Remove _dropck flag from SendError

The nomicon has been outdated about this since 2015,
see rust-lang/nomicon#363 for the update. 
We don't need to tell dropck that we are owning a T as it
can already infer that from the generic parameter in the drop impl.

Author: Noratrieb

src/errors.rs

@@ -1,6 +1,5 @@
 use super::{dealloc, Channel};
 use core::fmt;
-use core::marker::PhantomData;
 use core::mem;
 use core::ptr::NonNull;
 
@@ -10,41 +9,6 @@ use core::ptr::NonNull;
 /// The message that could not be sent can be retreived again with [`SendError::into_inner`].
 pub struct SendError<T> {
     channel_ptr: NonNull<Channel<T>>,
-    /// Required due to the reasons outlined in
-    /// [this section](https://doc.rust-lang.org/nomicon/dropck.html) of the nomicon, as well as
-    /// the next section.
-    ///
-    /// Without the phantom data, the following code would incorrectly compile. This code is
-    /// invalid because `error` gets dropped first (struct fields are dropped in declaration order)
-    /// but `oof` then accesses the data deallocated by `error`, causing a use-after-free.
-    ///
-    /// ```compile_fail
-    /// let (tx, rx) = oneshot::channel::<Box<u8>>();
-    /// drop(rx);
-    /// let error = tx.send(Box::new(0)).unwrap_err();
-    ///
-    /// struct Oof<'a>(&'a u8);
-    ///
-    /// impl<'a> Drop for Oof<'a> {
-    ///     fn drop(&mut self) {
-    ///         println!("{}", self.0);
-    ///     }
-    /// }
-    ///
-    /// struct Foo<'a> {
-    ///     error: SendError<Box<u8>>,
-    ///     oof: Option<Oof<'a>>,
-    /// }
-    ///
-    /// let mut foo = Foo {
-    ///     error,
-    ///     oof: None
-    /// };
-    ///
-    /// foo.oof = Some(Oof(&**foo.error.as_inner()));
-    /// drop(foo);
-    /// ```
-    _dropck: PhantomData<T>,
 }
 
 unsafe impl<T: Send> Send for SendError<T> {}
@@ -60,7 +24,6 @@ impl<T> SendError<T> {
     pub(crate) const unsafe fn new(channel_ptr: NonNull<Channel<T>>) -> Self {
         Self {
             channel_ptr,
-            _dropck: PhantomData,
         }
     }