Auto merge of #42802 - frewsxcv:rollup, r=frewsxcv

Rollup of 4 pull requests

- Successful merges: #42397, #42620, #42762, #42766
- Failed merges:

2 files changed, 10 insertions, 5 deletions

diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index 496c014f70e..82c4f0830a6 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -238,6 +238,16 @@ macro_rules! assert_approx_eq {
 /// into libsyntax itself.
 #[cfg(dox)]
 pub mod builtin {
+
+    /// Unconditionally causes compilation to fail with the given error message when encountered.
+    ///
+    /// For more information, see the [RFC].
+    ///
+    /// [RFC]: https://github.com/rust-lang/rfcs/blob/master/text/1695-add-error-macro.md
+    #[unstable(feature = "compile_error_macro", issue = "40872")]
+    #[macro_export]
+    macro_rules! compile_error { ($msg:expr) => ({ /* compiler built-in */ }) }
+
     /// The core macro for formatted string creation & output.
     ///
     /// This macro produces a value of type [`fmt::Arguments`]. This value can be
diff --git a/src/libstd/sync/mpsc/mod.rs b/src/libstd/sync/mpsc/mod.rs
index 7d5e1929cd2..dcd4c8dfdf5 100644
--- a/src/libstd/sync/mpsc/mod.rs
+++ b/src/libstd/sync/mpsc/mod.rs
@@ -499,8 +499,6 @@ unsafe impl<T: Send> Send for Sender<T> { }
 impl<T> !Sync for Sender<T> { }
 
 /// The sending-half of Rust's synchronous [`sync_channel`] type.
-/// This half can only be owned by one thread, but it can be cloned
-/// to send to other threads.
 ///
 /// Messages can be sent through this channel with [`send`] or [`try_send`].
 ///
@@ -555,9 +553,6 @@ pub struct SyncSender<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Send> Send for SyncSender<T> {}
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T> !Sync for SyncSender<T> {}
-
 /// An error returned from the [`Sender::send`] or [`SyncSender::send`]
 /// function on **channel**s.
 ///