macros: reduce usage of last statement spans in proc-macros (#5092)

This excludes the initial let statement of the proc-macro expansion
from receiving the last statement spans to aid completions in rust-analyzer.
The current span confuses rust-analyzer as it will map the tail expression
tokens to the let keyword (as this is the first token it finds with the
same span) which currently breaks completions. This commit should not
degrade the initial intent of the span reusages, as the type mismatch
parts are still spanned appropriately.

tokio-macros/src/entry.rs

@@ -383,17 +383,20 @@ fn parse_knobs(mut input: syn::ItemFn, is_test: bool, config: FinalConfig) -> To
 
     let body = &input.block;
     let brace_token = input.block.brace_token;
-    input.block = syn::parse2(quote_spanned! {last_stmt_end_span=>
+    let block_expr = quote_spanned! {last_stmt_end_span=>
+        #[allow(clippy::expect_used, clippy::diverging_sub_expression)]
+        {
+            return #rt
+                .enable_all()
+                .build()
+                .expect("Failed building the Runtime")
+                .block_on(body);
+        }
+    };
+    input.block = syn::parse2(quote! {
         {
             let body = async #body;
-            #[allow(clippy::expect_used, clippy::diverging_sub_expression)]
-            {
-                return #rt
-                    .enable_all()
-                    .build()
-                    .expect("Failed building the Runtime")
-                    .block_on(body);
-            }
+            #block_expr
         }
     })
     .expect("Parsing failure");