Merge #2257

2257: Added a comment to WasmPtr about structs. r=syrusakbary a=Earthmark

# Description
This adds a comment about how to use compound c-like structs using WasmPtr.

# Review
This is possibly too small for a changelog entry.

Example tests ensuring this behavior should possibly be added.

I'm also curious as to if this is correct, cause it seems to be working in a side project but I'm unsure as to if it's a good idea!

Co-authored-by: Daniel Miller (Earthmark) <earthmark.miller@gmail.com>
Co-authored-by: Daniel Miller <earthmark.miller@gmail.com>

lib/api/src/ptr.rs

@@ -32,6 +32,35 @@ pub struct Item;
 ///     derefed_ptr.set(inner_val + 1);
 /// }
 /// ```
+///
+/// This type can also be used with primitive-filled structs, but be careful of
+/// guarantees required by `ValueType`.
+/// ```
+/// # use wasmer::Memory;
+/// # use wasmer::WasmPtr;
+/// # use wasmer::ValueType;
+///
+/// #[derive(Copy, Clone, Debug)]
+/// #[repr(C)]
+/// struct V3 {
+///     x: f32,
+///     y: f32,
+///     z: f32
+/// }
+/// // This is safe as the 12 bytes represented by this struct
+/// // are valid for all bit combinations.
+/// unsafe impl ValueType for V3 {
+/// }
+///
+/// fn update_vector_3(memory: Memory, ptr: WasmPtr<V3>) {
+///     let derefed_ptr = ptr.deref(&memory).expect("pointer in bounds");
+///     let mut inner_val: V3 = derefed_ptr.get();
+///     println!("Got {:?} from Wasm memory address 0x{:X}", inner_val, ptr.offset());
+///     // update the value being pointed to
+///     inner_val.x = 10.4;
+///     derefed_ptr.set(inner_val);
+/// }
+/// ```
 #[repr(transparent)]
 pub struct WasmPtr<T: Copy, Ty = Item> {
     offset: u32,