Rust error E0407: method `b` is not a member of trait `Foo`

The compiler team's explanation

A definition of a method not in the implemented trait was given in a trait implementation.

Erroneous code example:

trait Foo {
    fn a();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
    fn b() {} // error: method `b` is not a member of trait `Foo`
}

Please verify you didn't misspell the method name and you used the correct trait. First example:

trait Foo {
    fn a();
    fn b();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
    fn b() {} // ok!
}

Second example:

trait Foo {
    fn a();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
}

impl Bar {
    fn b() {}
}

From rustc --explain E0407 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.