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    // Copyright 2018 Developers of the Rand project.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Error types

    use core::fmt;
    use core::num::NonZeroU32;

    /// Error type of random number generators
    ///
    /// In order to be compatible with `std` and `no_std`, this type has two
    /// possible implementations: with `std` a boxed `Error` trait object is stored,
    /// while with `no_std` we merely store an error code.
    #[derive(Debug)]
    pub struct Error {
        #[cfg(feature="std")]
        inner: Box<dyn std::error::Error + Send + Sync + 'static>,
        #[cfg(not(feature="std"))]
        code: NonZeroU32,
    }

    impl Error {
        /// Construct from any type supporting `std::error::Error`
        ///
        /// Available only when configured with `std`.
        ///
        /// See also `From<NonZeroU32>`, which is available with and without `std`.
        #[cfg(feature="std")]
        pub fn new<E>(err: E) -> Self
        where E: Into<Box<dyn std::error::Error + Send + Sync + 'static>>
        {
            Error { inner: err.into() }
        }

        /// Reference the inner error (`std` only)
        ///
        /// When configured with `std`, this is a trivial operation and never
        /// panics. Without `std`, this method is simply unavailable.
        #[cfg(feature="std")]
        pub fn inner(&self) -> &(dyn std::error::Error + Send + Sync + 'static) {
            &*self.inner
        }

        /// Unwrap the inner error (`std` only)
        ///
        /// When configured with `std`, this is a trivial operation and never
        /// panics. Without `std`, this method is simply unavailable.
        #[cfg(feature="std")]
        pub fn take_inner(self) -> Box<dyn std::error::Error + Send + Sync + 'static> {
            self.inner
        }

        /// Retrieve the error code, if any.
        ///
        /// If this `Error` was constructed via `From<NonZeroU32>`, then this method
        /// will return this `NonZeroU32` code (for `no_std` this is always the
        /// case). Otherwise, this method will return `None`.
        pub fn code(&self) -> Option<NonZeroU32> {
            #[cfg(feature="std")] {
                self.inner.downcast_ref::<ErrorCode>().map(|c| c.0)
            }
            #[cfg(not(feature="std"))] {
                Some(self.code)
            }
        }
    }

    impl fmt::Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            #[cfg(feature="std")] {
                write!(f, "{}", self.inner)
            }
            #[cfg(not(feature="std"))] {
                write!(f, "error code {}", self.code)
            }
        }
    }

    impl From<NonZeroU32> for Error {
        fn from(code: NonZeroU32) -> Self {
            #[cfg(feature="std")] {
                Error { inner: Box::new(ErrorCode(code)) }
            }
            #[cfg(not(feature="std"))] {
                Error { code }
            }
        }
    }

    #[cfg(feature="getrandom")]
    impl From<getrandom::Error> for Error {
        fn from(error: getrandom::Error) -> Self {
            #[cfg(feature="std")] {
                Error { inner: Box::new(error) }
            }
            #[cfg(not(feature="std"))] {
                Error { code: error.code() }
            }
        }
    }

    #[cfg(feature="std")]
    impl std::error::Error for Error {
        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
            self.inner.source()
        }
    }

    #[cfg(feature="std")]
    impl From<Error> for std::io::Error {
        fn from(error: Error) -> Self {
            std::io::Error::new(std::io::ErrorKind::Other, error)
        }
    }

    #[cfg(feature="std")]
    #[derive(Debug, Copy, Clone)]
    struct ErrorCode(NonZeroU32);

    #[cfg(feature="std")]
    impl fmt::Display for ErrorCode {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "error code {}", self.0)
        }
    }

    #[cfg(feature="std")]
    impl std::error::Error for ErrorCode {}

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