[dev.typeparams] cmd/compile/internal/types2: print "incomplete" for interfaces in debug mode only

The /* incomplete */ comment printed for interfaces that have not been
"completed" yet is not useful for end-users; it's here for type-checker
debugging. Rather than trying to pass through a debug flag through all
print routines (which may require new exported API), simply don't print
the comment unless we have the debug flag set inside the type-checker.

For #46167.

Change-Id: Ibd22edfe63001dfd2b814eeb94c2d54d35afd88c
Reviewed-on: https://go-review.googlesource.com/c/go/+/320150
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/types_test.go

@@ -7,3 +7,6 @@ package types2
 func init() {
 	acceptMethodTypeParams = true
 }
+
+// Debug is set if types2 is built with debug mode enabled.
+const Debug = debug

src/cmd/compile/internal/types2/typestring.go

@@ -226,7 +226,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				empty = false
 			}
 		}
-		if t.allMethods == nil || len(t.methods) > len(t.allMethods) {
+		if debug && (t.allMethods == nil || len(t.methods) > len(t.allMethods)) {
 			if !empty {
 				buf.WriteByte(' ')
 			}

src/cmd/compile/internal/types2/typestring_test.go

@@ -138,6 +138,10 @@ func TestTypeString(t *testing.T) {
 var nopos syntax.Pos
 
 func TestIncompleteInterfaces(t *testing.T) {
+	if !Debug {
+		t.Skip("requires type checker to be compiled with debug = true")
+	}
+
 	sig := NewSignature(nil, nil, nil, false)
 	m := NewFunc(nopos, nil, "m", sig)
 	for _, test := range []struct {