Fix Blob::tensor_desc() (#56)

* Fix and optimise Blob::tensor_desc()
* Load shared library for Blob::tensor_desc() test

Author: chemicstry

crates/openvino/src/blob.rs

@@ -1,12 +1,12 @@
 use crate::tensor_desc::TensorDesc;
 use crate::{drop_using_function, try_unsafe, util::Result, InferenceError};
-use crate::{Layout, Precision};
 use openvino_sys::{
-    self, dimensions_t, ie_blob_buffer__bindgen_ty_1, ie_blob_buffer_t, ie_blob_byte_size,
-    ie_blob_free, ie_blob_get_buffer, ie_blob_get_dims, ie_blob_get_layout, ie_blob_get_precision,
-    ie_blob_make_memory, ie_blob_size, ie_blob_t,
+    self, ie_blob_buffer__bindgen_ty_1, ie_blob_buffer_t, ie_blob_byte_size, ie_blob_free,
+    ie_blob_get_buffer, ie_blob_get_dims, ie_blob_get_layout, ie_blob_get_precision,
+    ie_blob_make_memory, ie_blob_size, ie_blob_t, tensor_desc_t,
 };
 use std::convert::TryFrom;
+use std::mem::MaybeUninit;
 
 /// See [`Blob`](https://docs.openvinotoolkit.org/latest/classInferenceEngine_1_1Blob.html).
 pub struct Blob {
@@ -53,22 +53,25 @@ impl Blob {
     pub fn tensor_desc(&self) -> Result<TensorDesc> {
         let blob = self.instance as *const ie_blob_t;
 
-        let mut layout = Layout::ANY;
-        try_unsafe!(ie_blob_get_layout(blob, std::ptr::addr_of_mut!(layout)))?;
+        let mut layout = MaybeUninit::uninit();
+        try_unsafe!(ie_blob_get_layout(blob, layout.as_mut_ptr()))?;
 
-        let mut dimensions = dimensions_t {
-            ranks: 0,
-            dims: [0; 8usize],
-        };
-        try_unsafe!(ie_blob_get_dims(blob, std::ptr::addr_of_mut!(dimensions)))?;
+        let mut dimensions = MaybeUninit::uninit();
+        try_unsafe!(ie_blob_get_dims(blob, dimensions.as_mut_ptr()))?;
 
-        let mut precision = Precision::UNSPECIFIED;
-        try_unsafe!(ie_blob_get_precision(
-            blob,
-            std::ptr::addr_of_mut!(precision)
-        ))?;
+        let mut precision = MaybeUninit::uninit();
+        try_unsafe!(ie_blob_get_precision(blob, precision.as_mut_ptr()))?;
 
-        Ok(TensorDesc::new(layout, &dimensions.dims, precision))
+        Ok(TensorDesc {
+            // Safety: all reads succeeded so values must be initialized
+            instance: unsafe {
+                tensor_desc_t {
+                    layout: layout.assume_init(),
+                    dims: dimensions.assume_init(),
+                    precision: precision.assume_init(),
+                }
+            },
+        })
     }
 
     /// Get the number of elements contained in the [`Blob`].
@@ -186,6 +189,7 @@ impl Blob {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::{Layout, Precision};
 
     #[test]
     #[should_panic]
@@ -223,4 +227,18 @@ mod tests {
             "we should have half as many items (u16 = f32 / 2)"
         );
     }
+
+    #[test]
+    fn tensor_desc() {
+        openvino_sys::library::load().expect("unable to find an OpenVINO shared library");
+
+        let desc = TensorDesc::new(Layout::NHWC, &[1, 2, 2, 2], Precision::U8);
+        let blob = Blob::new(&desc, &[0; 8]).unwrap();
+        let desc2 = blob.tensor_desc().unwrap();
+
+        // Both TensorDesc's should be equal
+        assert_eq!(desc.layout(), desc2.layout());
+        assert_eq!(desc.dims(), desc2.dims());
+        assert_eq!(desc.precision(), desc2.precision());
+    }
 }