lint: avoid suggesting to use a keyword as a valid name

In some circumstances the programmer is tempted to use a Go reserved
keyword as a name. A common ofender is the word "type". The Go compiler
will not accept that, of course. In those situations a common strategy
is to append an underline to the keyword, in order to make the compiler
happy. Thus, the name "type" would be changed to "type_" and so on.

The golint considered that use of underline as bad style and was wrongly
suggesting as a corrective action employing the original keyword as name:

astVarRef.go:6:2: don't use underscores in Go names; struct field type_ should be type
cmdIf.go:32:3: don't use underscores in Go names; var goto_ should be goto

This CL changes golint so that it doesn't make such invalid suggestion.

Fixes golang#393

lint.go

@@ -551,6 +551,11 @@ func isInTopLevel(f *ast.File, ident *ast.Ident) bool {
 	return true
 }
 
+// isGoKeyword returns true if "name" is a reserved keyword.
+func isGoKeyword(name string) bool {
+	return keywords[name]
+}
+
 // lintNames examines all names in the file.
 // It complains if any use underscores or incorrect known initialisms.
 func (f *file) lintNames() {
@@ -596,7 +601,7 @@ func (f *file) lintNames() {
 		}
 
 		if len(id.Name) > 2 && strings.Contains(id.Name[1:], "_") {
-			if keywords[should] {
+			if isGoKeyword(should) {
 				return // refrain from suggesting the use of a keyword as a valid name.
 			}
 			f.errorf(id, 0.9, link("http://golang.org/doc/effective_go.html#mixed-caps"), category("naming"), "don't use underscores in Go names; %s %s should be %s", thing, id.Name, should)

lint_test.go

@@ -252,6 +252,23 @@ func TestLintName(t *testing.T) {
 	}
 }
 
+func TestKeyword(t *testing.T) {
+	tests := []struct {
+		name string
+		want bool
+	}{
+		{"type", true},
+		{"goto", true},
+		{"while", false},
+	}
+	for _, test := range tests {
+		got := isGoKeyword(test.name)
+		if got != test.want {
+			t.Errorf("isGoKeyword(%q) = %t, want %t", test.name, got, test.want)
+		}
+	}
+}
+
 func TestExportedType(t *testing.T) {
 	tests := []struct {
 		typString string