Run proc macro expansion in a separate thread (for the thread-local interner)

Cargo.lock

@@ -247,6 +247,20 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "crossbeam"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4ae5588f6b3c3cb05239e90bd110f257254aecd01e4635400391aeae07497845"
+dependencies = [
+ "cfg-if",
+ "crossbeam-channel",
+ "crossbeam-deque",
+ "crossbeam-epoch",
+ "crossbeam-queue",
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-channel"
 version = "0.5.5"
@@ -282,6 +296,16 @@ dependencies = [
  "scopeguard",
 ]
 
+[[package]]
+name = "crossbeam-queue"
+version = "0.3.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1f25d8400f4a7a5778f0e4e52384a48cbd9b5c495d110786187fc750075277a2"
+dependencies = [
+ "cfg-if",
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-utils"
 version = "0.8.10"
@@ -1162,6 +1186,7 @@ dependencies = [
 name = "proc-macro-srv"
 version = "0.0.0"
 dependencies = [
+ "crossbeam",
  "expect-test",
  "libloading",
  "mbe",

crates/proc-macro-srv/Cargo.toml

@@ -24,6 +24,7 @@ tt = { path = "../tt", version = "0.0.0" }
 mbe = { path = "../mbe", version = "0.0.0" }
 paths = { path = "../paths", version = "0.0.0" }
 proc-macro-api = { path = "../proc-macro-api", version = "0.0.0" }
+crossbeam = "0.8.1"
 
 [dev-dependencies]
 expect-test = "1.4.0"

crates/proc-macro-srv/src/lib.rs

@@ -63,9 +63,16 @@ impl ProcMacroSrv {
 
         let macro_body = task.macro_body.to_subtree();
         let attributes = task.attributes.map(|it| it.to_subtree());
-        let result = expander
+        let result = crossbeam::scope(|s| {
+            s.spawn(|_| {
+                expander
                     .expand(&task.macro_name, &macro_body, attributes.as_ref())
-            .map(|it| FlatTree::new(&it));
+                    .map(|it| FlatTree::new(&it))
+            })
+            .join()
+            .unwrap()
+        })
+        .unwrap();
 
         prev_env.rollback();