Box<dyn Any> to sidestep Arc<[T]>provenance issues

1 extra alloc and indirection won't kill me. probably.
Closes #18

crates/thindx-xaudio2/src/xa28/ixaudio2sourcevoice_ext.rs

@@ -56,22 +56,7 @@ impl<Sample, Context: Send + Sync + Sized + 'static> IXAudio2SourceVoiceTyped<Sa
 
         b.pContext = Box::into_raw(Box::new(SourceBuffer::<Context> {
             context,
-            audio_len:  audio_data.len(),
-            audio_data: {
-                // ⚠️ WARNING: easy to muck up provenance of ArcInner<[Sample]> here.
-                let raw : *const [Sample] = Arc::into_raw(audio_data);
-                let raw : *const Sample = unsafe { (*raw).as_ptr() };
-                raw.cast()
-            },
-            audio_free: |data, len| {
-                // 🐞 BUG: core::slice::from_raw_parts here incorrectly narrows provenance from ArcInner<[Sample]> to [Sample] here.
-                // However, core::ptr::from_raw_parts is not yet stable:    https://github.com/rust-lang/rust/issues/81513
-                // Local tracking issue:                                    https://github.com/MaulingMonkey/thindx-xaudio2/issues/18
-                let audio_data : *const [Sample] = unsafe { core::slice::from_raw_parts(data as *const Sample, len) };
-
-                let audio_data = unsafe { Arc::from_raw(audio_data) };
-                drop(audio_data);
-            },
+            _audio_data: Box::new(audio_data),
         })).cast();
 
         unsafe { self.SubmitSourceBuffer(&b, null()) }.succeeded()

crates/thindx-xaudio2/src/xa28/source_buffer.rs

@@ -1,6 +1,6 @@
 #[allow(unused_imports)] use super::*;
 #[allow(unused_imports)] use super::xaudio2::sys::*;
-use core::ffi::c_void;
+use std::any::Any;
 
 
 
@@ -10,7 +10,5 @@ use core::ffi::c_void;
 /// *   [VoiceCallback::BufferContext] parameters
 pub(crate) struct SourceBuffer<Context: Send + Sync + Sized + 'static> {
     pub(crate) context:     Context,
-    pub(crate) audio_data:  *const c_void,
-    pub(crate) audio_len:   usize,
-    pub(crate) audio_free:  unsafe fn(*const c_void, usize),
+    pub(crate) _audio_data: Box<dyn Any>, // for keepalive only
 }

crates/thindx-xaudio2/src/xa28/voice_callback.rs

@@ -118,7 +118,6 @@ impl<VC: VoiceCallback> VoiceCallbackWrapper<VC> {
             let this : &Self = unsafe { &*sptr::from_exposed_addr(sptr::Strict::addr(this)) };
             let buffer_context = *unsafe { Box::from_raw(buffer_context as *mut SourceBuffer<VC::BufferContext>) };
             this.callbacks.on_buffer_end(buffer_context.context);
-            unsafe { (buffer_context.audio_free)(buffer_context.audio_data, buffer_context.audio_len) }
         })
     }