/// Return early with an error.
///
/// This macro is equivalent to `return Err(`[`anyhow!($args...)`][anyhow!]`)`.
///
/// The surrounding function's or closure's return value is required to be
/// `Result<_,`[`anyhow::Error`][crate::Error]`>`.
///
/// # Example
///
/// ```
/// # use anyhow::{bail, Result};
/// #
/// # fn has_permission(user: usize, resource: usize) -> bool {
/// # true
/// # }
/// #
/// # fn main() -> Result<()> {
/// # let user = 0;
/// # let resource = 0;
/// #
/// if !has_permission(user, resource) {
/// bail!("permission denied for accessing {}", resource);
/// }
/// # Ok(())
/// # }
/// ```
///
/// ```
/// # use anyhow::{bail, Result};
/// # use thiserror::Error;
/// #
/// # const MAX_DEPTH: usize = 1;
/// #
/// #[derive(Error, Debug)]
/// enum ScienceError {
/// #[error("recursion limit exceeded")]
/// RecursionLimitExceeded,
/// # #[error("...")]
/// # More = (stringify! {
/// ...
/// # }, 1).1,
/// }
///
/// # fn main() -> Result<()> {
/// # let depth = 0;
/// #
/// if depth > MAX_DEPTH {
/// bail!(ScienceError::RecursionLimitExceeded);
/// }
/// # Ok(())
/// # }
/// ```
/// Return early with an error if a condition is not satisfied.
///
/// This macro is equivalent to `if !$cond { return
/// Err(`[`anyhow!($args...)`][anyhow!]`); }`.
///
/// The surrounding function's or closure's return value is required to be
/// `Result<_,`[`anyhow::Error`][crate::Error]`>`.
///
/// Analogously to `assert!`, `ensure!` takes a condition and exits the function
/// if the condition fails. Unlike `assert!`, `ensure!` returns an `Error`
/// rather than panicking.
///
/// # Example
///
/// ```
/// # use anyhow::{ensure, Result};
/// #
/// # fn main() -> Result<()> {
/// # let user = 0;
/// #
/// ensure!(user == 0, "only user 0 is allowed");
/// # Ok(())
/// # }
/// ```
///
/// ```
/// # use anyhow::{ensure, Result};
/// # use thiserror::Error;
/// #
/// # const MAX_DEPTH: usize = 1;
/// #
/// #[derive(Error, Debug)]
/// enum ScienceError {
/// #[error("recursion limit exceeded")]
/// RecursionLimitExceeded,
/// # #[error("...")]
/// # More = (stringify! {
/// ...
/// # }, 1).1,
/// }
///
/// # fn main() -> Result<()> {
/// # let depth = 0;
/// #
/// ensure!(depth <= MAX_DEPTH, ScienceError::RecursionLimitExceeded);
/// # Ok(())
/// # }
/// ```
/// Construct an ad-hoc error from a string or existing non-`anyhow` error
/// value.
///
/// This evaluates to an [`Error`][crate::Error]. It can take either just a
/// string, or a format string with arguments. It also can take any custom type
/// which implements `Debug` and `Display`.
///
/// If called with a single argument whose type implements `std::error::Error`
/// (in addition to `Debug` and `Display`, which are always required), then that
/// Error impl's `source` is preserved as the `source` of the resulting
/// `anyhow::Error`.
///
/// # Example
///
/// ```
/// # type V = ();
/// #
/// use anyhow::{anyhow, Result};
///
/// fn lookup(key: &str) -> Result<V> {
/// if key.len() != 16 {
/// return Err(anyhow!("key length must be 16 characters, got {:?}", key));
/// }
///
/// // ...
/// # Ok(())
/// }
/// ```