Always set #![no_std] and remove redundant imports

```
error: the item `Box` is imported redundantly
 --> crossbeam-epoch/src/atomic.rs:1:5
  |
1 | use alloc::boxed::Box;
  |     ^^^^^^^^^^^^^^^^^
 --> /rustc/381d69953bb7c3390cec0fee200f24529cb6320f/library/std/src/prelude/mod.rs:125:13
  |
  = note: the item `Box` is already defined here
  |
  = note: `-D unused-imports` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: the item `Box` is imported redundantly
 --> crossbeam-epoch/src/deferred.rs:1:5
  |
1 | use alloc::boxed::Box;
  |     ^^^^^^^^^^^^^^^^^
 --> /rustc/381d69953bb7c3390cec0fee200f24529cb6320f/library/std/src/prelude/mod.rs:125:13
  |
  = note: the item `Box` is already defined here
```

crossbeam-channel/src/context.rs

@@ -41,7 +41,7 @@ impl Context {
     where
         F: FnOnce(&Self) -> R,
     {
-        thread_local! {
+        std::thread_local! {
             /// Cached thread-local context.
             static CONTEXT: Cell<Option<Context>> = Cell::new(Some(Context::new()));
         }

crossbeam-channel/src/counter.rs

@@ -1,5 +1,6 @@
 //! Reference counter for channels.
 
+use std::boxed::Box;
 use std::isize;
 use std::ops;
 use std::process;

crossbeam-channel/src/flavors/array.rs

@@ -8,6 +8,7 @@
 //!   - <http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue>
 //!   - <https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub>
 
+use std::boxed::Box;
 use std::cell::UnsafeCell;
 use std::mem::{self, MaybeUninit};
 use std::ptr;

crossbeam-channel/src/flavors/list.rs

@@ -1,5 +1,6 @@
 //! Unbounded channel implemented as a linked list.
 
+use std::boxed::Box;
 use std::cell::UnsafeCell;
 use std::marker::PhantomData;
 use std::mem::MaybeUninit;

crossbeam-channel/src/flavors/zero.rs

@@ -2,6 +2,7 @@
 //!
 //! This kind of channel is also known as *rendezvous* channel.
 
+use std::boxed::Box;
 use std::cell::UnsafeCell;
 use std::marker::PhantomData;
 use std::sync::atomic::{AtomicBool, Ordering};

crossbeam-channel/src/lib.rs

@@ -325,6 +325,7 @@
 //! [`iter`]: Receiver::iter
 //! [`try_iter`]: Receiver::try_iter
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -333,7 +334,9 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(feature = "std")]
 mod channel;

crossbeam-channel/src/select.rs

@@ -4,6 +4,7 @@ use std::fmt;
 use std::marker::PhantomData;
 use std::mem;
 use std::time::{Duration, Instant};
+use std::vec::Vec;
 
 use crossbeam_utils::Backoff;
 

crossbeam-channel/src/utils.rs

@@ -12,7 +12,7 @@ pub(crate) fn shuffle<T>(v: &mut [T]) {
         return;
     }
 
-    thread_local! {
+    std::thread_local! {
         static RNG: Cell<Wrapping<u32>> = const { Cell::new(Wrapping(1_406_868_647)) };
     }
 

crossbeam-channel/src/waker.rs

@@ -4,6 +4,7 @@ use std::ptr;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Mutex;
 use std::thread::{self, ThreadId};
+use std::vec::Vec;
 
 use crate::context::Context;
 use crate::select::{Operation, Selected};
@@ -275,7 +276,7 @@ impl Drop for SyncWaker {
 /// Returns the id of the current thread.
 #[inline]
 fn current_thread_id() -> ThreadId {
-    thread_local! {
+    std::thread_local! {
         /// Cached thread-local id.
         static THREAD_ID: ThreadId = thread::current().id();
     }

crossbeam-channel/tests/mpsc.rs

@@ -193,7 +193,7 @@ mod channel_tests {
 
     use std::env;
     use std::thread;
-    use std::time::{Duration, Instant};
+    use std::time::Instant;
 
     fn stress_factor() -> usize {
         match env::var("RUST_TEST_STRESS") {
@@ -969,7 +969,6 @@ mod sync_channel_tests {
 
     use std::env;
     use std::thread;
-    use std::time::Duration;
 
     fn stress_factor() -> usize {
         match env::var("RUST_TEST_STRESS") {

crossbeam-deque/src/deque.rs

@@ -1,3 +1,4 @@
+use std::boxed::Box;
 use std::cell::{Cell, UnsafeCell};
 use std::cmp;
 use std::fmt;

crossbeam-deque/src/lib.rs

@@ -82,6 +82,7 @@
 //! [`steal_batch()`]: Stealer::steal_batch
 //! [`steal_batch_and_pop()`]: Stealer::steal_batch_and_pop
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -90,7 +91,9 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(feature = "std")]
 mod deque;

