go/types: implement TypeList.String (debugging support)

This is a port of CL 345471 to go/types.

Change-Id: Icad5fb8b3b4375182f420a51c80607b88696561e
Reviewed-on: https://go-review.googlesource.com/c/go/+/346552
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/instantiate.go

@@ -69,7 +69,7 @@ func Instantiate(env *Environment, typ Type, targs []Type, validate bool) (Type,
 func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, posList []token.Pos) (res Type) {
 	assert(check != nil)
 	if trace {
-		check.trace(pos, "-- instantiating %s with %s", typ, typeListString(targs))
+		check.trace(pos, "-- instantiating %s with %s", typ, NewTypeList(targs))
 		check.indent++
 		defer func() {
 			check.indent--

src/go/types/subst.go

@@ -284,12 +284,6 @@ func instantiatedHash(typ *Named, targs []Type) string {
 	return string(res[:i])
 }
 
-func typeListString(list []Type) string {
-	var buf bytes.Buffer
-	writeTypeList(&buf, list, nil, nil)
-	return buf.String()
-}
-
 // typOrNil is like typ but if the argument is nil it is replaced with Typ[Invalid].
 // A nil type may appear in pathological cases such as type T[P any] []func(_ T([]_))
 // where an array/slice element is accessed before it is set up.

src/go/types/typelists.go

@@ -4,6 +4,8 @@
 
 package types
 
+import "bytes"
+
 // TParamList holds a list of type parameters.
 type TParamList struct{ tparams []*TypeParam }
 
@@ -52,6 +54,17 @@ func (l *TypeList) list() []Type {
 	return l.types
 }
 
+func (l *TypeList) String() string {
+	if l == nil || len(l.types) == 0 {
+		return "[]"
+	}
+	var buf bytes.Buffer
+	buf.WriteByte('[')
+	writeTypeList(&buf, l.types, nil, nil)
+	buf.WriteByte(']')
+	return buf.String()
+}
+
 // ----------------------------------------------------------------------------
 // Implementation