futures-macro: improve diagnostics on type mismatch (#2433)

Author: taiki-e

futures-macro/Cargo.toml

@@ -23,4 +23,4 @@ autocfg = "1"
 proc-macro2 = "1.0"
 proc-macro-hack = "0.5.19"
 quote = "1.0"
-syn = { version = "1.0", features = ["full"] }
+syn = { version = "1.0.56", features = ["full"] }

futures-macro/src/executor.rs

@@ -1,5 +1,6 @@
 use proc_macro::TokenStream;
-use quote::quote;
+use proc_macro2::Span;
+use quote::{quote, quote_spanned, ToTokens};
 
 pub(crate) fn test(args: TokenStream, item: TokenStream) -> TokenStream {
     if !args.is_empty() {
@@ -9,23 +10,45 @@ pub(crate) fn test(args: TokenStream, item: TokenStream) -> TokenStream {
     }
 
     let mut input = syn::parse_macro_input!(item as syn::ItemFn);
-    let attrs = &input.attrs;
-    let vis = &input.vis;
-    let sig = &mut input.sig;
-    let body = &input.block;
 
-    if sig.asyncness.take().is_none() {
-        return syn::Error::new_spanned(sig.fn_token, "Only async functions are supported")
+    if input.sig.asyncness.take().is_none() {
+        return syn::Error::new_spanned(input.sig.fn_token, "Only async functions are supported")
             .to_compile_error()
             .into();
     }
 
+    // If type mismatch occurs, the current rustc points to the last statement.
+    let (last_stmt_start_span, last_stmt_end_span) = {
+        let mut last_stmt = input
+            .block
+            .stmts
+            .last()
+            .map(ToTokens::into_token_stream)
+            .unwrap_or_default()
+            .into_iter();
+        // `Span` on stable Rust has a limitation that only points to the first
+        // token, not the whole tokens. We can work around this limitation by
+        // using the first/last span of the tokens like
+        // `syn::Error::new_spanned` does.
+        let start = last_stmt.next().map_or_else(Span::call_site, |t| t.span());
+        let end = last_stmt.last().map_or(start, |t| t.span());
+        (start, end)
+    };
+
+    let path = quote_spanned! {last_stmt_start_span=>
+        ::futures_test::__private
+    };
+    let body = &input.block;
+    input.block.stmts = vec![syn::Stmt::Expr(
+        syn::parse2(quote_spanned! {last_stmt_end_span=>
+            #path::block_on(async #body)
+        })
+        .unwrap(),
+    )];
+
     let gen = quote! {
         #[::core::prelude::v1::test]
-        #(#attrs)*
-        #vis #sig {
-            ::futures_test::__private::block_on(async move #body)
-        }
+        #input
     };
 
     gen.into()

futures/tests/test_macro.rs

@@ -13,3 +13,8 @@ async fn it_is_being_run() {
     let fut = async { false };
     assert!(fut.await);
 }
+
+#[futures_test::test]
+async fn return_ty() -> Result<(), ()> {
+    Ok(())
+}