crossbeam-epoch/src/collector.rs

@@ -114,6 +114,7 @@ impl fmt::Debug for LocalHandle {
 mod tests {
     use std::mem::ManuallyDrop;
     use std::sync::atomic::{AtomicUsize, Ordering};
+    use std::vec::Vec;
 
     use crossbeam_utils::thread;
 

crossbeam-epoch/src/default.rs

@@ -76,7 +76,7 @@ mod tests {
             }
         }
 
-        thread_local! {
+        std::thread_local! {
             static FOO: Foo = const { Foo };
         }
 

crossbeam-epoch/src/deferred.rs

@@ -90,8 +90,11 @@ impl Deferred {
 #[cfg(all(test, not(crossbeam_loom)))]
 mod tests {
     use super::Deferred;
+    use std::boxed::Box;
     use std::cell::Cell;
     use std::convert::identity;
+    use std::string::ToString;
+    use std::vec;
 
     #[test]
     fn on_stack() {

crossbeam-epoch/src/internal.rs

@@ -558,7 +558,7 @@ impl IsElement<Self> for Local {
 
 #[cfg(all(test, not(crossbeam_loom)))]
 mod tests {
-    use std::sync::atomic::{AtomicUsize, Ordering};
+    use std::sync::atomic::AtomicUsize;
 
     use super::*;
 

crossbeam-epoch/src/lib.rs

@@ -48,6 +48,7 @@
 //! For majority of use cases, just use the default garbage collector by invoking [`pin`]. If you
 //! want to create your own garbage collector, use the [`Collector`] API.
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -56,10 +57,11 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
 
 #[cfg(crossbeam_loom)]
 extern crate loom_crate as loom;
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(crossbeam_loom)]
 #[allow(unused_imports, dead_code)]

crossbeam-epoch/src/sync/list.rs

@@ -301,6 +301,7 @@ mod tests {
     use crate::{Collector, Owned};
     use crossbeam_utils::thread;
     use std::sync::Barrier;
+    use std::vec::Vec;
 
     impl IsElement<Self> for Entry {
         fn entry_of(entry: &Self) -> &Entry {

crossbeam-epoch/src/sync/queue.rs

@@ -219,6 +219,7 @@ mod test {
     use super::*;
     use crate::pin;
     use crossbeam_utils::thread;
+    use std::vec;
 
     struct Queue<T> {
         queue: super::Queue<T>,

crossbeam-queue/src/lib.rs

@@ -5,6 +5,7 @@
 //! * [`ArrayQueue`], a bounded MPMC queue that allocates a fixed-capacity buffer on construction.
 //! * [`SegQueue`], an unbounded MPMC queue that allocates small buffers, segments, on demand.
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -13,10 +14,11 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
 
 #[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
 extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
 mod array_queue;

crossbeam-skiplist/src/lib.rs

@@ -228,6 +228,7 @@
 //! }
 //! ```
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -236,10 +237,11 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
 
 #[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
 extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
 pub mod base;

crossbeam-utils/src/lib.rs

@@ -24,6 +24,7 @@
 //! [`WaitGroup`]: sync::WaitGroup
 //! [`scope`]: thread::scope
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -32,7 +33,9 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(crossbeam_loom)]
 #[allow(unused_imports)]

crossbeam-utils/src/sync/sharded_lock.rs

@@ -1,3 +1,4 @@
+use std::boxed::Box;
 use std::cell::UnsafeCell;
 use std::collections::HashMap;
 use std::fmt;
@@ -8,6 +9,7 @@ use std::panic::{RefUnwindSafe, UnwindSafe};
 use std::sync::{LockResult, PoisonError, TryLockError, TryLockResult};
 use std::sync::{Mutex, RwLock, RwLockReadGuard, RwLockWriteGuard};
 use std::thread::{self, ThreadId};
+use std::vec::Vec;
 
 use crate::sync::once_lock::OnceLock;
 use crate::CachePadded;
@@ -613,7 +615,7 @@ impl Drop for Registration {
     }
 }
 
-thread_local! {
+std::thread_local! {
     static REGISTRATION: Registration = {
         let thread_id = thread::current().id();
         let mut indices = thread_indices().lock().unwrap();

crossbeam-utils/src/thread.rs

@@ -111,13 +111,16 @@
 //! }).unwrap();
 //! ```
 
+use std::boxed::Box;
 use std::fmt;
 use std::io;
 use std::marker::PhantomData;
 use std::mem;
 use std::panic;
+use std::string::String;
 use std::sync::{Arc, Mutex};
 use std::thread;
+use std::vec::Vec;
 
 use crate::sync::WaitGroup;
 

src/lib.rs

@@ -38,6 +38,7 @@
 //! [`Backoff`]: utils::Backoff
 //! [`CachePadded`]: utils::CachePadded
 
+#![no_std]
 #![doc(test(
     no_crate_inject,
     attr(
@@ -46,7 +47,9 @@
     )
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
-#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "std")]
+extern crate std;
 
 pub use crossbeam_utils::atomic;