build: use unsafe.Slice to reduce memory usage of overlayFiles

internal/build/build.go

@@ -30,6 +30,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"unsafe"
 
 	"golang.org/x/tools/go/ssa"
 
@@ -111,10 +112,6 @@ const (
 	loadSyntax  = loadTypes | packages.NeedSyntax | packages.NeedTypesInfo
 )
 
-var overlayFiles = map[string]string{
-	"math/exp_amd64.go": "package math;",
-}
-
 func Do(args []string, conf *Config) {
 	flags, patterns, verbose := ParseArgs(args, buildFlags)
 	cfg := &packages.Config{
@@ -126,7 +123,8 @@ func Do(args []string, conf *Config) {
 	if len(overlayFiles) > 0 {
 		cfg.Overlay = make(map[string][]byte)
 		for file, src := range overlayFiles {
-			cfg.Overlay[filepath.Join(runtime.GOROOT(), "src", file)] = []byte(src)
+			overlay := unsafe.Slice(unsafe.StringData(src), len(src))
+			cfg.Overlay[filepath.Join(runtime.GOROOT(), "src", file)] = overlay
 		}
 	}
 
@@ -765,6 +763,10 @@ var hasAltPkg = map[string]none{
 	"runtime":              {},
 }
 
+var overlayFiles = map[string]string{
+	"math/exp_amd64.go": "package math;",
+}
+
 func check(err error) {
 	if err != nil {
 		panic(err)