fix confusing diagnostic for reserved ##

compiler/rustc_parse/src/errors.rs

@@ -2151,6 +2151,15 @@ pub(crate) enum UnknownPrefixSugg {
     },
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_reserved_multihash)]
+#[note]
+pub(crate) struct ReservedMultihash {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sugg: Option<GuardedStringSugg>,
+}
 #[derive(Diagnostic)]
 #[diag(parse_reserved_string)]
 #[note]