Does Rust allow you define Ord or PartialOrd for your type and external types?

Question

Let's say I want to define my own type..

struct MyString(String);

Does Rust let me define the behavior when comparing the string against another type, such as an Option<MyString>? I want to write something like this,

impl std::cmp::PartialOrd<Option<MyString>> {
    fn partial_cmp(self, other: Option<MyString> ) {
    }
}

But I'm getting,

error[E0116]: cannot define inherent impl for a type outside of the crate where the type is defined [...] impl for type defined outside of crate. [...] define and implement a trait or new type instead

This is confusing to me because MyString is my type. Is this a violation of coherence?

Answer 1

The error

impl for a type outside of the crate where the type is defined

Is because I left off for MyString in impl ... for MyString {}. An easy syntax error to make that can be fixed by adding that,

impl std::cmp::PartialOrd<Option<MyString>> for MyString {
    fn partial_cmp(self, other: Option<MyString> ) {
`

Then I get the much more reasonable error

error[E0277]: can't compare `MyString` with `Option<MyString>`
   --> src/main.rs:6:6
    |
6   | impl std::cmp::PartialOrd<Option<MyString>> for MyString {
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `MyString == Option<MyString>`
    |
    = help: the trait `PartialEq<Option<MyString>>` is not implemented for `MyString`