// =============================================================================
// Copyright (c) 2025 - 2026 Haixing Hu.
//
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0.
// =============================================================================
//! Relationships used by string and collection length constraints.
/// A numeric relationship required of a measured string or collection length.
///
/// [`LengthMetric`](super::LengthMetric) identifies the measurement unit in a
/// structured validation error.