Remove unnecessary Default bound from DeviceSlice::as_host_vec. (Rust-GPU#68)

Co-authored-by: Christian Legnitto <LegNeato@users.noreply.github.com>

Author: chr1sj0nes

crates/cust/src/memory/device/device_slice.rs

@@ -13,6 +13,7 @@ use std::ops::{
     Index, IndexMut, Range, RangeFrom, RangeFull, RangeInclusive, RangeTo, RangeToInclusive,
 };
 use std::os::raw::c_void;
+use std::slice;
 use std::ptr::{slice_from_raw_parts, slice_from_raw_parts_mut};
 
 /// Fixed-size device-side slice.
@@ -35,14 +36,6 @@ impl<T: DeviceCopy> Debug for DeviceSlice<T> {
     }
 }
 
-impl<T: DeviceCopy + Default + Clone> DeviceSlice<T> {
-    pub fn as_host_vec(&self) -> CudaResult<Vec<T>> {
-        let mut vec = vec![T::default(); self.len()];
-        self.copy_to(&mut vec)?;
-        Ok(vec)
-    }
-}
-
 // This works by faking a regular slice out of the device raw-pointer and the length and transmuting
 // I have no idea if this is safe or not. Probably not, though I can't imagine how the compiler
 // could possibly know that the pointer is not de-referenceable. I'm banking that we get proper
@@ -94,6 +87,17 @@ impl<T: DeviceCopy> DeviceSlice<T> {
         DevicePointer::from_raw(self as *const _ as *const () as usize as u64)
     }
 
+    pub fn as_host_vec(&self) -> CudaResult<Vec<T>> {
+        let mut vec = Vec::with_capacity(self.len());
+        // SAFETY: The slice points to uninitialized memory, but we only write to it. Once it is
+        // written, all values are valid, so we can (and must) change the length of the vector.
+        unsafe {
+            self.copy_to(slice::from_raw_parts_mut(vec.as_mut_ptr(), self.len()))?;
+            vec.set_len(self.len())
+        }
+        Ok(vec)
+    }
+
     /* TODO (AL): keep these?
     /// Divides one DeviceSlice into two at a given index.
     ///