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use super::BitFlag;
use super::BitFlags;
use core::convert::TryFrom;
use core::fmt;

// Coherence doesn't let us use a generic type here. Work around by implementing
// for each integer type manually.
for_each_uint! { $ty $hide_docs =>
    impl<T> TryFrom<$ty> for BitFlags<T>
    where
        T: BitFlag<Numeric=$ty>,
    {
        type Error = FromBitsError<T>;

        fn try_from(bits: T::Numeric) -> Result<Self, Self::Error> {
            Self::from_bits(bits)
        }
    }
}

/// The error struct used by [`BitFlags::from_bits`]
/// and the [`TryFrom`] implementation for invalid values.
///
/// Note that the implementation of [`std::error::Error`]
/// for this type is gated on the `std` feature flag.
///
/// ```
/// # use std::convert::TryInto;
/// # use enumflags2::{bitflags, BitFlags};
/// #[bitflags]
/// #[derive(Clone, Copy, Debug)]
/// #[repr(u8)]
/// enum MyFlags {
///     A = 0b0001,
///     B = 0b0010,
///     C = 0b0100,
///     D = 0b1000,
/// }
///
/// let result: Result<BitFlags<MyFlags>, _> = 0b10101u8.try_into();
/// assert!(result.is_err());
/// let error = result.unwrap_err();
/// assert_eq!(error.truncate(), MyFlags::C | MyFlags::A);
/// assert_eq!(error.invalid_bits(), 0b10000);
/// ```
#[derive(Debug, Copy, Clone)]
pub struct FromBitsError<T: BitFlag> {
    pub(crate) flags: BitFlags<T>,
    pub(crate) invalid: T::Numeric,
}

impl<T: BitFlag> FromBitsError<T> {
    /// Return the truncated result of the conversion.
    pub fn truncate(self) -> BitFlags<T> {
        self.flags
    }

    /// Return the bits that didn't correspond to any flags.
    pub fn invalid_bits(self) -> T::Numeric {
        self.invalid
    }
}

impl<T: BitFlag + fmt::Debug> fmt::Display for FromBitsError<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            fmt,
            "Invalid bits for {:?}: {:#b}",
            self.flags, self.invalid
        )
    }
}

#[cfg(feature = "std")]
impl<T: BitFlag + fmt::Debug> std::error::Error for FromBitsError<T> {
    fn description(&self) -> &str {
        "invalid bitflags representation"
    }
}