cmd/vet: check argument types in printf formats

Fixes #4404.

R=gri, rsc
CC=golang-dev
https://golang.org/cl/7378061

Author: robpike

src/cmd/vet/Makefile

@@ -3,6 +3,6 @@
 # license that can be found in the LICENSE file.
 
 test testshort:
-	go build
+	go build -tags unsafe
 	../../../test/errchk ./vet -printfuncs='Warn:1,Warnf:1' *.go
 

src/cmd/vet/atomic.go

@@ -10,7 +10,7 @@ import (
 	"sync/atomic"
 )
 
-// checkAtomicAssignment walks the assignment statement checking for comomon
+// checkAtomicAssignment walks the assignment statement checking for common
 // mistaken usage of atomic package, such as: x = atomic.AddUint64(&x, 1)
 func (f *File) checkAtomicAssignment(n *ast.AssignStmt) {
 	if !vet("atomic") {

src/cmd/vet/main.go

@@ -160,7 +160,8 @@ func doPackageDir(directory string) {
 }
 
 type Package struct {
-	types map[ast.Expr]types.Type
+	types  map[ast.Expr]types.Type
+	values map[ast.Expr]interface{}
 }
 
 // doPackage analyzes the single package constructed from the named files.
@@ -188,8 +189,12 @@ func doPackage(names []string) {
 	}
 	pkg := new(Package)
 	pkg.types = make(map[ast.Expr]types.Type)
+	pkg.values = make(map[ast.Expr]interface{})
 	exprFn := func(x ast.Expr, typ types.Type, val interface{}) {
 		pkg.types[x] = typ
+		if val != nil {
+			pkg.values[x] = val
+		}
 	}
 	context := types.Context{
 		Expr: exprFn,