internal/goversion: add new package, move Go 1.x constant there out of go/build

Found by Josh, who says in the bug that it shrinks cmd/compile by 1.6 MB (6.5%).

Fixes #31563

Change-Id: I35127af539630e628a0a4f2273af519093536c38
Reviewed-on: https://go-review.googlesource.com/c/go/+/172997
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/gc/dep_test.go

@@ -0,0 +1,26 @@
+// Copyright 2019 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 gc
+
+import (
+	"internal/testenv"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+func TestDeps(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	out, err := exec.Command("go", "list", "-f", "{{.Deps}}", "cmd/compile/internal/gc").Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, dep := range strings.Fields(strings.Trim(string(out), "[]")) {
+		switch dep {
+		case "go/build", "go/token":
+			t.Errorf("undesired dependency on %q", dep)
+		}
+	}
+}

src/cmd/compile/internal/gc/main.go

@@ -19,7 +19,7 @@ import (
 	"cmd/internal/sys"
 	"flag"
 	"fmt"
-	"go/build"
+	"internal/goversion"
 	"io"
 	"io/ioutil"
 	"log"
@@ -1426,8 +1426,7 @@ var flag_lang string
 
 // currentLang returns the current language version.
 func currentLang() string {
-	tags := build.Default.ReleaseTags
-	return tags[len(tags)-1]
+	return fmt.Sprintf("go1.%d", goversion.Version)
 }
 
 // goVersionRE is a regular expression that matches the valid

src/cmd/dist/buildtool.go

@@ -89,6 +89,7 @@ var bootstrapDirs = []string{
 	"debug/elf",
 	"debug/macho",
 	"debug/pe",
+	"internal/goversion",
 	"internal/xcoff",
 	"math/big",
 	"math/bits",

src/go/build/build.go

@@ -13,6 +13,7 @@ import (
 	"go/parser"
 	"go/token"
 	"internal/goroot"
+	"internal/goversion"
 	"io"
 	"io/ioutil"
 	"log"
@@ -292,15 +293,14 @@ func defaultContext() Context {
 	c.GOPATH = envOr("GOPATH", defaultGOPATH())
 	c.Compiler = runtime.Compiler
 
-	// Each major Go release in the Go 1.x series should add a tag here.
-	// Old tags should not be removed. That is, the go1.x tag is present
-	// in all releases >= Go 1.x. Code that requires Go 1.x or later should
-	// say "+build go1.x", and code that should only be built before Go 1.x
-	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
-	// NOTE: If you add to this list, also update the doc comment in doc.go.
-	// NOTE: The last element in ReleaseTags should be the current release.
-	const version = 13 // go1.13
-	for i := 1; i <= version; i++ {
+	// Each major Go release in the Go 1.x series adds a new
+	// "go1.x" release tag. That is, the go1.x tag is present in
+	// all releases >= Go 1.x. Code that requires Go 1.x or later
+	// should say "+build go1.x", and code that should only be
+	// built before Go 1.x (perhaps it is the stub to use in that
+	// case) should say "+build !go1.x".
+	// The last element in ReleaseTags is the current release.
+	for i := 1; i <= goversion.Version; i++ {
 		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
 	}
 

src/go/build/deps_test.go

@@ -268,7 +268,7 @@ var pkgDeps = map[string][]string{
 	"encoding/pem":                   {"L4"},
 	"encoding/xml":                   {"L4", "encoding"},
 	"flag":                           {"L4", "OS"},
-	"go/build":                       {"L4", "OS", "GOPARSER", "internal/goroot"},
+	"go/build":                       {"L4", "OS", "GOPARSER", "internal/goroot", "internal/goversion"},
 	"html":                           {"L4"},
 	"image/draw":                     {"L4", "image/internal/imageutil"},
 	"image/gif":                      {"L4", "compress/lzw", "image/color/palette", "image/draw"},

src/internal/goversion/goversion.go

@@ -0,0 +1,13 @@
+// Copyright 2019 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 goversion
+
+// Version is the current Go 1.x version. During development cycles on
+// the master branch it changes to be the version of the next Go 1.x
+// release.
+//
+// When incrementing this, also add to the list at src/go/build/doc.go
+// (search for "onward").
+const Version = 13