asset: Store a PhantomData lifetime inside AssetManager to repres…

…ent missing ownership semantics

ndk/src/asset.rs

@@ -7,6 +7,7 @@
 use std::{
     ffi::{CStr, CString},
     io,
+    marker::PhantomData,
     os::fd::{FromRawFd, OwnedFd},
     ptr::NonNull,
 };
@@ -16,24 +17,28 @@ use std::{
 /// [`AAssetManager *`]: https://developer.android.com/ndk/reference/group/asset#aassetmanager
 #[derive(Debug)]
 #[doc(alias = "AAssetManager")]
-pub struct AssetManager {
+pub struct AssetManager<'a> {
     ptr: NonNull<ffi::AAssetManager>,
+    _marker: PhantomData<&'a ()>,
 }
 
 // AAssetManager is thread safe.
 // See https://developer.android.com/ndk/reference/group/asset#aassetmanager
-unsafe impl Send for AssetManager {}
-unsafe impl Sync for AssetManager {}
+unsafe impl<'a> Send for AssetManager<'a> {}
+unsafe impl<'a> Sync for AssetManager<'a> {}
 
-impl AssetManager {
+impl<'a> AssetManager<'a> {
     /// Wraps a pointer to [`ffi::AssetManager`]
     ///
     /// # Safety
     /// `ptr` must be a valid pointer to an Android [`ffi::AAssetManager`]. The caller is
     /// responsible for guaranteeing the lifetime of this pointer, and should drop the structure
     /// _before_ the pointer becomes invalid.
     pub unsafe fn from_ptr(ptr: NonNull<ffi::AAssetManager>) -> Self {
-        Self { ptr }
+        Self {
+            ptr,
+            _marker: PhantomData,
+        }
     }
 
     /// Returns the pointer to the native [`ffi::AAssetManager`].

ndk/src/configuration.rs

@@ -100,7 +100,7 @@ impl Configuration {
         self.ptr
     }
 
-    pub fn from_asset_manager(am: &AssetManager) -> Self {
+    pub fn from_asset_manager(am: &AssetManager<'_>) -> Self {
         let config = Self::new();
         unsafe {
             ffi::AConfiguration_fromAssetManager(config.ptr().as_mut(), am.ptr().as_mut());

ndk/src/native_activity.rs

@@ -107,7 +107,9 @@ impl NativeActivity {
     }
 
     /// This app's asset manager, which can be used to access assets from the `.apk` file.
-    pub fn asset_manager(&self) -> crate::asset::AssetManager {
+    pub fn asset_manager(&self) -> crate::asset::AssetManager<'_> {
+        // SAFETY: Android initializes this field to a valid pointer, and the lifetime of the
+        // returned AssetManager is constrained to the lifetime of Self.
         unsafe {
             crate::asset::AssetManager::from_ptr(
                 NonNull::new(self.ptr.as_ref().assetManager).unwrap(),