attributes: change order of async and unsafe modifier (tokio-rs#2864)

When using `#[tracing::instrument]` and the `async unsafe` modifiers
the generated function read `unsafe async fn`, which is wrong. Corrected
the order and added a test.

Fixes: tokio-rs#2576

Signed-off-by: Gabriel Goller <[email protected]>

Author: kaffarell

tracing-attributes/src/expand.rs

@@ -92,7 +92,7 @@ pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
 
     quote!(
         #(#outer_attrs) *
-        #vis #constness #unsafety #asyncness #abi fn #ident<#gen_params>(#params) #output
+        #vis #constness #asyncness #unsafety #abi fn #ident<#gen_params>(#params) #output
         #where_clause
         {
             #(#inner_attrs) *

tracing-attributes/tests/async_fn.rs

@@ -31,6 +31,9 @@ async fn test_ret_impl_trait_err(n: i32) -> Result<impl Iterator<Item = i32>, &'
 #[instrument]
 async fn test_async_fn_empty() {}
 
+#[instrument]
+async unsafe fn test_async_unsafe_fn_empty() {}
+
 // Reproduces a compile error when an instrumented function body contains inner
 // attributes (https://github.com/tokio-rs/tracing/issues/2294).
 #[deny(unused_variables)]