tests: ensure that the data cannot be accessed

Similarly to guards, the pin-init data generated by the `#[pin_data]`
attribute macro and instantiated by an initializer macro should not be
accessible by user-controlled code.

Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

tests/ui/compile-fail/init/data_access.rs

@@ -0,0 +1,12 @@
+use pin_init::*;
+
+#[pin_data]
+struct Foo {}
+
+fn main() {
+    let _ = pin_init!(Foo {
+        _: {
+            let _ = __data;
+        },
+    });
+}

tests/ui/compile-fail/init/data_access.stderr

@@ -0,0 +1,5 @@
+error[E0425]: cannot find value `__data` in this scope
+ --> tests/ui/compile-fail/init/data_access.rs:9:21
+  |
+9 |             let _ = __data;
+  |                     ^^^^^^ not found in this scope