cmd/vet: replace hasMethod with isFormatter

The former checks if a type has a method called "Format". The latter
checks if a type satisfies fmt.Formatter.

isFormatter does exactly what we want, so it's both simpler and more
accurate. Remove the only use of hasMethod in its favor.

Change-Id: Idc156a99081c3308f98512b87011a04aa8c6638d
Reviewed-on: https://go-review.googlesource.com/91215
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/print.go

@@ -657,8 +657,7 @@ func (f *File) recursiveStringer(e ast.Expr) bool {
 
 	// It's unlikely to be a recursive stringer if it has a Format method.
 	if typ := f.pkg.types[e].Type; typ != nil {
-		// Not a perfect match; see issue 6259.
-		if f.hasMethod(typ, "Format") {
+		if f.isFormatter(typ) {
 			return false
 		}
 	}

src/cmd/vet/types.go

@@ -310,15 +310,4 @@ func (f *File) matchStructArgType(t printfArgType, typ *types.Struct, arg ast.Ex
 	return true
 }
 
-// hasMethod reports whether the type contains a method with the given name.
-// It is part of the workaround for Formatters and should be deleted when
-// that workaround is no longer necessary.
-// TODO: This could be better once issue 6259 is fixed.
-func (f *File) hasMethod(typ types.Type, name string) bool {
-	// assume we have an addressable variable of type typ
-	obj, _, _ := types.LookupFieldOrMethod(typ, true, f.pkg.typesPkg, name)
-	_, ok := obj.(*types.Func)
-	return ok
-}
-
 var archSizes = types.SizesFor("gc", build.Default.GOARCH)