compile_error().into())
 }