Add regression test

Author: oli-obk

tests/ui/privacy/generic_struct_field_projection.rs

@@ -0,0 +1,26 @@
+mod baz {
+    struct Foo;
+
+    pub trait Trait {
+        type Assoc;
+    }
+
+    impl Trait for Foo {
+        type Assoc = ();
+    }
+
+    pub struct Bar<'a, T: Trait> {
+        source: &'a T::Assoc,
+        //~^ ERROR: type `Foo` is private
+    }
+
+    pub struct Baz<'a> {
+        mode: Bar<'a, Foo>,
+    }
+}
+
+pub struct Struct<'a> {
+    lexer: baz::Baz<'a>,
+}
+
+fn main() {}

tests/ui/privacy/generic_struct_field_projection.stderr

@@ -0,0 +1,8 @@
+error: type `Foo` is private
+  --> $DIR/generic_struct_field_projection.rs:13:9
+   |
+LL |         source: &'a T::Assoc,
+   |         ^^^^^^^^^^^^^^^^^^^^ private type
+
+error: aborting due to 1 previous error
+