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! }