Understanding Rust Error E0210

Rust Error E0210 Explained

Rust Error E0210 occurs when there's a violation of one of Rust's orphan rules related to implementing foreign traits. The error occurs when a type parameter is implemented for a foreign trait (a trait defined in another crate), and the type parameter isn't covered by a local type. To better understand the issue, let's discuss a few examples.

Invalid Example - Uncovered Type Parameter

extern crate foo;
use foo::ForeignTrait;

impl<T> ForeignTrait for T { } // error
fn main() {}

In the example above, we try to implement the ForeignTrait for a type parameter T. Since T is not covered by a local type, Rust Error E0210 occurs. To fix the error, you can cover it with a local type:

Valid Example - Covered Type Parameter

use std::panic::UnwindSafe as ForeignTrait;
struct MyType<T>(T);
impl<T> ForeignTrait for MyType<T> { } // Ok
fn main() {}

In this example, we create a new local struct MyType, which covers the type parameter T. This resolves Error E0210 in the given scenario.

It's important to note that simply using a type alias is insufficient - you must use a proper covering local type.

To better comprehend the rule and its reasons for existence, let's go through general rules of orphan impls and type parameters.

General Rules for Orphan Impls and Type Parameters

Consider an impl:
impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }

Among the types T0, ..., Tn, one of them must be a local type. The following two conditions must be true:

1. At least one of the types T0..=Tn must be a local type.
2. Let Ti be the first such type. No uncovered type parameters P1..=Pm may appear in T0..Ti (excluding Ti).

The design of these orphan rules is detailed in RFC 2451 and RFC 1023. Adhering to these rules ensures that your Rust code will not encounter Error E0210.