go/packages: use go115UsesCgo instead of UsesCgo

x/tools half of golang.org/cl/237417.

Updates #16623.
Updates #39072.

Change-Id: Ic31c2cf64dc1b71b70912b9641cf468f60d116f8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/237418
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

go/packages/packages.go

@@ -19,7 +19,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"reflect"
 	"strings"
 	"sync"
 	"time"
@@ -27,6 +26,7 @@ import (
 	"golang.org/x/tools/go/gcexportdata"
 	"golang.org/x/tools/internal/gocommand"
 	"golang.org/x/tools/internal/packagesinternal"
+	"golang.org/x/tools/internal/typesinternal"
 )
 
 // A LoadMode controls the amount of detail to return when loading.
@@ -922,17 +922,13 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 		Sizes: ld.sizes,
 	}
 	if (ld.Mode & TypecheckCgo) != 0 {
-		// TODO: remove this when we stop supporting 1.14.
-		rtc := reflect.ValueOf(tc).Elem()
-		usesCgo := rtc.FieldByName("UsesCgo")
-		if !usesCgo.IsValid() {
+		if !typesinternal.SetUsesCgo(tc) {
 			appendError(Error{
 				Msg:  "TypecheckCgo requires Go 1.15+",
 				Kind: ListError,
 			})
 			return
 		}
-		usesCgo.SetBool(true)
 	}
 	types.NewChecker(tc, ld.Fset, lpkg.Types, lpkg.TypesInfo).Files(lpkg.Syntax)
 

internal/lsp/cache/check.go

@@ -12,7 +12,6 @@ import (
 	"go/token"
 	"go/types"
 	"path"
-	"reflect"
 	"sort"
 	"strings"
 	"sync"
@@ -23,6 +22,7 @@ import (
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/memoize"
 	"golang.org/x/tools/internal/span"
+	"golang.org/x/tools/internal/typesinternal"
 	errors "golang.org/x/xerrors"
 )
 
@@ -404,9 +404,7 @@ func typeCheck(ctx context.Context, fset *token.FileSet, m *metadata, mode sourc
 	}
 	// We want to type check cgo code if go/types supports it.
 	// We passed TypecheckCgo to go/packages when we Loaded.
-	if usescgo := reflect.ValueOf(cfg).Elem().FieldByName("UsesCgo"); usescgo.IsValid() {
-		usescgo.SetBool(true)
-	}
+	typesinternal.SetUsesCgo(cfg)
 
 	check := types.NewChecker(cfg, fset, pkg.types, pkg.typesInfo)
 

internal/lsp/cache/snapshot.go

@@ -12,7 +12,6 @@ import (
 	"go/types"
 	"os"
 	"path/filepath"
-	"reflect"
 	"sort"
 	"strings"
 	"sync"
@@ -24,6 +23,7 @@ import (
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/packagesinternal"
 	"golang.org/x/tools/internal/span"
+	"golang.org/x/tools/internal/typesinternal"
 	errors "golang.org/x/xerrors"
 )
 
@@ -122,7 +122,7 @@ func (s *snapshot) Config(ctx context.Context) *packages.Config {
 		Tests: true,
 	}
 	// We want to type check cgo code if go/types supports it.
-	if reflect.ValueOf(&types.Config{}).Elem().FieldByName("UsesCgo").IsValid() {
+	if typesinternal.SetUsesCgo(&types.Config{}) {
 		cfg.Mode |= packages.TypecheckCgo
 	}
 	packagesinternal.SetGoCmdRunner(cfg, s.view.gocmdRunner)

internal/typesinternal/types.go

@@ -0,0 +1,28 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package typesinternal
+
+import (
+	"go/types"
+	"reflect"
+	"unsafe"
+)
+
+func SetUsesCgo(conf *types.Config) bool {
+	v := reflect.ValueOf(conf).Elem()
+
+	f := v.FieldByName("go115UsesCgo")
+	if !f.IsValid() {
+		f = v.FieldByName("UsesCgo")
+		if !f.IsValid() {
+			return false
+		}
+	}
+
+	addr := unsafe.Pointer(f.UnsafeAddr())
+	*(*bool)(addr) = true
+
+	return true
+}