from_arg.rs

use crate::func::args::{Arg, ArgError};
use crate::{Reflect, TypePath};

/// A trait for types that can be created from an [`Arg`].
///
/// This trait exists so that types can be automatically converted into an [`Arg`]
/// so they can be put into an [`ArgList`] and passed to a [`DynamicFunction`] or
/// [`DynamicFunctionMut`].
///
/// This trait is used instead of a blanket [`From`] implementation due to coherence issues:
/// we can't implement `From<T>` for both `T` and `&T`/`&mut T`.
///
/// This trait is automatically implemented for non-reference types when using the `Reflect`
/// [derive macro]. Blanket impls cover `&T` and `&mut T`.
///
/// [`ArgList`]: crate::func::args::ArgList
/// [`DynamicFunction`]: crate::func::DynamicFunction
/// [`DynamicFunctionMut`]: crate::func::DynamicFunctionMut
/// [derive macro]: derive@crate::Reflect
pub trait FromArg {
    /// The type to convert into.
    ///
    /// This should almost always be the same as `Self`, but with the lifetime `'a`.
    ///
    /// The reason we use a separate associated type is to allow for the lifetime
    /// to be tied to the argument, rather than the type itself.
    type This<'a>;

    /// Creates an item from an argument.
    ///
    /// The argument must be of the expected type and ownership.
    fn from_arg(arg: Arg<'_>) -> Result<Self::This<'_>, ArgError>;
}

// Blanket impl.
impl<T: Reflect + TypePath> FromArg for &'static T {
    type This<'a> = &'a T;
    fn from_arg(arg: Arg<'_>) -> Result<Self::This<'_>, ArgError> {
        arg.take_ref()
    }
}

// Blanket impl.
impl<T: Reflect + TypePath> FromArg for &'static mut T {
    type This<'a> = &'a mut T;
    fn from_arg(arg: Arg<'_>) -> Result<Self::This<'_>, ArgError> {
        arg.take_mut()
    }
}

/// Implements the [`FromArg`] trait for the given type.
///
/// This will implement it for `$ty`, `&$ty`, and `&mut $ty`.
///
/// See [`impl_function_traits`] for details on syntax.
///
/// [`impl_function_traits`]: crate::func::macros::impl_function_traits
macro_rules! impl_from_arg {
    (
        $ty: ty
        $(;
            < $($T: ident $(: $T1: tt $(+ $T2: tt)*)?),* >
        )?
        $(
            [ $(const $N: ident : $size: ident),* ]
        )?
        $(
            where $($U: ty $(: $U1: tt $(+ $U2: tt)*)?),*
        )?
    ) => {
        impl <
            $($($T $(: $T1 $(+ $T2)*)?),*)?
            $(, $(const $N : $size),*)?
        > $crate::func::args::FromArg for $ty
        $(
            where $($U $(: $U1 $(+ $U2)*)?),*
        )?
        {
            type This<'from_arg> = $ty;
            fn from_arg(arg: $crate::func::args::Arg<'_>) ->
                Result<Self::This<'_>, $crate::func::args::ArgError>
            {
                arg.take_owned()
            }
        }
    };
}

pub(crate) use impl_from_arg;