Introduce VecAssumeInit to convert Vec<MaybeUninit<T>> to Vec<T>

Use ManuallyDrop to implement VecAssumeInit

lax/src/lib.rs

@@ -156,6 +156,30 @@ impl_as_ptr!(MaybeUninit<f64>, f64);
 impl_as_ptr!(MaybeUninit<c32>, lapack_sys::__BindgenComplex<f32>);
 impl_as_ptr!(MaybeUninit<c64>, lapack_sys::__BindgenComplex<f64>);
 
+pub(crate) trait VecAssumeInit {
+    type Target;
+    unsafe fn assume_init(self) -> Self::Target;
+}
+
+macro_rules! impl_vec_assume_init {
+    ($e:ty) => {
+        impl VecAssumeInit for Vec<MaybeUninit<$e>> {
+            type Target = Vec<$e>;
+            unsafe fn assume_init(self) -> Self::Target {
+                // FIXME use Vec::into_raw_parts instead after stablized
+                // https://doc.rust-lang.org/std/vec/struct.Vec.html#method.into_raw_parts
+                let mut me = std::mem::ManuallyDrop::new(self);
+                Vec::from_raw_parts(me.as_mut_ptr() as *mut $e, me.len(), me.capacity())
+            }
+        }
+    };
+}
+
+impl_vec_assume_init!(f32);
+impl_vec_assume_init!(f64);
+impl_vec_assume_init!(c32);
+impl_vec_assume_init!(c64);
+
 /// Upper/Lower specification for seveal usages
 #[derive(Debug, Clone, Copy)]
 #[repr(u8)]