macros: fix wrong error messages (#4067)

tests-build/tests/fail/macros_type_mismatch.rs

@@ -7,25 +7,11 @@ async fn missing_semicolon_or_return_type() {
 
 #[tokio::main]
 async fn missing_return_type() {
-    /* TODO(taiki-e): one of help messages still wrong
-    help: consider using a semicolon here
-       |
-    16 |     return Ok(());;
-       |
-    */
     return Ok(());
 }
 
 #[tokio::main]
 async fn extra_semicolon() -> Result<(), ()> {
-    /* TODO(taiki-e): help message still wrong
-    help: try using a variant of the expected enum
-       |
-    29 |     Ok(Ok(());)
-       |
-    29 |     Err(Ok(());)
-       |
-    */
     Ok(());
 }
 

tests-build/tests/fail/macros_type_mismatch.stderr

@@ -16,36 +16,23 @@ help: try adding a return type
   |                                             ^^^^^^^^^^^^^^^^
 
 error[E0308]: mismatched types
-  --> $DIR/macros_type_mismatch.rs:16:5
+  --> $DIR/macros_type_mismatch.rs:10:5
    |
-16 |     return Ok(());
+9  | async fn missing_return_type() {
+   |                                - help: try adding a return type: `-> Result<(), _>`
+10 |     return Ok(());
    |     ^^^^^^^^^^^^^^ expected `()`, found enum `Result`
    |
    = note: expected unit type `()`
                    found enum `Result<(), _>`
-help: consider using a semicolon here
-   |
-16 |     return Ok(());;
-   |                   ^
-help: try adding a return type
-   |
-9  | async fn missing_return_type() -> Result<(), _> {
-   |                                ^^^^^^^^^^^^^^^^
 
 error[E0308]: mismatched types
-  --> $DIR/macros_type_mismatch.rs:29:5
+  --> $DIR/macros_type_mismatch.rs:14:31
    |
-20 | async fn extra_semicolon() -> Result<(), ()> {
-   |                               -------------- expected `Result<(), ()>` because of return type
-...
-29 |     Ok(());
-   |     ^^^^^^^ expected enum `Result`, found `()`
+14 | async fn extra_semicolon() -> Result<(), ()> {
+   |          ---------------      ^^^^^^^^^^^^^^ expected enum `Result`, found `()`
+   |          |
+   |          implicitly returns `()` as its body has no tail or `return` expression
    |
    = note:   expected enum `Result<(), ()>`
            found unit type `()`
-help: try using a variant of the expected enum
-   |
-29 |     Ok(Ok(());)
-   |
-29 |     Err(Ok(());)
-   |

tests-build/tests/macros.rs

@@ -5,6 +5,12 @@ fn compile_fail_full() {
     #[cfg(feature = "full")]
     t.pass("tests/pass/forward_args_and_output.rs");
 
+    #[cfg(feature = "full")]
+    t.pass("tests/pass/macros_main_return.rs");
+
+    #[cfg(feature = "full")]
+    t.pass("tests/pass/macros_main_loop.rs");
+
     #[cfg(feature = "full")]
     t.compile_fail("tests/fail/macros_invalid_input.rs");
 

tests-build/tests/macros_clippy.rs

@@ -0,0 +1,7 @@
+#[cfg(feature = "full")]
+#[tokio::test]
+async fn test_with_semicolon_without_return_type() {
+    #![deny(clippy::semicolon_if_nothing_returned)]
+
+    dbg!(0);
+}

tests-build/tests/pass/macros_main_loop.rs

@@ -0,0 +1,14 @@
+use tests_build::tokio;
+
+#[tokio::main]
+async fn main() -> Result<(), ()> {
+    loop {
+        if !never() {
+            return Ok(());
+        }
+    }
+}
+
+fn never() -> bool {
+    std::time::Instant::now() > std::time::Instant::now()
+}

tests-build/tests/pass/macros_main_return.rs

@@ -0,0 +1,6 @@
+use tests_build::tokio;
+
+#[tokio::main]
+async fn main() -> Result<(), ()> {
+    return Ok(());
+}

tokio-macros/src/entry.rs

@@ -323,15 +323,27 @@ fn parse_knobs(
 
     let body = &input.block;
     let brace_token = input.block.brace_token;
+    let (tail_return, tail_semicolon) = match body.stmts.last() {
+        Some(syn::Stmt::Semi(expr, _)) => (
+            match expr {
+                syn::Expr::Return(_) => quote! { return },
+                _ => quote! {},
+            },
+            quote! {
+                ;
+            },
+        ),
+        _ => (quote! {}, quote! {}),
+    };
     input.block = syn::parse2(quote_spanned! {last_stmt_end_span=>
         {
             let body = async #body;
             #[allow(clippy::expect_used)]
-            #rt
+            #tail_return #rt
                 .enable_all()
                 .build()
                 .expect("Failed building the Runtime")
-                .block_on(body)
+                .block_on(body)#tail_semicolon
         }
     })
     .expect("Parsing failure");