attributes: fix warnings and test failures (#3399)

Due to a change in the error messages produced by rustc, one of the UI
tests for failing compilation of the `#[instrument]` attribute when
applied to async functions was failing.

This change updates the expected output so that the test passes.

Some lint failures and errors in core were also fixed.

Co-authored-by: Hayden Stainsby <hds@caffeineconcepts.com>

Author: jstarks

tracing-attributes/tests/ui/fail/async_instrument.stderr

@@ -14,15 +14,17 @@ error[E0308]: mismatched types
   --> tests/ui/fail/async_instrument.rs:10:5
    |
 10 |     ""
-   |     ^^- help: try using a conversion method: `.to_string()`
-   |     |
-   |     expected `String`, found `&str`
+   |     ^^ expected `String`, found `&str`
    |
 note: return type inferred to be `String` here
   --> tests/ui/fail/async_instrument.rs:9:31
    |
-9  | async fn simple_mismatch() -> String {
+ 9 | async fn simple_mismatch() -> String {
    |                               ^^^^^^
+help: try using a conversion method
+   |
+10 |     "".to_string()
+   |       ++++++++++++
 
 error[E0277]: `(&str,)` doesn't implement `std::fmt::Display`
   --> tests/ui/fail/async_instrument.rs:14:57
@@ -76,15 +78,17 @@ error[E0308]: mismatched types
   --> tests/ui/fail/async_instrument.rs:35:16
    |
 35 |         return "";
-   |                ^^- help: try using a conversion method: `.to_string()`
-   |                |
-   |                expected `String`, found `&str`
+   |                ^^ expected `String`, found `&str`
    |
 note: return type inferred to be `String` here
   --> tests/ui/fail/async_instrument.rs:33:28
    |
 33 | async fn early_return() -> String {
    |                            ^^^^^^
+help: try using a conversion method
+   |
+35 |         return "".to_string();
+   |                  ++++++++++++
 
 error[E0308]: mismatched types
   --> tests/ui/fail/async_instrument.rs:40:1

tracing-core/src/field.rs

@@ -1128,8 +1128,10 @@ mod test {
     use crate::metadata::{Kind, Level, Metadata};
 
     // Make sure TEST_CALLSITE_* have non-zero size, so they can't be located at the same address.
-    struct TestCallsite1();
-    static TEST_CALLSITE_1: TestCallsite1 = TestCallsite1();
+    struct TestCallsite1 {
+        _unused: u8,
+    }
+    static TEST_CALLSITE_1: TestCallsite1 = TestCallsite1 { _unused: 0 };
     static TEST_META_1: Metadata<'static> = metadata! {
         name: "field_test1",
         target: module_path!(),
@@ -1149,8 +1151,10 @@ mod test {
         }
     }
 
-    struct TestCallsite2();
-    static TEST_CALLSITE_2: TestCallsite2 = TestCallsite2();
+    struct TestCallsite2 {
+        _unused: u8,
+    }
+    static TEST_CALLSITE_2: TestCallsite2 = TestCallsite2 { _unused: 0 };
     static TEST_META_2: Metadata<'static> = metadata! {
         name: "field_test2",
         target: module_path!(),
@@ -1238,7 +1242,7 @@ mod test {
 
         struct MyVisitor;
         impl Visit for MyVisitor {
-            fn record_debug(&mut self, field: &Field, _: &dyn (fmt::Debug)) {
+            fn record_debug(&mut self, field: &Field, _: &dyn fmt::Debug) {
                 assert_eq!(field.callsite(), TEST_META_1.callsite())
             }
         }
@@ -1257,7 +1261,7 @@ mod test {
 
         struct MyVisitor;
         impl Visit for MyVisitor {
-            fn record_debug(&mut self, field: &Field, _: &dyn (fmt::Debug)) {
+            fn record_debug(&mut self, field: &Field, _: &dyn fmt::Debug) {
                 assert_eq!(field.name(), "bar")
             }
         }