Auto merge of #99123 - mystor:crossbeam_bridge, r=eddyb

proc_macro: use crossbeam channels for the proc_macro cross-thread bridge

This is done by having the crossbeam dependency inserted into the `proc_macro` server code from the server side, to avoid adding a dependency to `proc_macro`.

In addition, this introduces a -Z command-line option which will switch rustc to run proc-macros using this cross-thread executor. With the changes to the bridge in #98186, #98187, #98188 and #98189, the performance of the executor should be much closer to same-thread execution.

In local testing, the crossbeam executor was substantially more performant than either of the two existing `CrossThread` strategies, so they have been removed to keep things simple.

r? `@eddyb`

compiler/rustc_session/src/config.rs

@@ -2972,3 +2972,13 @@ impl OomStrategy {
         }
     }
 }
+
+/// How to run proc-macro code when building this crate
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum ProcMacroExecutionStrategy {
+    /// Run the proc-macro code on the same thread as the server.
+    SameThread,
+
+    /// Run the proc-macro code on a different thread.
+    CrossThread,
+}