How do I have a trait field in a struct? - enums

Question

I have some code that I want to turn into a crate. But it includes a structure that contains a field that I want to be provided by the user of the crate. But I need functionality from that field, so I want to specify it as a trait.
pub trait RoleTrait {
fn owner<T: RoleTrait>() -> T;
fn order<T: RoleTrait>(&self) -> usize;
}
pub struct RequestInfo<Role: RoleTrait + PartialEq> {
role: Option<Role>,
name: String,
}
impl<Role: RoleTrait> RequestInfo<Role>
where
Role: std::cmp::PartialEq,
{
fn name(&self) -> String {
self.name.to_string()
}
fn role(&self) -> &Option<Role> {
&self.role
}
fn is_owner(&self) -> bool {
if let Some(role) = self.role {
role == Role::owner()
} else {
false
}
}
fn order(&self) -> usize {
if let Some(role) = self.role {
role.order() + 1
} else {
0
}
}
fn from(name: String) -> RequestInfo<Role> {
RequestInfo::<Role> {
role: None,
name: name,
}
}
fn with_role(name: String, role: Role) -> RequestInfo<Role> {
RequestInfo::<Role> {
role: Some(role),
name: name,
}
}
}
With two implementations of RoleTrait:
#[derive(PartialEq)]
pub enum CourseRole {
Professor,
Marker,
Student,
}
impl RoleTrait for CourseRole {
fn owner<T: RoleTrait>() -> T {
CourseRole::Professor
}
fn order<T: RoleTrait>(&self) -> usize {
if *self == CourseRole::Professor {
0
} else {
1
}
}
}
#[derive(PartialEq)]
pub enum BlogRole {
Owner,
Blogger,
}
impl RoleTrait for BlogRole {
fn owner<T: RoleTrait>() -> T {
BlogRole::Owner
}
fn order<T: RoleTrait>(&self) -> usize {
if *self == BlogRole::Owner {
0
} else {
1
}
}
}
I get 3 errors with this.
error[E0282]: type annotations needed
--> src/main.rs:28:18
|
28 | role.order() + 1
| ^^^^^ cannot infer type for `T`
error[E0308]: mismatched types
--> src/main.rs:55:9
|
54 | fn owner<T: RoleTrait>() -> T {
| - expected `T` because of return type
55 | CourseRole::Professor
| ^^^^^^^^^^^^^^^^^^^^^ expected type parameter, found enum `CourseRole`
|
= note: expected type `T`
found type `CourseRole`
error[E0308]: mismatched types
--> src/main.rs:72:9
|
71 | fn owner<T: RoleTrait>() -> T {
| - expected `T` because of return type
72 | BlogRole::Owner
| ^^^^^^^^^^^^^^^ expected type parameter, found enum `BlogRole`
|
= note: expected type `T`
found type `BlogRole`
(and the second error repeated for the other enum)
Frankly, I'm surprised (and pleased!) that some of my code is valid (like the references to owner in the trait). I had a lot more errors when I started writing this question, but I can't figure out these remaining ones since T looks so clear and rustc seems to have already figured out harder things. In the last 2 errors, it's almost like it doesn't realize that there is an implementation of the trait for the enum because it's in the middle of defining that implementation (but it obviously understands that in other places).

Answer 1

Something feels a little "off" with this trait:
pub trait RoleTrait {
fn owner<T: RoleTrait>() -> T;
fn order<T: RoleTrait>(&self) -> usize;
}
The owner method doesn't have a receiver (e.g. self), so it seems unnecessary to introduce a new type parameter; Self will do the same thing.
In order, having a separate T is not exactly the same thing as just using Self - it allows T and Self to be a completely different implementations of RoleTrait. But this feels like quite a strange and unusual requirement, especially since T doesn't appear in the method signature.
Your code can be fixed quite simply by following the more typical pattern:
pub trait RoleTrait {
fn owner() -> Self;
fn order(&self) -> usize;
}
This small change leads to all the type errors being resolved, and just leaving a couple of small borrow errors (playground), which can be quite easily addressed (playground).