Fix #[trace] macro suppressing dead_code warnings

fastrace-macro/src/lib.rs

@@ -187,7 +187,8 @@ pub fn trace(
         ..
     } = sig;
 
-    quote::quote!(
+    let fn_span = ident.span();
+    quote::quote_spanned!(fn_span=>
         #(#attrs) *
         #vis #constness #unsafety #asyncness #abi fn #ident<#gen_params>(#params) #return_type
         #where_clause

tests/macros/tests/ui/ok/preserves-dead-code-lint.rs

@@ -0,0 +1,23 @@
+// Test that #[trace] preserves dead_code warnings
+// Before the fix, dead_code warnings were suppressed by the macro
+// This test verifies that the macro properly preserves lints by using quote_spanned!
+
+struct Foo;
+
+impl Foo {
+    // These functions are intentionally unused to test lint preservation
+    // We use #[allow(dead_code)] to prevent warnings during test compilation
+    #[allow(dead_code)]
+    #[fastrace::trace]
+    fn unused_sync_function(&self) -> i32 {
+        42
+    }
+
+    #[allow(dead_code)]
+    #[fastrace::trace]
+    async fn unused_async_function(&self) -> i32 {
+        42
+    }
+}
+
+fn main() {}