CODEWITHSUNDEEP
compiler-errorsrust
uorbe001
uorbe001

Reputation: 187

Rust "cannot determine a type for this bounded type parameter: unconstrained type"

I just started fiddling with Rust and I found an error I don't understand, trying to build something like a DSL.

I've defined a struct like this:

struct Thing<T> {
     a: T
}

impl<T: Show> Thing<T> {
   fn selfie<T>(&self) -> Thing<T> { Thing { a: self.a } }
   fn say<T>(&self) { println!("sing") }
}

Then I call it like this and get the error:

let a = Thing { a: 1i }; // works
a.say(); // works
let s = a.selfie(); //works
s.say(); // error

main.rs:49:5: 49:12 error: cannot determine a type for this bounded type parameter: unconstrained type
main.rs:49     s.say();
               ^~~~~~~

Does anyone know what this error means? I'm having a hard time figuring it out, and have tried a few different casts and nothing seems to work.

Alternatively, is there a better way to return a reference to "self" (which is what I really wanted to do in the "selfie" method but I was getting errors due to the reference lifespan) or to "an equivalent struct of the same kind"?

Upvotes: 1

Views: 378

Answers (1)

A.B.
A.B.

Reputation: 16630

This compiles with a recent Rust version.

use std::fmt::Show;

struct Thing<T> {
     a: T
}

impl<T: Show + Clone> Thing<T> {
   fn selfie(&self) -> Thing<T> { Thing { a: self.a.clone() } }
   fn say(&self) { println!("sing: {}", self.a) }
}

I'm assuming that the say method should actually print a (because otherwise the Show bound makes no sense).

The problem was that in say<T> a generic type T exists but cannot be inferred because it's not used. In the selfie method, the <T> is not needed. We do need to restrict T to those types that implement Clone though if we want to clone it.

I should also point out that the selfie method is reinventing clone. You could just do:

#[deriving(Clone)]
struct Thing<T> {
     a: T
}

and then call clone() rather than selfie.

Upvotes: 2

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