Refactor lifetime name into an enum

src/test/run-pass/underscore-lifetimes.rs

@@ -26,10 +26,22 @@ fn foo3(x: &'_ u8) -> Foo {
 
 fn foo4(_: Foo<'_>) {}
 
+struct Foo2<'a, 'b> {
+    a: &'a u8,
+    b: &'b u8,
+}
+fn foo5<'b>(foo: Foo2<'_, 'b>) -> &'b u8 {
+    foo.b
+}
+
 fn main() {
     let x = &5;
     let _ = foo(x);
     let _ = foo2(x);
     let _ = foo3(x);
     foo4(Foo(x));
+    let _ = foo5(Foo2 {
+        a: x,
+        b: &6,
+    });
 }