context-trait

Runtime-synthesized trait instances using function pointer tables, scoped to a callback.

This crate provides [WithContext], a wrapper that pairs a value with a context supplying trait implementations. Built-in contexts include [OrdContext], [HashContext], and [DisplayContext].

Declarative macros [with_ord!], [with_hash!], and [with_display!] make it easy to use non-default trait implementations in a scoped block.

The [impl_context_trait!] macro lets users define new context types for arbitrary traits.

Examples

use context_trait::{with_ord, OrdContext, WithContext};

let items = vec![3i32, 1, 4, 1, 5];
with_ord!(items, |a: &i32, b: &i32| b.cmp(a), |wrapped: &[WithContext<i32, OrdContext<i32>>]| {
    let mut sorted = wrapped.to_vec();
    sorted.sort();
    let values: Vec<i32> = sorted.into_iter().map(|w| w.inner).collect();
    assert_eq!(values, vec![5, 4, 3, 1, 1]);
});