text/template: use sync.OnceValue for builtinFuncs

Replaced sync.Once with sync.OnceValue to simplify code and reduce globals.

Change-Id: I0586df379b855950eacc5b98baad68f6ba0ba129
GitHub-Last-Rev: 7540b1efba
GitHub-Pull-Request: golang/go#73689
Reviewed-on: https://go-review.googlesource.com/c/go/+/672235
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/text/template/funcs.go

@@ -62,26 +62,13 @@ func builtins() FuncMap {
 	}
 }
 
-var builtinFuncsOnce struct {
+// builtinFuncs lazily computes & caches the builtinFuncs map.
-	sync.Once
+var builtinFuncs = sync.OnceValue(func() map[string]reflect.Value {
-	v map[string]reflect.Value
+	funcMap := builtins()
-}
+	m := make(map[string]reflect.Value, len(funcMap))
-
-// builtinFuncsOnce lazily computes & caches the builtinFuncs map.
-// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
-func builtinFuncs() map[string]reflect.Value {
-	builtinFuncsOnce.Do(func() {
-		builtinFuncsOnce.v = createValueFuncs(builtins())
-	})
-	return builtinFuncsOnce.v
-}
-
-// createValueFuncs turns a FuncMap into a map[string]reflect.Value
-func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
-	m := make(map[string]reflect.Value)
 	addValueFuncs(m, funcMap)
 	return m
-}
+})
 
 // addValueFuncs adds to values the functions in funcs, converting them to reflect.Values.
 func addValueFuncs(out map[string]reflect.Value, in FuncMap) {