Use targetted diagnostic for borrow across yield error

compiler/rustc_borrowck/src/borrowck_errors.rs

@@ -373,11 +373,12 @@ pub(crate) fn cannot_borrow_across_coroutine_yield(
         span: Span,
         yield_span: Span,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let coroutine_kind = self.body.coroutine.as_ref().unwrap().coroutine_kind;
         let mut err = struct_span_err!(
             self,
             span,
             E0626,
-            "borrow may still be in use when coroutine yields",
+            "borrow may still be in use when {coroutine_kind:#} yields",
         );
         err.span_label(yield_span, "possible yield occurs here");
         err

tests/ui/coroutine/self_referential_gen_block.rs

@@ -6,7 +6,7 @@ fn main() {
     let mut x = {
         let mut x = gen {
             let y = 42;
-            let z = &y; //~ ERROR: borrow may still be in use when coroutine yields
+            let z = &y; //~ ERROR: borrow may still be in use when `gen` block yields
             yield 43;
             panic!("{z}");
         };

tests/ui/coroutine/self_referential_gen_block.stderr

@@ -1,4 +1,4 @@
-error[E0626]: borrow may still be in use when coroutine yields
+error[E0626]: borrow may still be in use when `gen` block yields
   --> $DIR/self_referential_gen_block.rs:9:21
    |
 LL |             let z = &y;