Environment variable best practices

When implementing environment variable configuration:

1. Follow standard environment variable conventions:
   • Use uppercase with underscores (e.g., NO_COLOR, FORCE_COLOR)
   • Validate non-empty values for boolean flags
   • Support standard environment variables where applicable (e.g., NO_COLOR, FORCE_COLOR, CLICOLOR_FORCE)
2. Document environment variables clearly:
   • Specify expected value formats and constraints
   • Document the precedence order when multiple configuration methods exist
   • Include examples of valid values

Example:

// Bad - No validation or documentation
if std::env::var_os("FORCE_COLOR").is_some() {
    ColorChoice::Always
}

// Good - Validates non-empty value and documents behavior
/// Controls color output. When set to a non-empty value, forces color output
/// regardless of terminal capabilities.
/// Takes precedence over NO_COLOR and --color settings.
if let Some(force_color) = std::env::var_os("FORCE_COLOR") {
    if !force_color.is_empty() {
        ColorChoice::Always
    }
}

1. Establish clear precedence rules:
   • Document which settings take precedence (e.g., CLI flags vs environment variables)
   • Consider standard environment variables first
   • Fall back to custom environment variables before defaults
2. Provide validation and feedback:
   • Validate environment variable values early
   • Provide clear error messages for invalid values
   • Consider warning on deprecated or conflicting configurations