chore: new clippy/rust lints (#2948)

## Motivation

clippy is failing with recent Rust toolchain up to 1.77.2.

## Solution

Give clippy what it wants.

Author: dpc

tracing-attributes/src/expand.rs

@@ -795,6 +795,7 @@ impl<'a> VisitMut for IdentAndTypesRenamer<'a> {
 }
 
 // A visitor struct that replace an async block by its patched version
+#[allow(dead_code)]
 struct AsyncTraitBlockReplacer<'a> {
     block: &'a Block,
     patched_block: Block,

tracing-attributes/tests/instrument.rs

@@ -100,6 +100,7 @@ fn fields() {
 
 #[test]
 fn skip() {
+    #[allow(dead_code)]
     struct UnDebug(pub u32);
 
     #[instrument(target = "my_target", level = "debug", skip(_arg2, _arg3))]

tracing-core/src/dispatch.rs

@@ -510,6 +510,7 @@ pub(crate) fn get_global() -> &'static Dispatch {
     unsafe {
         // This is safe given the invariant that setting the global dispatcher
         // also sets `GLOBAL_INIT` to `INITIALIZED`.
+        #[allow(static_mut_refs)]
         &GLOBAL_DISPATCH
     }
 }

tracing-core/src/field.rs

@@ -966,6 +966,7 @@ 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.
+    #[allow(dead_code)]
     struct TestCallsite1(u8);
     static TEST_CALLSITE_1: TestCallsite1 = TestCallsite1(0);
     static TEST_META_1: Metadata<'static> = metadata! {
@@ -987,6 +988,7 @@ mod test {
         }
     }
 
+    #[allow(dead_code)]
     struct TestCallsite2(u8);
     static TEST_CALLSITE_2: TestCallsite2 = TestCallsite2(0);
     static TEST_META_2: Metadata<'static> = metadata! {

tracing-futures/tests/std_future.rs

@@ -55,6 +55,7 @@ fn span_on_drop() {
         }
     }
 
+    #[allow(dead_code)]
     struct Fut(Option<AssertSpanOnDrop>);
 
     impl Future for Fut {

tracing-subscriber/src/registry/sharded.rs

@@ -592,6 +592,7 @@ mod tests {
         closed: Vec<(&'static str, Weak<()>)>,
     }
 
+    #[allow(dead_code)]
     struct SetRemoved(Arc<()>);
 
     impl<C> Subscribe<C> for CloseSubscriber

tracing/tests/instrument.rs

@@ -21,6 +21,7 @@ fn span_on_drop() {
         }
     }
 
+    #[allow(dead_code)]
     struct Fut(Option<AssertSpanOnDrop>);
 
     impl Future for Fut {