macros: improve diagnostics on type mismatch (#3766)

tests-build/tests/fail/macros_dead_code.rs

@@ -0,0 +1,8 @@
+#![deny(dead_code)]
+
+use tests_build::tokio;
+
+#[tokio::main]
+async fn f() {}
+
+fn main() {}

tests-build/tests/fail/macros_dead_code.stderr

@@ -0,0 +1,11 @@
+error: function is never used: `f`
+ --> $DIR/macros_dead_code.rs:6:10
+  |
+6 | async fn f() {}
+  |          ^
+  |
+note: the lint level is defined here
+ --> $DIR/macros_dead_code.rs:1:9
+  |
+1 | #![deny(dead_code)]
+  |         ^^^^^^^^^

tests-build/tests/fail/macros_type_mismatch.rs

@@ -0,0 +1,32 @@
+use tests_build::tokio;
+
+#[tokio::main]
+async fn missing_semicolon_or_return_type() {
+    Ok(())
+}
+
+#[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(());
+}
+
+fn main() {}

tests-build/tests/fail/macros_type_mismatch.stderr

@@ -0,0 +1,51 @@
+error[E0308]: mismatched types
+ --> $DIR/macros_type_mismatch.rs:5:5
+  |
+5 |     Ok(())
+  |     ^^^^^^ expected `()`, found enum `Result`
+  |
+  = note: expected unit type `()`
+                  found enum `Result<(), _>`
+help: consider using a semicolon here
+  |
+5 |     Ok(());
+  |           ^
+help: try adding a return type
+  |
+4 | async fn missing_semicolon_or_return_type() -> Result<(), _> {
+  |                                             ^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/macros_type_mismatch.rs:16:5
+   |
+16 |     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
+   |
+20 | async fn extra_semicolon() -> Result<(), ()> {
+   |                               -------------- expected `Result<(), ()>` because of return type
+...
+29 |     Ok(());
+   |     ^^^^^^^ expected enum `Result`, found `()`
+   |
+   = 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

@@ -8,6 +8,12 @@ fn compile_fail_full() {
     #[cfg(feature = "full")]
     t.compile_fail("tests/fail/macros_invalid_input.rs");
 
+    #[cfg(feature = "full")]
+    t.compile_fail("tests/fail/macros_dead_code.rs");
+
+    #[cfg(feature = "full")]
+    t.compile_fail("tests/fail/macros_type_mismatch.rs");
+
     #[cfg(all(feature = "rt", not(feature = "full")))]
     t.compile_fail("tests/fail/macros_core_no_default.rs");