go.tools/cmd/vet: verify printf for String and Error implementers

Fixes golang/go#5624.

R=adonovan
CC=gobot, golang-dev, gri
https://golang.org/cl/9657048

cmd/vet/testdata/print.go

@@ -75,6 +75,8 @@ func PrintfTests() {
 	fmt.Printf("%x %x %x %x", 3, i, "hi", s)
 	fmt.Printf("%X %X %X %X", 3, i, "hi", s)
 	fmt.Printf("%.*s %d %g", 3, "hi", 23, 2.3)
+	fmt.Printf("%s", stringerv)
+	fmt.Printf("%T", stringerv)
 	fmt.Printf("%*%", 2) // Ridiculous but allowed.
 	// Some bad format/argTypes
 	fmt.Printf("%b", "hi")                     // ERROR "arg .hi. for printf verb %b of wrong type"
@@ -94,6 +96,7 @@ func PrintfTests() {
 	fmt.Printf("%U", x)                        // ERROR "arg x for printf verb %U of wrong type"
 	fmt.Printf("%x", nil)                      // ERROR "arg nil for printf verb %x of wrong type"
 	fmt.Printf("%X", 2.3)                      // ERROR "arg 2.3 for printf verb %X of wrong type"
+	fmt.Printf("%t", stringerv)                // ERROR "arg stringerv for printf verb %t of wrong type"
 	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"
@@ -116,7 +119,7 @@ func PrintfTests() {
 	const format = "%s %s\n"
 	Printf(format, "hi", "there")
 	Printf(format, "hi") // ERROR "wrong number of args for format in Printf call"
-	f := new(File)
+	f := new(stringer)
 	f.Warn(0, "%s", "hello", 3)  // ERROR "possible formatting directive in Warn call"
 	f.Warnf(0, "%s", "hello", 3) // ERROR "wrong number of args for format in Warnf call"
 	f.Warnf(0, "%r", "hello")    // ERROR "unrecognized printf verb"
@@ -158,7 +161,28 @@ func Printf(format string, args ...interface{}) {
 	panic("don't call - testing only")
 }
 
+// printf is used by the test so we must declare it.
+func printf(format string, args ...interface{}) {
+	panic("don't call - testing only")
+}
+
 // multi is used by the test.
 func multi() []interface{} {
 	panic("don't call - testing only")
 }
+
+type stringer float64
+
+var stringerv stringer
+
+func (*stringer) String() string {
+	return "string"
+}
+
+func (*stringer) Warn(int, ...interface{}) string {
+	return "warn"
+}
+
+func (*stringer) Warnf(int, string, ...interface{}) string {
+	return "warnf"
+}

cmd/vet/types.go

@@ -57,11 +57,33 @@ func (pkg *Package) isStruct(c *ast.CompositeLit) (bool, string) {
 }
 
 func (f *File) matchArgType(t printfArgType, arg ast.Expr) bool {
-	// TODO: for now, we can only test builtin types and untyped constants.
+	// TODO: for now, we can only test builtin types, untyped constants, and Stringer/Errors.
 	typ := f.pkg.types[arg]
 	if typ == nil {
 		return true
 	}
+	// If we can use a string and this is a named type, does it implement the Stringer or Error interface?
+	// TODO: Simplify when we have the IsAssignableTo predicate in go/types.
+	if named, ok := typ.(*types.Named); ok && t&argString != 0 {
+		for i := 0; i < named.NumMethods(); i++ {
+			method := named.Method(i)
+			// Method must be either String or Error.
+			if method.Name() != "String" && method.Name() != "Error" {
+				continue
+			}
+			sig := method.Type().(*types.Signature)
+			// There must be zero arguments and one return.
+			if sig.Params().Len() != 0 || sig.Results().Len() != 1 {
+				continue
+			}
+			// Result must be string.
+			if !isUniverseString(sig.Results().At(0).Type()) {
+				continue
+			}
+			// It's a Stringer and we can print it.
+			return true
+		}
+	}
 	basic, ok := typ.Underlying().(*types.Basic)
 	if !ok {
 		return true
@@ -161,9 +183,10 @@ func (f *File) isErrorMethodCall(call *ast.CallExpr) bool {
 		return false
 	}
 	// It must have return type "string" from the universe.
-	result := sig.Results().At(0).Type()
-	if types.IsIdentical(result, types.Typ[types.String]) {
-		return true
-	}
-	return false
+	return isUniverseString(sig.Results().At(0).Type())
+}
+
+// isUniverseString reports whether the type is the predeclared type "string".
+func isUniverseString(typ types.Type) bool {
+	return types.IsIdentical(typ, types.Typ[types.String])
 }