cmd/go: vet support for upcoming cmd/vet fixes

Two minor changes to allow fixes in cmd/vet's printf checking.

1. Pass package import path in vet config, so that vet knows
whether it is, for example, vetting "fmt".

2. Add new, but undocumented and for now unsupported
flag -vettool to control which vet binary is invoked during go vet.
This lets the cmd/vet tests build and test a throwaway vet.exe
using cmd/go to ensure type checking information, all without
installing a potentially buggy cmd/vet.

For #22936.

Change-Id: I18df7c796ebc711361c847c63eb3ee17fb041ff7
Reviewed-on: https://go-review.googlesource.com/83837
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/vet/vet.go

@@ -9,6 +9,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/load"
 	"cmd/go/internal/work"
+	"path/filepath"
 )
 
 var CmdVet = &base.Command{
@@ -38,6 +39,13 @@ func runVet(cmd *base.Command, args []string) {
 
 	work.BuildInit()
 	work.VetFlags = vetFlags
+	if vetTool != "" {
+		var err error
+		work.VetTool, err = filepath.Abs(vetTool)
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+	}
 
 	pkgs := load.PackagesForBuild(pkgArgs)
 	if len(pkgs) == 0 {

src/cmd/go/internal/vet/vetflag.go

@@ -55,10 +55,13 @@ var vetFlagDefn = []*cmdflag.Defn{
 	{Name: "unusedstringmethods"},
 }
 
+var vetTool string
+
 // add build flags to vetFlagDefn.
 func init() {
 	var cmd base.Command
 	work.AddBuildFlags(&cmd)
+	cmd.Flag.StringVar(&vetTool, "vettool", "", "path to vet tool binary") // for cmd/vet tests; undocumented for now
 	cmd.Flag.VisitAll(func(f *flag.Flag) {
 		vetFlagDefn = append(vetFlagDefn, &cmdflag.Defn{
 			Name:  f.Name,
@@ -87,8 +90,13 @@ func vetFlags(args []string) (passToVet, packageNames []string) {
 			}
 			switch f.Name {
 			// Flags known to the build but not to vet, so must be dropped.
-			case "x", "n":
-				args = append(args[:i], args[i+1:]...)
+			case "x", "n", "vettool":
+				if extraWord {
+					args = append(args[:i], args[i+2:]...)
+					extraWord = false
+				} else {
+					args = append(args[:i], args[i+1:]...)
+				}
 				i--
 			}
 		}

src/cmd/go/internal/work/exec.go

@@ -495,6 +495,7 @@ func (b *Builder) build(a *Action) (err error) {
 			Compiler:    cfg.BuildToolchainName,
 			Dir:         a.Package.Dir,
 			GoFiles:     mkAbsFiles(a.Package.Dir, gofiles),
+			ImportPath:  a.Package.ImportPath,
 			ImportMap:   make(map[string]string),
 			PackageFile: make(map[string]string),
 		}
@@ -643,10 +644,15 @@ type vetConfig struct {
 	GoFiles     []string
 	ImportMap   map[string]string
 	PackageFile map[string]string
+	ImportPath  string
 
 	SucceedOnTypecheckFailure bool
 }
 
+// VetTool is the path to an alternate vet tool binary.
+// The caller is expected to set it (if needed) before executing any vet actions.
+var VetTool string
+
 // VetFlags are the flags to pass to vet.
 // The caller is expected to set them before executing any vet actions.
 var VetFlags []string
@@ -687,7 +693,11 @@ func (b *Builder) vet(a *Action) error {
 	}
 
 	p := a.Package
-	return b.run(a, p.Dir, p.ImportPath, nil, cfg.BuildToolexec, base.Tool("vet"), VetFlags, a.Objdir+"vet.cfg")
+	tool := VetTool
+	if tool == "" {
+		tool = base.Tool("vet")
+	}
+	return b.run(a, p.Dir, p.ImportPath, nil, cfg.BuildToolexec, tool, VetFlags, a.Objdir+"vet.cfg")
 }
 
 // linkActionID computes the action ID for a link action.