compiler/rustc_error_codes/src/error_codes/E0210.md

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:

```compile_fail,E0210
# #[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`:

```
# 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:

```ignore (cannot-doctest-multicrate-project)
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:

```ignore (cannot-doctest-multicrate-project)
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`:

```ignore (only-for-syntax-highlight)
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].

[RFC 2451]: https://rust-lang.github.io/rfcs/2451-re-rebalancing-coherence.html
[RFC 1023]: https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md