cmd/vet: disable complaint about 0 flag in print

The problem is that vet complains about 0 as a Printf flag in some
situations where fmt allows it but probably shouldn't. The two
need to be brought in line, but it's too late in the release cycle.

The situation is messy and should be resolved properly in 1.11. This
CL is a simple fix to disable a spurious complaint for 1.10 that will be
resolved in a more thorough way in 1.11.

The workaround is just to be silent about flag 0, as suggested in
issue 23605.

Fixes #23605
Update #23498

Change-Id: Ice1a4f4d86845d70c1340a0a6430d74e5de9afd4
Reviewed-on: https://go-review.googlesource.com/90695
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/vet/print.go

@@ -569,6 +569,11 @@ func (f *File) okPrintfArg(call *ast.CallExpr, state *formatState) (ok bool) {
 			return false
 		}
 		for _, flag := range state.flags {
+			// TODO: Disable complaint about '0' for Go 1.10. To be fixed properly in 1.11.
+			// See issues 23598 and 23605.
+			if flag == '0' {
+				continue
+			}
 			if !strings.ContainsRune(v.flags, rune(flag)) {
 				f.Badf(call.Pos(), "%s format %s has unrecognized flag %c", state.name, state.format, flag)
 				return false

src/cmd/vet/testdata/print.go

@@ -533,3 +533,9 @@ func UnexportedStringerOrError() {
 	fmt.Println("foo\\n")       // not an error
 	fmt.Println(`foo\n`)        // not an error
 }
+
+// TODO: Disable complaint about '0' for Go 1.10. To be fixed properly in 1.11.
+// See issues 23598 and 23605.
+func DisableErrorForFlag0() {
+	fmt.Printf("%0t", true)
+}