E0404 - Error codes index (original) (raw)

Error codes index

Error code E0404

A type that is not a trait was used in a trait position, such as a bound or impl.

Erroneous code example:

#![allow(unused)]
fn main() {
struct Foo;
struct Bar;

impl Foo for Bar {} // error: `Foo` is not a trait
fn baz<T: Foo>(t: T) {} // error: `Foo` is not a trait
}

Another erroneous code example:

#![allow(unused)]
fn main() {
type Foo = Iterator<Item=String>;

fn bar<T: Foo>(t: T) {} // error: `Foo` is a type alias
}

Please verify that the trait's name was not misspelled or that the right identifier was used. Example:

#![allow(unused)]
fn main() {
trait Foo {
    // some functions
}
struct Bar;

impl Foo for Bar { // ok!
    // functions implementation
}

fn baz<T: Foo>(t: T) {} // ok!
}

Alternatively, you could introduce a new trait with your desired restrictions as a super trait:

#![allow(unused)]
fn main() {
trait Foo {}
struct Bar;
impl Foo for Bar {}
trait Qux: Foo {} // Anything that implements Qux also needs to implement Foo
fn baz<T: Qux>(t: T) {} // also ok!
}

Finally, if you are on nightly and want to use a trait alias instead of a type alias, you should use #![feature(trait_alias)]:

#![allow(unused)]
#![feature(trait_alias)]
fn main() {
trait Foo = Iterator<Item=String>;

fn bar<T: Foo>(t: T) {} // ok!
}