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(());)
-   |