Add test for public struct with private fields

tests/ui/xcrate/auxiliary/xcrate_unit_struct.rs

@@ -18,6 +18,11 @@ pub enum Unit {
 
 #[derive(Copy, Clone)]
 pub struct StructWithFields {
+    pub foo: isize,
+}
+
+#[derive(Copy, Clone)]
+pub struct StructWithPrivFields {
     foo: isize,
 }
 

tests/ui/xcrate/xcrate-unit-struct.rs

@@ -8,5 +8,7 @@
 fn main() {
     let _ = xcrate_unit_struct::StructWithFields;
     //~^ ERROR expected value, found struct `xcrate_unit_struct::StructWithFields`
+    let _ = xcrate_unit_struct::StructWithPrivFields;
+    //~^ ERROR expected value, found struct `xcrate_unit_struct::StructWithPrivFields`
     let _ = xcrate_unit_struct::Struct;
 }

tests/ui/xcrate/xcrate-unit-struct.stderr

@@ -9,6 +9,17 @@ LL |     let _ = xcrate_unit_struct::StructWithFields;
 LL | pub struct StructWithFields {
    | --------------------------- `xcrate_unit_struct::StructWithFields` defined here
 
-error: aborting due to previous error
+error[E0423]: expected value, found struct `xcrate_unit_struct::StructWithPrivFields`
+  --> $DIR/xcrate-unit-struct.rs:11:13
+   |
+LL |     let _ = xcrate_unit_struct::StructWithPrivFields;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use struct literal syntax instead: `xcrate_unit_struct::StructWithPrivFields { foo: val }`
+   |
+  ::: $DIR/auxiliary/xcrate_unit_struct.rs:25:1
+   |
+LL | pub struct StructWithPrivFields {
+   | ------------------------------- `xcrate_unit_struct::StructWithPrivFields` defined here
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0423`.