I am modifying some code to depend on rand version 0.5. At first, I was worried how I would enable generating random values of my own types with Standard, but I found out this is legal:
impl ::rand::distributions::Distribution<MyType> for ::rand::distributions::Standard {
// ...
}
Why is it legal? I thought implementing an external trait for an external type is illegal.
The entire purpose of these rules (called the orphan rules or coherence rules) is to avoid having any conflicting implementations for a given trait/type combination. Since Rust 1.0, it is believed that it would not be in line with a language striving for solid stability to be "willy-nilly" about who is allowed to implement a trait for a type.
A very common example of this particular type of implementation is From:
impl From<MyType> for i32 {
// ...
}
By parameterizing the trait with a local type, there's still no ambiguity about who is implementing the trait. One way of thinking about it would be treating Distribution as not a trait but a trait constructor 1. Each instance of Distribution creates a new trait, one that's custom-made for the case.
See also:
1 — This isn't true, but it's a reasonable analogy.