//! Provide a .diag() function for diagnostic output if requested.
//!
//! The [`ConfigDiag`] trait may be used to decorate an object that
//! stores some kind of configuration information. It adds
//! a [`diag`][`ConfigDiag::diag`] method that checks whether
//! verbose output has been requested and,
//! if so, displays a message to the standard error stream.
//! If the [`diag_to_stderr`][`ConfigDiag::diag_to_stderr`] method is
//! overridden and it returns false, any diagnostic messages selected for
//! display are sent to the standard output stream instead.
//!
//! Example:
//!
//! ```rust
//! # use std::error;
//! #
//! use config_diag::ConfigDiag;
//!
//! struct Config {
//! verbose: bool,
//! }
//!
//! impl ConfigDiag for Config {
//! fn diag_is_verbose(&self) -> bool {
//! self.verbose
//! }
//! }
//!
//! # pub fn main() -> Result<(), Box<dyn error::Error>> {
//! let config = Config { verbose: true };
//! # let event = 42;
//! config.diag(|| format!("Something happened: {}", event));
//! config.diag_("Got here!");
//! # Ok(())
//! # }
/*
* Copyright (c) 2021, 2022 Peter Pentchev <roam@ringlet.net>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/// Provide a `.diag()` function for verbose output if requested.
/// Useful for decorating configuration-like classes.