tests: ensure the guards cannot be accessed

The guards created in order to drop already initialized fields correctly
when the initializer errors or panics must not be accessible by user
controlled code. (They could for example `mem::forget` the guard and
thus prevent its normal function, making the initializer macros
unsound.) Thus add a test to ensure that this is true.

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

Author: BennoLossin

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

@@ -0,0 +1,14 @@
+use pin_init::*;
+
+struct Foo {
+    x: usize,
+}
+
+fn main() {
+    let _ = init!(Foo {
+        x: 0,
+        _: {
+            let _ = __x_guard;
+        },
+    });
+}

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

@@ -0,0 +1,5 @@
+error[E0425]: cannot find value `__x_guard` in this scope
+  --> tests/ui/compile-fail/init/guard_access.rs:11:21
+   |
+11 |             let _ = __x_guard;
+   |                     ^^^^^^^^^ not found in this scope