Crate contextual

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A small crate to deal with data in context.

use contextual::{WithContext, AsRefWithContext, DisplayWithContext};
use std::fmt;

/// Index of an element in some array.
pub struct Index(usize);

impl<T, C: AsRef<[T]>> AsRefWithContext<T, C> for Index {
  fn as_ref_with<'a>(&'a self, context: &'a C) -> &'a T {

impl<'a, C: AsRef<[&'a str]>> DisplayWithContext<C> for Index {
  fn fmt_with(&self, context: &C, f: &mut fmt::Formatter) -> fmt::Result {
    use fmt::Display;

let i = Index(1);
let context = ["a", "b", "c"];

print!("index: {}", i.with(&context));
assert_eq!(*i.with(&context).as_ref(), "b")