Add safety comments to binder tests.

These will soon be required by a lint.

Bug: 290018030
Test: m rust
Change-Id: I807496ec5262145342ad9a40943094bfbe3cccc5
diff --git a/libs/binder/rust/tests/integration.rs b/libs/binder/rust/tests/integration.rs
index ca2cedc..c049b80 100644
--- a/libs/binder/rust/tests/integration.rs
+++ b/libs/binder/rust/tests/integration.rs
@@ -545,6 +545,11 @@
     }
 
     fn get_expected_selinux_context() -> &'static str {
+        // SAFETY: The pointer we pass to `getcon` is valid because it comes from a reference, and
+        // `getcon` doesn't retain it after it returns. If `getcon` succeeds then `out_ptr` will
+        // point to a valid C string, otherwise it will remain null. We check for null, so the
+        // pointer we pass to `CStr::from_ptr` must be a valid pointer to a C string. There is a
+        // memory leak as we don't call `freecon`, but that's fine because this is just a test.
         unsafe {
             let mut out_ptr = ptr::null_mut();
             assert_eq!(selinux_sys::getcon(&mut out_ptr), 0);