Macro z_impl_from_structural

macro_rules! z_impl_from_structural {
    (
        impl[ $($impl_params:tt)* ] FromStructural<$from:ident> for $self:ty
        where [ $($where_preds:tt)* ]
        {
            fn from_structural($from_var:ident){
                $($code:tt)*
            }
        }
    ) => { ... };
}

For implementing FromStructural, and delegating the implementation of TryFromStructural to it.

Example

This example demonstrates how you can implement FromStructural in a more general way than necessary,

use structural::{FP, IntoField, Structural, StructuralExt, fp, make_struct};

use std::borrow::Cow;

{
    let this = make_struct!{
        encoding: "utf8",
        contents: &[0,1,2,3,4][..],
    };
    assert_eq!(
        this.into_struc::<Message>(),
        Message{encoding: "utf8".to_string(), contents: vec![0,1,2,3,4]}
    );
}
{
    let this = HttpMessage{
        encoding: Cow::from("utf16"),
        contents: Cow::from(vec![5,7,8]),
        valid_until: 0o40002,
    };
    assert_eq!(
        this.into_struc::<Message>(),
        Message{encoding: "utf16".to_string(), contents: vec![5,7,8]}
    );
}

#[derive(Structural, Debug)]
#[struc(no_trait, public, access="move")]
pub struct HttpMessage<'a> {
    encoding: Cow<'a,str>,
    contents: Cow<'a,[u8]>,
    valid_until: u32,
}

#[derive(Structural, Debug, PartialEq)]
#[struc(no_trait, public, access="move")]
pub struct Message {
    encoding: String,
    contents: Vec<u8>,
}

// This macro generates the TryFromStructural impl based on the passed
// FromStructural implementation.
structural::z_impl_from_structural! {
    impl[F, E, C] FromStructural<F> for Message
    where[
        // The bounds here would usually just be `F: Message_SI`
        // (Message_SI being a trait generated by the Structural derive,
        //  aliasing the accessor traits implemented by Message),
        // but I decided to make this example different.
        F: IntoField<FP!(encoding), Ty = C>,
        F: IntoField<FP!(contents), Ty = E>,
        C: Into<String>,
        E: Into<Vec<u8>>,
    ]{
        fn from_structural(this){
            let (encoding, contents) = this.into_fields(fp!(encoding, contents));
            Self {
                encoding: encoding.into(),
                contents: contents.into(),
            }
        }
    }
}