cmd/vet: don't treat trailing % as possible formatting directive

Eliminates the following false positive:

cmd/go/go_test.go:1916: possible formatting directive in Error call

The line in question:

tg.t.Error("some coverage results are 0.0%")

Updates #11041

Change-Id: I3b7611fa3e0245714a19bd5388f21e39944f5296
Reviewed-on: https://go-review.googlesource.com/27128
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/print.go

@@ -626,7 +626,10 @@ func (f *File) checkPrint(call *ast.CallExpr, name string) {
 	}
 	arg := args[0]
 	if lit, ok := arg.(*ast.BasicLit); ok && lit.Kind == token.STRING {
-		if strings.Contains(lit.Value, "%") {
+		// Ignore trailing % character in lit.Value.
+		// The % in "abc 0.0%" couldn't be a formatting directive.
+		s := strings.TrimSuffix(lit.Value, `%"`)
+		if strings.Contains(s, "%") {
 			f.Badf(call.Pos(), "possible formatting directive in %s call", name)
 		}
 	}

src/cmd/vet/testdata/print.go

@@ -133,6 +133,7 @@ func PrintfTests() {
 	fmt.Printf("%.*s %d %g", 3, "hi", 23, 'x') // ERROR "arg 'x' for printf verb %g of wrong type"
 	fmt.Println()                              // not an error
 	fmt.Println("%s", "hi")                    // ERROR "possible formatting directive in Println call"
+	fmt.Println("0.0%")                        // correct (trailing % couldn't be a formatting directive)
 	fmt.Printf("%s", "hi", 3)                  // ERROR "wrong number of args for format in Printf call"
 	_ = fmt.Sprintf("%"+("s"), "hi", 3)        // ERROR "wrong number of args for format in Sprintf call"
 	fmt.Printf("%s%%%d", "hi", 3)              // correct