cmd/vet: skip printf check for non-constant format string during failed import

Fixes #17006

Change-Id: I3c2060ca5384a4b9782a7d804305d2cf4388dd5a
Reviewed-on: https://go-review.googlesource.com/29014
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/print.go

@@ -94,7 +94,7 @@ func formatString(f *File, call *ast.CallExpr) (string, int) {
 	if typ != nil {
 		if sig, ok := typ.(*types.Signature); ok {
 			if !sig.Variadic() {
-				// Skip checking non-variadic functions
+				// Skip checking non-variadic functions.
 				return "", -1
 			}
 			idx := sig.Params().Len() - 2
@@ -103,30 +103,36 @@ func formatString(f *File, call *ast.CallExpr) (string, int) {
 				// fixed arguments.
 				return "", -1
 			}
-			s, ok := stringLiteralArg(f, call, idx)
+			s, ok := stringConstantArg(f, call, idx)
 			if !ok {
-				// The last argument before variadic args isn't a string
+				// The last argument before variadic args isn't a string.
 				return "", -1
 			}
 			return s, idx
 		}
 	}
 
-	// Cannot determine call's signature. Fallback to scanning for the first
-	// string argument in the call
+	// Cannot determine call's signature. Fall back to scanning for the first
+	// string constant in the call.
 	for idx := range call.Args {
-		if s, ok := stringLiteralArg(f, call, idx); ok {
+		if s, ok := stringConstantArg(f, call, idx); ok {
 			return s, idx
 		}
+		if f.pkg.types[call.Args[idx]].Type == types.Typ[types.String] {
+			// Skip checking a call with a non-constant format
+			// string argument, since its contents are unavailable
+			// for validation.
+			return "", -1
+		}
 	}
 	return "", -1
 }
 
-// stringLiteralArg returns call's string constant argument at the index idx.
+// stringConstantArg returns call's string constant argument at the index idx.
 //
 // ("", false) is returned if call's argument at the index idx isn't a string
-// literal.
-func stringLiteralArg(f *File, call *ast.CallExpr, idx int) (string, bool) {
+// constant.
+func stringConstantArg(f *File, call *ast.CallExpr, idx int) (string, bool) {
 	if idx >= len(call.Args) {
 		return "", false
 	}

src/cmd/vet/testdata/print.go

@@ -238,6 +238,9 @@ func PrintfTests() {
 	externalprintf.Logf(level, "%d", 42)                        // OK
 	externalprintf.Errorf(level, level, "foo %q bar", "foobar") // OK
 	externalprintf.Logf(level, "%d")                            // ERROR "format reads arg 1, have only 0 args"
+	var formatStr = "%s %s"
+	externalprintf.Sprintf(formatStr, "a", "b")     // OK
+	externalprintf.Logf(level, formatStr, "a", "b") // OK
 
 	// user-defined Println-like functions
 	ss := &someStruct{}