Rust error E0404: expected trait, found struct `Foo`

The compiler team's explanation

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

Erroneous code example:

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:

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:

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:

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)]:

#![feature(trait_alias)]
trait Foo = Iterator<Item=String>;

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

From rustc --explain E0404 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.