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//! # Using generic types
//!
//! Error types enhanced by SNAFU may contain generic type and lifetime parameters.
//!
//! ## Types
//!
//! ```rust
//! # use snafu::{Snafu, ensure};
//! #
//! #[derive(Debug, Snafu)]
//! enum Error<T>
//! where
//!     T: std::fmt::Display,
//! {
//!     #[snafu(display("The value {} was too large", value))]
//!     TooLarge { value: T, limit: u32 },
//!
//!     #[snafu(display("The value {} was too small", value))]
//!     TooSmall { value: T, limit: u32 },
//! }
//!
//! fn validate_number(value: u8) -> Result<u8, Error<u8>> {
//!     ensure!(value <= 200, TooLarge { value, limit: 100u32 });
//!     ensure!(value >= 100, TooSmall { value, limit: 200u32 });
//!     Ok(value)
//! }
//!
//! fn validate_string(value: &str) -> Result<&str, Error<String>> {
//!     ensure!(value.len() <= 20, TooLarge { value, limit: 10u32 });
//!     ensure!(value.len() >= 10, TooSmall { value, limit: 20u32 });
//!     Ok(value)
//! }
//! ```
//!
//! ## Lifetimes
//!
//! ```rust
//! # use snafu::{Snafu, ensure};
//! #
//! #[derive(Debug, Snafu)]
//! enum Error<'a> {
//!     #[snafu(display("The username {} contains the bad word {}", value, word))]
//!     BadWord { value: &'a str, word: &'static str },
//! }
//!
//! fn validate_username<'a>(value: &'a str) -> Result<&'a str, Error<'a>> {
//!     ensure!(!value.contains("stinks"), BadWord { value, word: "stinks" });
//!     ensure!(!value.contains("smells"), BadWord { value, word: "smells" });
//!     Ok(value)
//! }
//! ```
//!
//! ## Caveats
//!
//! A SNAFU [opaque type](crate::guide::opaque) requires that the
//! contained type implements several traits, such as
//! `Display`. However, type constraints cannot be automatically added
//! to the opaque type because they are not allowed to reference the
//! inner type without also exposing it publicly.
//!
//! The best option is to avoid using a generic opaque error. If you
//! choose to expose a generic opaque error, you will likely need to add
//! explicit duplicate type constraints:
//!
//! ```rust
//! use snafu:: Snafu;
//!
//! #[derive(Debug, Snafu)]
//! struct ApiError<T>(Error<T>)
//! where                        // These lines are required to
//!    T: std::fmt::Debug;       // ensure that delegation can work.
//!
//! #[derive(Debug, Snafu)]
//! enum Error<T>
//! where
//!     T: std::fmt::Debug,
//! {
//!     #[snafu(display("Boom: {:?}", value))]
//!     Boom { value: T },
//! }
//! ```