fix: better spans in #[tokio::main] output

tests-build/tests/fail/macros_type_mismatch.stderr

@@ -1,47 +1,46 @@
 error[E0308]: mismatched types
  --> $DIR/macros_type_mismatch.rs:5:5
   |
-4 | async fn missing_semicolon_or_return_type() {
-  |                                             - help: a return type might be missing here: `-> _`
 5 |     Ok(())
-  |     ^^^^^^ expected `()`, found enum `Result`
+  |     ^^^^^^- help: consider using a semicolon here: `;`
+  |     |
+  |     expected `()`, found enum `Result`
   |
   = note: expected unit type `()`
                   found enum `Result<(), _>`
 
 error[E0308]: mismatched types
-  --> $DIR/macros_type_mismatch.rs:10:5
-   |
-9  | async fn missing_return_type() {
-   |                                - help: a return type might be missing here: `-> _`
-10 |     return Ok(());
-   |     ^^^^^^^^^^^^^^ expected `()`, found enum `Result`
-   |
-   = note: expected unit type `()`
-                   found enum `Result<(), _>`
+ --> $DIR/macros_type_mismatch.rs:8:1
+  |
+8 | #[tokio::main]
+  | ^^^^^^^^^^^^^^ expected `()`, found enum `Result`
+9 | async fn missing_return_type() {
+  |                                - help: a return type might be missing here: `-> _`
+  |
+  = note: expected unit type `()`
+                  found enum `Result<(), _>`
+  = note: this error originates in the attribute macro `tokio::main` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
-  --> $DIR/macros_type_mismatch.rs:23:5
+  --> $DIR/macros_type_mismatch.rs:14:31
    |
 14 | async fn extra_semicolon() -> Result<(), ()> {
-   |                               -------------- expected `Result<(), ()>` because of return type
+   |          ---------------      ^^^^^^^^^^^^^^ expected enum `Result`, found `()`
+   |          |
+   |          implicitly returns `()` as its body has no tail or `return` expression
 ...
 23 |     Ok(());
-   |     ^^^^^^^ expected enum `Result`, found `()`
+   |           - help: remove this semicolon to return this value
    |
    = note:   expected enum `Result<(), ()>`
            found unit type `()`
-help: try adding an expression at the end of the block
-   |
-23 ~     Ok(());;
-24 +     Ok(())
-   |
 
 error[E0308]: mismatched types
-  --> $DIR/macros_type_mismatch.rs:32:5
+  --> $DIR/macros_type_mismatch.rs:29:1
    |
+29 | #[tokio::main]
+   | ^^^^^^^^^^^^^^ expected `()`, found integer
 30 | async fn issue_4635() {
    |                       - help: try adding a return type: `-> i32`
-31 |     return 1;
-32 |     ;
-   |     ^ expected `()`, found integer
+   |
+   = note: this error originates in the attribute macro `tokio::main` (in Nightly builds, run with -Z macro-backtrace for more info)

tokio-macros/src/entry.rs

@@ -376,7 +376,7 @@ fn parse_knobs(mut input: syn::ItemFn, is_test: bool, config: FinalConfig) -> To
     let body = &input.block;
     let brace_token = input.block.brace_token;
     let body_ident = quote! { body };
-    let block_expr = quote_spanned! {last_stmt_end_span=>
+    let block_expr = quote_spanned! {Span::call_site()=>
         #[allow(clippy::expect_used, clippy::diverging_sub_expression)]
         {
             return #rt
@@ -396,22 +396,48 @@ fn parse_knobs(mut input: syn::ItemFn, is_test: bool, config: FinalConfig) -> To
     //
     // We don't do this for the main function as it should only be used once so
     // there will be no benefit.
+
+    // FIXME: this is ... rather repetitive
+    let (output_type, body_cast) = match &input.sig.output {
+        // For functions with no return value syn doesn't print anything,
+        // but that doesn't work as `Output` for our boxed `Future`, so
+        // default to `()` (the same type as the function output).
+        syn::ReturnType::Default => {
+            let t = quote! { () };
+            let b = quote! {
+                async {
+                    #[allow(unreachable_code)]
+                    {
+                        let _body_cast: () = #body;
+                        _body_cast;
+                    }
+                }
+            };
+            (t, b)
+        }
+        syn::ReturnType::Type(_, ret_type) => {
+            let t = quote! { #ret_type };
+            let b = quote! {
+                async {
+                    #[allow(unreachable_code)]
+                    {
+                        let _body_cast: #ret_type = #body;
+                        _body_cast
+                    }
+                }
+            };
+            (t, b)
+        }
+    };
     let body = if is_test {
-        let output_type = match &input.sig.output {
-            // For functions with no return value syn doesn't print anything,
-            // but that doesn't work as `Output` for our boxed `Future`, so
-            // default to `()` (the same type as the function output).
-            syn::ReturnType::Default => quote! { () },
-            syn::ReturnType::Type(_, ret_type) => quote! { #ret_type },
-        };
-        quote! {
-            let body = async #body;
+        quote_spanned! {last_stmt_end_span=>
+            let body = #body_cast;
             #crate_path::pin!(body);
             let body: ::std::pin::Pin<&mut dyn ::std::future::Future<Output = #output_type>> = body;
         }
     } else {
-        quote! {
-            let body = async #body;
+        quote_spanned! {last_stmt_end_span=>
+            let body = #body_cast;
         }
     };