Error code E0210

This error indicates a violation of one of Rust's orphan rules for trait implementations. The rule concerns the use of type parameters in an implementation of a foreign trait (a trait defined in another crate), and states that type parameters must be "covered" by a local type.

When implementing a foreign trait for a foreign type, the trait must have one or more type parameters. A type local to your crate must appear before any use of any type parameters.

To understand what this means, it is perhaps easier to consider a few examples.

If ForeignTrait is a trait defined in some external crate foo, then the following trait impl is an error:

#[cfg(for_demonstration_only)]
extern crate foo;
#[cfg(for_demonstration_only)]
use foo::ForeignTrait;
use std::panic::UnwindSafe as ForeignTrait;

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

To work around this, it can be covered with a local type, MyType:

#![allow(unused)]
fn main() {
use std::panic::UnwindSafe as ForeignTrait;
struct MyType<T>(T);
impl<T> ForeignTrait for MyType<T> { } // Ok
}

Please note that a type alias is not sufficient.

For another example of an error, suppose there's another trait defined in foo named ForeignTrait2 that takes two type parameters. Then this impl results in the same rule violation:

struct MyType2;
impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error

The reason for this is that there are two appearances of type parameter T in the impl header, both as parameters for ForeignTrait2. The first appearance is uncovered, and so runs afoul of the orphan rule.

Consider one more example:

impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok

This only differs from the previous impl in that the parameters T and MyType<T> for ForeignTrait2 have been swapped. This example does not violate the orphan rule; it is permitted.

To see why that last example was allowed, you need to understand the general rule. Unfortunately this rule is a bit tricky to state. Consider an impl:

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

where P1, ..., Pm are the type parameters of the impl and T0, ..., Tn are types. One of the types T0, ..., Tn must be a local type (this is another orphan rule, see the explanation for E0117).

Both of the following must be true:

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

For information on the design of the orphan rules, see RFC 2451 and RFC 1023.