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/// Return early with an error.
///
/// This macro is equivalent to `return Err(From::from($err))`.
///
/// # Example
///
/// ```
/// # use anyhow::{bail, Result};
/// #
/// # fn has_permission(user: usize, resource: usize) -> bool {
/// # true
/// # }
/// #
/// # fn main() -> Result<()> {
/// # let user = 0;
/// # let resource = 0;
/// #
/// if !has_permission(user, resource) {
/// bail!("permission denied for accessing {}", resource);
/// }
/// # Ok(())
/// # }
/// ```
///
/// ```
/// # use anyhow::{bail, Result};
/// # use std::fmt::{self, Display};
/// #
/// # #[derive(Debug)]
/// # enum ScienceError {
/// # RecursionLimitExceeded,
/// # }
/// #
/// # impl std::error::Error for ScienceError {}
/// #
/// # impl Display for ScienceError {
/// # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
/// # unimplemented!()
/// # }
/// # }
/// #
/// # const MAX_DEPTH: usize = 1;
/// #
/// # const IGNORE: &str = stringify! {
/// #[derive(Error, Debug)]
/// enum ScienceError {
/// #[error(display = "recursion limit exceeded")]
/// RecursionLimitExceeded,
/// ...
/// }
/// # };
///
/// # fn main() -> Result<()> {
/// # let depth = 0;
/// #
/// if depth > MAX_DEPTH {
/// bail!(ScienceError::RecursionLimitExceeded);
/// }
/// # Ok(())
/// # }
/// ```
/// Return early with an error if a condition is not satisfied.
///
/// This macro is equivalent to `if !$cond { return Err(From::from($err)); }`.
///
/// 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 std::fmt::{self, Display};
/// #
/// # #[derive(Debug)]
/// # enum ScienceError {
/// # RecursionLimitExceeded,
/// # }
/// #
/// # impl std::error::Error for ScienceError {}
/// #
/// # impl Display for ScienceError {
/// # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
/// # unimplemented!()
/// # }
/// # }
/// #
/// # const MAX_DEPTH: usize = 1;
/// #
/// # const IGNORE: &str = stringify! {
/// #[derive(Error, Debug)]
/// enum ScienceError {
/// #[error(display = "recursion limit exceeded")]
/// RecursionLimitExceeded,
/// ...
/// }
/// # };
///
/// # fn main() -> Result<()> {
/// # let depth = 0;
/// #
/// ensure!(depth <= MAX_DEPTH, ScienceError::RecursionLimitExceeded);
/// # Ok(())
/// # }
/// ```
/// Construct an ad-hoc error from a string.
///
/// This evaluates to an `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`.
///
/// # 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(())
/// }
/// ```