Add CollectRows and RowTo* functions

Collect functionality was originally developed in pgxutil

rows.go

@@ -395,3 +395,124 @@ func ForEachScannedRow(rows Rows, scans []any, fn func() error) (pgconn.CommandT
 
 	return rows.CommandTag(), nil
 }
+
+// CollectableRow is the subset of Rows methods that a RowToFunc is allowed to call.
+type CollectableRow interface {
+	FieldDescriptions() []pgproto3.FieldDescription
+	Scan(dest ...any) error
+	Values() ([]any, error)
+	RawValues() [][]byte
+}
+
+// RowToFunc is a function that scans or otherwise converts row to a T.
+type RowToFunc[T any] func(row CollectableRow) (T, error)
+
+// CollectRows iterates through rows, calling fn for each row, and collecting the results into a slice of T.
+func CollectRows[T any](rows Rows, fn RowToFunc[T]) ([]T, error) {
+	defer rows.Close()
+
+	slice := []T{}
+
+	for rows.Next() {
+		value, err := fn(rows)
+		if err != nil {
+			return nil, err
+		}
+		slice = append(slice, value)
+	}
+
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+
+	return slice, nil
+}
+
+// RowTo returns a T scanned from row.
+func RowTo[T any](row CollectableRow) (T, error) {
+	var value T
+	err := row.Scan(&value)
+	return value, err
+}
+
+// RowTo returns a the address of a T scanned from row.
+func RowToAddrOf[T any](row CollectableRow) (*T, error) {
+	var value T
+	err := row.Scan(&value)
+	return &value, err
+}
+
+// RowToMap returns a map scanned from row.
+func RowToMap(row CollectableRow) (map[string]any, error) {
+	var value map[string]any
+	err := row.Scan((*mapRowScanner)(&value))
+	return value, err
+}
+
+type mapRowScanner map[string]any
+
+func (rs *mapRowScanner) ScanRow(rows Rows) error {
+	values, err := rows.Values()
+	if err != nil {
+		return err
+	}
+
+	*rs = make(mapRowScanner, len(values))
+
+	for i := range values {
+		(*rs)[string(rows.FieldDescriptions()[i].Name)] = values[i]
+	}
+
+	return nil
+}
+
+// RowToStructByPos returns a T scanned from row. T must be a struct. T must have the same number a public fields as row
+// has fields. The row and T fields will by matched by position.
+func RowToStructByPos[T any](row CollectableRow) (T, error) {
+	var value T
+	err := row.Scan(&positionalStructRowScanner{ptrToStruct: &value})
+	return value, err
+}
+
+// RowToAddrOfStructByPos returns the address of a T scanned from row. T must be a struct. T must have the same number a
+// public fields as row has fields. The row and T fields will by matched by position.
+func RowToAddrOfStructByPos[T any](row CollectableRow) (*T, error) {
+	var value T
+	err := row.Scan(&positionalStructRowScanner{ptrToStruct: &value})
+	return &value, err
+}
+
+type positionalStructRowScanner struct {
+	ptrToStruct any
+}
+
+func (rs *positionalStructRowScanner) ScanRow(rows Rows) error {
+	dst := rs.ptrToStruct
+	dstValue := reflect.ValueOf(dst)
+	if dstValue.Kind() != reflect.Ptr {
+		return fmt.Errorf("dst not a pointer")
+	}
+
+	dstElemValue := dstValue.Elem()
+	dstElemType := dstElemValue.Type()
+
+	exportedFields := make([]int, 0, dstElemType.NumField())
+	for i := 0; i < dstElemType.NumField(); i++ {
+		sf := dstElemType.Field(i)
+		if sf.PkgPath == "" {
+			exportedFields = append(exportedFields, i)
+		}
+	}
+
+	rowFieldCount := len(rows.RawValues())
+	if rowFieldCount > len(exportedFields) {
+		return fmt.Errorf("got %d values, but dst struct has only %d fields", rowFieldCount, len(exportedFields))
+	}
+
+	scanTargets := make([]any, rowFieldCount)
+	for i := 0; i < rowFieldCount; i++ {
+		scanTargets[i] = dstElemValue.Field(exportedFields[i]).Addr().Interface()
+	}
+
+	return rows.Scan(scanTargets...)
+}