Add new UI test for private types in public APIs

Author: GuillaumeGomez

tests/ui/privacy/local-public-in-public-api.rs

@@ -0,0 +1,42 @@
+// This ui test checks that private types present in public APIs get
+// a warning.
+
+#![crate_type = "lib"]
+#![deny(private_in_public)]
+
+mod my_type {
+    pub struct Field;
+    pub struct MyType {
+        pub field: Field,
+    }
+
+    impl MyType {
+        pub fn builder() -> MyTypeBuilder {
+                            //~^ ERROR
+                            //~| WARN
+            MyTypeBuilder { field: Field }
+        }
+
+        pub fn with_builder(_: MyTypeBuilder) {}
+                               //~^ ERROR
+                               //~| WARN
+    }
+
+    pub struct MyTypeBuilder {
+        field: Field,
+    }
+
+    impl MyTypeBuilder {
+        pub fn field(mut self, val: Field) -> Self {
+            self.field = val;
+
+            self
+        }
+
+        pub fn build(self) -> MyType {
+            MyType { field: self.field }
+        }
+    }
+}
+
+pub use my_type::{MyType, Field};

tests/ui/privacy/local-public-in-public-api.stderr

@@ -0,0 +1,25 @@
+error: public but not reexported type `MyTypeBuilder` in public interface (error E0446)
+  --> $DIR/local-public-in-public-api.rs:14:9
+   |
+LL |         pub fn builder() -> MyTypeBuilder {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
+note: the lint level is defined here
+  --> $DIR/local-public-in-public-api.rs:5:9
+   |
+LL | #![deny(private_in_public)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: public but not reexported type `MyTypeBuilder` in public interface (error E0446)
+  --> $DIR/local-public-in-public-api.rs:20:9
+   |
+LL |         pub fn with_builder(_: MyTypeBuilder) {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
+
+error: aborting due to 2 previous errors
+