You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: structured.go
    path of file: ./repos/cockroach/pkg/sql/catalog/tabledesc
    
    the code of the file until where you have to start completion: // Copyright 2015 The Cockroach Authors.
//
// Use of this software is governed by the Business Source License
// included in the file licenses/BSL.txt.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0, included in the file
// licenses/APL.txt.

package tabledesc

import (
	"context"
	"fmt"
	"strconv"
	"strings"

	"github.com/cockroachdb/cockroach/pkg/clusterversion"
	"github.com/cockroachdb/cockroach/pkg/docs"
	"github.com/cockroachdb/cockroach/pkg/keys"
	"github.com/cockroachdb/cockroach/pkg/roachpb"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/catenumpb"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/catpb"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/colinfo"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/internal/validate"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/schemaexpr"
	"github.com/cockroachdb/cockroach/pkg/sql/catalog/typedesc"
	"github.com/cockroachdb/cockroach/pkg/sql/lexbase"
	"github.com/cockroachdb/cockroach/pkg/sql/parser"
	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgerror"
	"github.com/cockroachdb/cockroach/pkg/sql/rowenc"
	"github.com/cockroachdb/cockroach/pkg/sql/schemachanger/scpb"
	"github.com/cockroachdb/cockroach/pkg/sql/sem/catid"
	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
	"github.com/cockroachdb/cockroach/pkg/sql/sqlerrors"
	"github.com/cockroachdb/cockroach/pkg/sql/types"
	"github.com/cockroachdb/cockroach/pkg/util/errorutil/unimplemented"
	"github.com/cockroachdb/cockroach/pkg/util/hlc"
	"github.com/cockroachdb/cockroach/pkg/util/intsets"
	"github.com/cockroachdb/cockroach/pkg/util/iterutil"
	"github.com/cockroachdb/cockroach/pkg/util/protoutil"
	"github.com/cockroachdb/errors"
	"github.com/lib/pq/oid"
)

// Mutable is a custom type for TableDescriptors
// going through schema mu
    
func ForEachExprStringInTableDesc(descI catalog.TableDescriptor, f func(expr *string) error) error {
	var desc *wrapper
	switch descV := descI.(type) {
	case *wrapper:
		desc = descV
	case *immutable:
		desc = &descV.wrapper
	case *Mutable:
		desc = &descV.wrapper
	default:
		return errors.AssertionFailedf("unexpected type of table %T", descI)
	}
	// Helpers for each schema element type that can contain an expression.
	doCol := func(c *descpb.ColumnDescriptor) error {
		if c.HasDefault() {
			if err := f(c.DefaultExpr); err != nil {
				return err
			}
		}
		if c.IsComputed() {
			if err := f(c.ComputeExpr); err != nil {
				return err
			}
		}
		if c.HasOnUpdate() {
			if err := f(c.OnUpdateExpr); err != nil {
				return err
			}
		}
		return nil
	}
	doIndex := func(i catalog.Index) error {
		if i.IsPartial() {
			return f(&i.IndexDesc().Predicate)
		}
		return nil
	}
	doCheck := func(c *descpb.TableDescriptor_CheckConstraint) error {
		return f(&c.Expr)
	}
	doUwi := func(uwi *descpb.UniqueWithoutIndexConstraint) error {
		if uwi.Predicate != "" {
			return f(&uwi.Predicate)
		}
		return nil
	}

	// Process columns.
	for i := range desc.Columns {
		if err := doCol(&desc.Columns[i]); err != nil {
			return err
		}
	}

	// Process all indexes.
	if err := catalog.ForEachIndex(descI, catalog.IndexOpts{
		NonPhysicalPrimaryIndex: true,
		DropMutations:           true,
		AddMutations:            true,
	}, doIndex); err != nil {
		return err
	}

	// Process checks.
	for i := range desc.Checks {
		if err := doCheck(desc.Checks[i]); err != nil {
			return err
		}
	}

	// Process uwis.
	for i := range desc.UniqueWithoutIndexConstraints {
		if err := doUwi(&desc.UniqueWithoutIndexConstraints[i]); err != nil {
			return err
		}
	}

	// Process all non-index mutations.
	for _, mut := range desc.Mutations {
		if c := mut.GetColumn(); c != nil {
			if err := doCol(c); err != nil {
				return err
			}
		}
		if c := mut.GetConstraint(); c != nil &&
			c.ConstraintType == descpb.ConstraintToUpdate_CHECK {
			if err := doCheck(&c.Check); err != nil {
				return err
			}
		}
		if c := mut.GetConstraint(); c != nil &&
			c.ConstraintType == descpb.ConstraintToUpdate_UNIQUE_WITHOUT_INDEX {
			if err := doUwi(&c.UniqueWithoutIndexConstraint); err != nil {
				return err
			}
		}
	}
	return nil
}