tree: audit blind assertions to *DCollatedString

Now that we support CIText via the DOidWrapper on top of
DCollatedString, we need to be careful when using "blind" type
assertions like `cs := d.(*tree.DCollatedString)` since it might fail.
This commit audits all of these assertions in non-test code and fixes
them to use `AsDCollatedString` where appropriate.

Release note: None

Author: yuzefovich

pkg/ccl/changefeedccl/avro/avro.go

@@ -450,7 +450,11 @@ func typeToSchema(typ *types.T) (*SchemaField, error) {
 		setNullable(
 			SchemaTypeString,
 			func(d tree.Datum, _ interface{}) (interface{}, error) {
-				return string(*d.(*tree.DString)), nil
+				s, ok := tree.AsDString(d)
+				if !ok {
+					return nil, errors.Newf("expected string type, got %T", d)
+				}
+				return string(s), nil
 			},
 			func(x interface{}) (tree.Datum, error) {
 				return tree.NewDString(x.(string)), nil
@@ -460,7 +464,11 @@ func typeToSchema(typ *types.T) (*SchemaField, error) {
 		setNullable(
 			SchemaTypeString,
 			func(d tree.Datum, _ interface{}) (interface{}, error) {
-				return d.(*tree.DCollatedString).Contents, nil
+				cs, ok := tree.AsDCollatedString(d)
+				if !ok {
+					return nil, errors.Newf("expected collated string type, got %T", d)
+				}
+				return cs.Contents, nil
 			},
 			func(x interface{}) (tree.Datum, error) {
 				return tree.NewDCollatedString(x.(string), typ.Locale(), &tree.CollationEnvironment{})

pkg/sql/rowenc/valueside/legacy.go

@@ -203,10 +203,7 @@ func MarshalLegacy(colType *types.T, val tree.Datum) (roachpb.Value, error) {
 			return r, nil
 		}
 	case types.CollatedStringFamily:
-		if colType.Oid() == oidext.T_citext {
-			val = tree.UnwrapDOidWrapper(val)
-		}
-		if v, ok := val.(*tree.DCollatedString); ok {
+		if v, ok := tree.AsDCollatedString(val); ok {
 			if lex.LocaleNamesAreEqual(v.Locale, colType.Locale()) {
 				r.SetString(v.Contents)
 				return r, nil

pkg/sql/sem/tree/datum.go

@@ -6572,7 +6572,7 @@ func AdjustValueToType(typ *types.T, inVal Datum) (outVal Datum, err error) {
 		if v, ok := AsDString(inVal); ok {
 			sv = string(v)
 			isString = true
-		} else if v, ok := inVal.(*DCollatedString); ok {
+		} else if v, ok := AsDCollatedString(inVal); ok {
 			sv = v.Contents
 			isCollatedString = true
 		}
@@ -6619,6 +6619,8 @@ func AdjustValueToType(typ *types.T, inVal Datum) (outVal Datum, err error) {
 				}
 				return NewDString(sv), nil
 			} else if isCollatedString {
+				// Note that we cannot have CITEXT here (because of Oid check
+				// above).
 				return NewDCollatedString(sv, typ.Locale(), &CollationEnvironment{})
 			}
 		}