Add ui test with nonempty string containing no lifetimes

test_suite/tests/ui/borrow/empty_lifetimes.rs

@@ -3,6 +3,8 @@ use serde_derive::Deserialize;
 #[derive(Deserialize)]
 struct Test<'a> {
     #[serde(borrow = "")]
+    r: &'a str,
+    #[serde(borrow = "  ")]
     s: &'a str,
 }
 

test_suite/tests/ui/borrow/empty_lifetimes.stderr

@@ -3,3 +3,9 @@ error: at least one lifetime must be borrowed
   |
 5 |     #[serde(borrow = "")]
   |                      ^^
+
+error: failed to parse borrowed lifetimes: "  "
+ --> tests/ui/borrow/empty_lifetimes.rs:7:22
+  |
+7 |     #[serde(borrow = "  ")]
+  |                      ^^^^