chore: fix clippy warnings (#4727)

tokio/src/future/poll_fn.rs

@@ -35,6 +35,6 @@ where
     type Output = T;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
-        (&mut self.f)(cx)
+        (self.f)(cx)
     }
 }

tokio/src/io/util/vec_with_initialized.rs

@@ -1,19 +1,18 @@
 use crate::io::ReadBuf;
 use std::mem::MaybeUninit;
 
-mod private {
-    pub trait Sealed {}
-
-    impl Sealed for Vec<u8> {}
-    impl Sealed for &mut Vec<u8> {}
-}
+/// Something that looks like a `Vec<u8>`.
+///
+/// # Safety
+///
+/// The implementor must guarantee that the vector returned by the
+/// `as_mut` and `as_mut` methods do not change from one call to
+/// another.
+pub(crate) unsafe trait VecU8: AsRef<Vec<u8>> + AsMut<Vec<u8>> {}
 
-/// A sealed trait that constrains the generic type parameter in `VecWithInitialized<V>`.  That struct's safety relies
-/// on certain invariants upheld by `Vec<u8>`.
-pub(crate) trait VecU8: AsMut<Vec<u8>> + private::Sealed {}
+unsafe impl VecU8 for Vec<u8> {}
+unsafe impl VecU8 for &mut Vec<u8> {}
 
-impl VecU8 for Vec<u8> {}
-impl VecU8 for &mut Vec<u8> {}
 /// This struct wraps a `Vec<u8>` or `&mut Vec<u8>`, combining it with a
 /// `num_initialized`, which keeps track of the number of initialized bytes
 /// in the unused capacity.
@@ -64,8 +63,8 @@ where
     }
 
     #[cfg(feature = "io-util")]
-    pub(crate) fn is_empty(&mut self) -> bool {
-        self.vec.as_mut().is_empty()
+    pub(crate) fn is_empty(&self) -> bool {
+        self.vec.as_ref().is_empty()
     }
 
     pub(crate) fn get_read_buf<'a>(&'a mut self) -> ReadBuf<'a> {

tokio/src/net/addr.rs

@@ -136,16 +136,22 @@ impl sealed::ToSocketAddrsPriv for &[SocketAddr] {
     type Future = ReadyFuture<Self::Iter>;
 
     fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
-        // Clippy doesn't like the `to_vec()` call here (as it will allocate,
-        // while `self.iter().copied()` would not), but it's actually necessary
-        // in order to ensure that the returned iterator is valid for the
-        // `'static` lifetime, which the borrowed `slice::Iter` iterator would
-        // not be.
+        #[inline]
+        fn slice_to_vec(addrs: &[SocketAddr]) -> Vec<SocketAddr> {
+            addrs.to_vec()
+        }
+
+        // This uses a helper method because clippy doesn't like the `to_vec()`
+        // call here (it will allocate, whereas `self.iter().copied()` would
+        // not), but it's actually necessary in order to ensure that the
+        // returned iterator is valid for the `'static` lifetime, which the
+        // borrowed `slice::Iter` iterator would not be.
+        //
         // Note that we can't actually add an `allow` attribute for
         // `clippy::unnecessary_to_owned` here, as Tokio's CI runs clippy lints
         // on Rust 1.52 to avoid breaking LTS releases of Tokio. Users of newer
         // Rust versions who see this lint should just ignore it.
-        let iter = self.to_vec().into_iter();
+        let iter = slice_to_vec(self).into_iter();
         future::ready(Ok(iter))
     }
 }