cmd/vet: fix ironic misuse of fmt.Sprintf

Move badf helper into top-level function so that prints from buildtag.go
are once again themselves printf-format-checked by vet.
Also, fix implementation, which was missing a ... in the Sprintf call and
produced messages like:

/Users/rsc/x_test.go:1: +build comment must appear before package clause and be followed by a blank line%!(EXTRA []interface {}=[])

These were introduced in CL 111415.

Change-Id: I000af3a4e01dc99fc79c9146aa68a71dace1460f
Reviewed-on: https://go-review.googlesource.com/121300
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/buildtag.go

@@ -18,18 +18,17 @@ var (
 	plusBuild  = []byte("+build")
 )
 
+func badfLine(f *File, line int, format string, args ...interface{}) {
+	msg := fmt.Sprintf(format, args...)
+	fmt.Fprintf(os.Stderr, "%s:%d: %s\n", f.name, line, msg)
+	setExit(1)
+}
+
 // checkBuildTag checks that build tags are in the correct location and well-formed.
 func checkBuildTag(f *File) {
 	if !vet("buildtags") {
 		return
 	}
-	// badf is like File.Badf, but it uses a line number instead of
-	// token.Pos.
-	badf := func(line int, format string, args ...interface{}) {
-		msg := fmt.Sprintf(format, args)
-		fmt.Fprintf(os.Stderr, "%s:%d: %s\n", f.name, line, msg)
-		setExit(1)
-	}
 
 	// we must look at the raw lines, as build tags may appear in non-Go
 	// files such as assembly files.
@@ -92,11 +91,11 @@ func checkBuildTag(f *File) {
 			fields := bytes.Fields(text)
 			if !bytes.Equal(fields[0], plusBuild) {
 				// Comment is something like +buildasdf not +build.
-				badf(i+1, "possible malformed +build comment")
+				badfLine(f, i+1, "possible malformed +build comment")
 				continue
 			}
 			if i >= cutoff {
-				badf(i+1, "+build comment must appear before package clause and be followed by a blank line")
+				badfLine(f, i+1, "+build comment must appear before package clause and be followed by a blank line")
 				continue
 			}
 			// Check arguments.
@@ -104,13 +103,13 @@ func checkBuildTag(f *File) {
 			for _, arg := range fields[1:] {
 				for _, elem := range strings.Split(string(arg), ",") {
 					if strings.HasPrefix(elem, "!!") {
-						badf(i+1, "invalid double negative in build constraint: %s", arg)
+						badfLine(f, i+1, "invalid double negative in build constraint: %s", arg)
 						break Args
 					}
 					elem = strings.TrimPrefix(elem, "!")
 					for _, c := range elem {
 						if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
-							badf(i+1, "invalid non-alphanumeric build constraint: %s", arg)
+							badfLine(f, i+1, "invalid non-alphanumeric build constraint: %s", arg)
 							break Args
 						}
 					}
@@ -120,7 +119,7 @@ func checkBuildTag(f *File) {
 		}
 		// Comment with +build but not at beginning.
 		if i < cutoff {
-			badf(i+1, "possible malformed +build comment")
+			badfLine(f, i+1, "possible malformed +build comment")
 			continue
 		}
 	}

src/cmd/vet/testdata/buildtag/buildtag.go

@@ -9,7 +9,7 @@
 
 package testdata
 
-// +build toolate // ERROR "build comment must appear before package clause and be followed by a blank line"
+// +build toolate // ERROR "build comment must appear before package clause and be followed by a blank line$"
 
 var _ = 3