Improve span of non-Send async block error

Author: dtolnay

src/expand.rs

@@ -341,7 +341,7 @@ fn transform_block(sig: &mut Signature, block: &mut Block) {
  ReturnType::Type(_, ret) => quote!(#ret),
  };
 
- let box_pin = quote_spanned!(ret_ty.span()=>
+ let box_pin = quote_spanned!(block.brace_token.span=>
  Box::pin(async move {
  let __ret: #ret_ty = {
  #(#decls)*

tests/ui/send-not-implemented.stderr

@@ -22,20 +22,26 @@ note: future is not `Send` as this value is used across an await
  = note: required for the cast to the object type `dyn Future<Output = ()> + Send`
 
 error: future cannot be sent between threads safely
- --> $DIR/send-not-implemented.rs:14:33
+ --> $DIR/send-not-implemented.rs:14:38
  |
-14 | async fn test_ret(&self) -> bool {
- | ^^^^ future created by async block is not `Send`
+14 | async fn test_ret(&self) -> bool {
+ | ______________________________________^
+15 | | let mutex = Mutex::new(());
+16 | | let _guard = mutex.lock().unwrap();
+17 | | f().await;
+18 | | true
+19 | | }
+ | |_____^ future created by async block is not `Send`
  |
  = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
 note: future is not `Send` as this value is used across an await
  --> $DIR/send-not-implemented.rs:17:9
  |
-14 | async fn test_ret(&self) -> bool {
- | - `_guard` is later dropped here
-15 | let mutex = Mutex::new(());
 16 | let _guard = mutex.lock().unwrap();
  | ------ has type `MutexGuard<'_, ()>` which is not `Send`
 17 | f().await;
  | ^^^^^^^^^ await occurs here, with `_guard` maybe used later
+18 | true
+19 | }
+ | - `_guard` is later dropped here
  = note: required for the cast to the object type `dyn Future<Output = bool> + Send`