Macro impl_context_trait 

macro_rules! impl_context_trait {
    (
        $(#[$ctx_meta:meta])*
        context $ctx_name:ident<$t:ident> {
            field $field:ident : $field_ty:ty
        }
        impl $trait_name:ident for WithContext<$t2:ident, $ctx_name2:ident<$t3:ident>> {
            $(
                fn $method:ident(&$self:ident $(, $arg:ident : $arg_ty:ty)* ) -> $ret:ty {
                    $($body:tt)*
                }
            )+
        }
    ) => { ... };
}

Define a new context type and its trait implementation for WithContext.

This macro generates a context struct holding a function pointer and implements the specified trait for WithContext<T, YourContext<T>>.

Examples

use context_trait::{impl_context_trait, WithContext};
use std::fmt;

// Define a trait we want to contextualize
trait Summarize {
    fn summarize(&self) -> String;
}

// Generate SummarizeContext and impl Summarize for WithContext
impl_context_trait! {
    /// A context for custom summarization.
    context SummarizeContext<T> {
        field summarize_fn: fn(&T) -> String
    }
    impl Summarize for WithContext<T, SummarizeContext<T>> {
        fn summarize(&self) -> String {
            (self.ctx.summarize_fn)(&self.inner)
        }
    }
}

struct Article { title: String, body: String }

let ctx = SummarizeContext {
    summarize_fn: |a: &Article| format!("{}...", &a.title),
};
let wrapped = WithContext {
    inner: Article { title: "Hello".into(), body: "World".into() },
    ctx,
};
assert_eq!(wrapped.summarize(), "Hello...");