go/packages: report a distinguished ErrGoTooOld error for older go commands

We should add support for older go commands using the multiple-calls
approach of earlier drafts of go/packages.

Also, tag tests for go1.11 to make 1.10 builder happy.

Change-Id: Ia04979528af25cbcd4b4fa5b21cb91d014d530c1
Reviewed-on: https://go-review.googlesource.com/123756
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go/packages/doc.go

@@ -13,6 +13,9 @@ recursively loading dependencies from source code.
 
 THIS INTERFACE IS EXPERIMENTAL AND IS LIKELY TO CHANGE.
 
+This package currently requires a go1.11 version of go list;
+its functions will return a GoTooOldError for older toolchains.
+
 This package is intended to replace golang.org/x/tools/go/loader.
 It provides a simpler interface to the same functionality and serves
 as a foundation for analysis tools that work with 'go build',

go/packages/golist.go

@@ -1,3 +1,7 @@
+// Copyright 2018 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 packages
 
 // This file defines the "go list" implementation of the Packages metadata query.
@@ -13,6 +17,12 @@ import (
 	"strings"
 )
 
+// A GoTooOldError indicates that the go command predates the Go
+// 1.11 features needed by this package. This error is a stopgap measure
+// until the necessary features can be emulated in terms of an older go
+// command, at which point this error will no longer be used.
+type GoTooOldError struct{ error }
+
 // golistPackages uses the "go list" command to expand the
 // pattern words and return metadata for the specified packages.
 func golistPackages(ctx context.Context, gopath string, cgo, export bool, words []string) ([]*Package, error) {
@@ -201,7 +211,7 @@ func golist(ctx context.Context, gopath string, cgo, export bool, args []string)
 
 		// Old go list?
 		if strings.Contains(fmt.Sprint(cmd.Stderr), "flag provided but not defined") {
-			return nil, fmt.Errorf("unsupported version of go list: %s: %s", exitErr, cmd.Stderr)
+			return nil, GoTooOldError{fmt.Errorf("unsupported version of go list: %s: %s", exitErr, cmd.Stderr)}
 		}
 
 		// Export mode entails a build.

go/packages/packages110_test.go

@@ -0,0 +1,21 @@
+// Copyright 2018 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.
+
+// +build !go1.11
+
+package packages_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/packages"
+)
+
+func TestGoIsTooOld(t *testing.T) {
+	_, err := packages.Metadata(nil, "errors")
+
+	if _, ok := err.(packages.GoTooOldError); !ok {
+		t.Fatalf("using go/packages with pre-Go 1.11 go: err=%v, want ErrGoTooOld", err)
+	}
+}

go/packages/packages_test.go

@@ -1,3 +1,9 @@
+// Copyright 2018 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.
+
+// +build go1.11
+
 package packages_test
 
 import (
@@ -44,7 +50,7 @@ func TestMetadataImportGraph(t *testing.T) {
 		"src/a/a.go":             `package a; const A = 1`,
 		"src/b/b.go":             `package b; import ("a"; _ "errors"); var B = a.A`,
 		"src/c/c.go":             `package c; import (_ "b"; _ "unsafe")`,
-		"src/c/c2.go":            "//+build ignore\n\n" + `package c; import _ "fmt"`,
+		"src/c/c2.go":            "// +build ignore\n\n" + `package c; import _ "fmt"`,
 		"src/subdir/d/d.go":      `package d`,
 		"src/subdir/d/d_test.go": `package d; import _ "math/bits"`,
 		"src/subdir/d/x_test.go": `package d_test; import _ "subdir/d"`, // TODO(adonovan): test bad import here

go/packages/stdlib_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.11
+
 package packages_test
 
 import (