internal/goexperiment: put generic methods behind GOEXPERIMENT

This feature will not launch behind a goexperiment. This is only used
to let in interested users and gate tests during development without
disrupting x/tools.

Change-Id: I97a944bdc3db610ee8a4409f8cdd9a6ddddd59d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/772441
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/noder/unified.go

@@ -7,6 +7,7 @@ package noder
 import (
 	"cmp"
 	"fmt"
+	"internal/goexperiment"
 	"internal/pkgbits"
 	"internal/types/errors"
 	"io"
@@ -25,8 +26,13 @@ import (
 )
 
 // uirVersion is the unified IR version to use for encoding/decoding.
-// Use V4 for generic methods; keep at V3 while we refine and test.
-const uirVersion = pkgbits.V3
+// Use V4 for generic methods if the GOEXPERIMENT is enabled.
+var uirVersion = func() pkgbits.Version {
+	if goexperiment.GenericMethods {
+		return pkgbits.V4
+	}
+	return pkgbits.V3
+}()
 
 // localPkgReader holds the package reader used for reading the local
 // package. It exists so the unified IR linker can refer back to it

src/internal/goexperiment/exp_genericmethods_off.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.genericmethods
+
+package goexperiment
+
+const GenericMethods = false
+const GenericMethodsInt = 0

src/internal/goexperiment/exp_genericmethods_on.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.genericmethods
+
+package goexperiment
+
+const GenericMethods = true
+const GenericMethodsInt = 1

src/internal/goexperiment/flags.go

@@ -133,4 +133,7 @@ type Flags struct {
 	// from interleaved key/elem slots (KVKVKVKV) to split key and elem
 	// arrays (KKKKVVVV).
 	MapSplitGroup bool
+
+	// GenericMethods enables use of generic methods.
+	GenericMethods bool
 }