Remove crossbeam-channel

The standard library's std::sync::mpsc basically is a crossbeam channel,
and for the use case here will definitely suffice. This drops this
dependency from librustc_driver.

Cargo.lock

@@ -3797,7 +3797,6 @@ dependencies = [
 name = "rustc_expand"
 version = "0.0.0"
 dependencies = [
- "crossbeam-channel",
  "rustc_ast",
  "rustc_ast_passes",
  "rustc_ast_pretty",

compiler/rustc_expand/Cargo.toml

@@ -9,7 +9,6 @@ doctest = false
 
 [dependencies]
 # tidy-alphabetical-start
-crossbeam-channel = "0.5.0"
 rustc_ast = { path = "../rustc_ast" }
 rustc_ast_passes = { path = "../rustc_ast_passes" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }

compiler/rustc_expand/src/proc_macro.rs

@@ -13,16 +13,16 @@ use rustc_session::config::ProcMacroExecutionStrategy;
 use rustc_span::profiling::SpannedEventArgRecorder;
 use rustc_span::{Span, DUMMY_SP};
 
-struct CrossbeamMessagePipe<T> {
-    tx: crossbeam_channel::Sender<T>,
-    rx: crossbeam_channel::Receiver<T>,
+struct MessagePipe<T> {
+    tx: std::sync::mpsc::SyncSender<T>,
+    rx: std::sync::mpsc::Receiver<T>,
 }
 
-impl<T> pm::bridge::server::MessagePipe<T> for CrossbeamMessagePipe<T> {
+impl<T> pm::bridge::server::MessagePipe<T> for MessagePipe<T> {
     fn new() -> (Self, Self) {
-        let (tx1, rx1) = crossbeam_channel::bounded(1);
-        let (tx2, rx2) = crossbeam_channel::bounded(1);
-        (CrossbeamMessagePipe { tx: tx1, rx: rx2 }, CrossbeamMessagePipe { tx: tx2, rx: rx1 })
+        let (tx1, rx1) = std::sync::mpsc::sync_channel(1);
+        let (tx2, rx2) = std::sync::mpsc::sync_channel(1);
+        (MessagePipe { tx: tx1, rx: rx2 }, MessagePipe { tx: tx2, rx: rx1 })
     }
 
     fn send(&mut self, value: T) {
@@ -35,7 +35,7 @@ impl<T> pm::bridge::server::MessagePipe<T> for CrossbeamMessagePipe<T> {
 }
 
 fn exec_strategy(ecx: &ExtCtxt<'_>) -> impl pm::bridge::server::ExecutionStrategy {
-    pm::bridge::server::MaybeCrossThread::<CrossbeamMessagePipe<_>>::new(
+    pm::bridge::server::MaybeCrossThread::<MessagePipe<_>>::new(
         ecx.sess.opts.unstable_opts.proc_macro_execution_strategy
             == ProcMacroExecutionStrategy::CrossThread,
     )