Use GeneratorKind::descr() instead of it's Display impl

Those are basically the same but the first one seems to fit better

compiler/rustc_infer/src/errors.rs

@@ -73,7 +73,7 @@ pub struct AmbigousReturn<'a> {
 pub struct NeedTypeInfoInGenerator<'a> {
     #[primary_span]
     pub span: Span,
-    pub generator_kind: String,
+    pub generator_kind: &'static str,
     #[subdiagnostic]
     pub bad_label: InferenceBadError<'a>,
 }

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -568,7 +568,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         NeedTypeInfoInGenerator {
             bad_label: data.make_bad_error(span),
             span,
-            generator_kind: kind.to_string(),
+            generator_kind: kind.descr(),
         }
         .into_diagnostic(&self.tcx.sess.parse_sess)
     }