mainthread: optimize memory usage

name     old time/op                   new time/op                   delta
Call-16  3.08µs ±0%  2.98µs ±0%   -3.23%  (p=0.000 n=9+9)

name     old alloc/op                  new alloc/op                  delta
Call-16    120B ±0%     24B ±0%  -80.00%  (p=0.000 n=10+10)

name     old allocs/op                 new allocs/op                 delta
Call-16    2.00 ±0%    1.00 ±0%  -50.00%  (p=0.000 n=10+10)

Author: changkun

mainthread.go

@@ -4,7 +4,10 @@
 
 package mainthread // import "golang.design/x/mainthread"
 
-import "runtime"
+import (
+	"runtime"
+	"sync"
+)
 
 var funcQ = make(chan func(), runtime.GOMAXPROCS(0))
 
@@ -17,7 +20,9 @@ func init() {
 //
 // Init must be called in the main package.
 func Init(main func()) {
-	done := make(chan struct{})
+	done := donePool.Get().(chan struct{})
+	defer donePool.Put(done)
+
 	go func() {
 		defer func() {
 			done <- struct{}{}
@@ -37,7 +42,9 @@ func Init(main func()) {
 
 // Call calls f on the main thread and blocks until f finishes.
 func Call(f func()) {
-	done := make(chan struct{})
+	done := donePool.Get().(chan struct{})
+	defer donePool.Put(done)
+
 	funcQ <- func() {
 		defer func() {
 			done <- struct{}{}
@@ -46,3 +53,7 @@ func Call(f func()) {
 	}
 	<-done
 }
+
+var donePool = sync.Pool{
+	New: func() interface{} { return make(chan struct{}) },
+}