Add test for default_field_values and const_default

Add a test showing `#![feature(default_field_values)]` using `#[const_trait] trait Default` (`#![feature(const_default)]` + `#![feature(const_trait_impl)]`).

Author: estebank

tests/ui/structs/default-field-values/const-trait-default-field-value.rs

@@ -0,0 +1,28 @@
+//@ check-pass
+
+// Ensure that `default_field_values` and `const_default` interact properly.
+
+#![feature(default_field_values)]
+#![feature(const_trait_impl)]
+#![feature(const_default)]
+
+#[derive(Default, PartialEq, Eq, Debug)]
+struct S {
+    r: Option<String> = <Option<_> as Default>::default(),
+    s: String = String::default(),
+    o: Option<String> = Option::<String>::default(),
+    p: std::marker::PhantomData<()> = std::marker::PhantomData::default(),
+    q: Option<String> = <Option<String> as Default>::default(),
+    t: Option<String> = Option::default(),
+}
+
+fn main() {
+    let s = S { .. };
+    assert_eq!(s.r, None);
+    assert_eq!(&s.s, "");
+    assert_eq!(s.o, None);
+    assert_eq!(s.p, std::marker::PhantomData);
+    assert_eq!(s.q, None);
+    assert_eq!(s.t, None);
+    assert_eq!(s, S::default());
+}