treaty -


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pub mod hint {
    use crate::protocol::{Implementer, Protocol};

    /// Protocol for giving a hint to a walker.
    ///
    /// This protocol is implemented by walkers.
    ///
    /// A hint is for a particular protocol `P`.
    pub struct Hint<P: Meta>(P);

    /// Meta information for the hint.
    ///
    /// This gives the visitor more information to work from when selecting a hint.
    pub trait Meta: Protocol {
        /// Information known by the walker.
        ///
        /// This should be information easy to get without changing the state of the walker
        /// in a irreversable way.
        type Known<'a, 'ctx: 'a>;

        /// Extra information the visitor can give to the walker about what it is expecting.
        type Hint<'a, 'ctx: 'a>;
    }

    /// Object implementing the [`Hint`] protocol.
    pub trait Object<'ctx, P: Meta> {
        /// Hint to the walker to use the `P` protocol.
        ///
        /// This should only be called once per [`RequestHint`].
        fn hint(
            &mut self,
            visitor: &mut dyn Implementer<'ctx>,
            hint: P::Hint<'_, 'ctx>,
        ) -> Result<(), ()>;

        /// Ask the walker for information about it's support of the protocol.
        fn known(&mut self, hint: &P::Hint<'_, 'ctx>) -> Result<P::Known<'_, 'ctx>, ()>;
    }

    impl<P: Meta> Protocol for Hint<P> {
        type Object<'a, 'ctx: 'a> = &'a mut dyn Object<'ctx, P>;
    }
}