Why do I get the error "the trait `Foo` is not implemented for `&mut T`" even though T implements the trait?

I have this source:

pub fn draw<G, C>(&self, font: &mut C, draw_state: &DrawState, transform: Matrix2d, g: &mut G)
where
    C: CharacterCache,
    G: Graphics<Texture = <C as CharacterCache>::Texture>,
{
    self.properties.draw(
        self.text.as_str(),
        &mut font,
        &draw_state,
        transform,
        g,
    );
}

And the error

the trait bound `&mut C: graphics::character::CharacterCache` is not satisfied 
(the trait `graphics::character::CharacterCache` is not implemented for `&mut C`) 

The only aspect of C that is defined is that it implements CharacterCache, yet the error says the opposite.

DrawState, Matrix2d, CharacterCache and its implementations, Texture, and self.properties (Text) are provided by the Piston 2d graphics library. There must be something about traits in general that I'm misunderstanding.

The Text::draw function signature:

fn draw<C, G>(
    &self,
    text: &str,
    cache: &mut C,
    draw_state: &DrawState,
    transform: Matrix2d,
    g: &mut G,
) where
    C: CharacterCache,
    G: Graphics<Texture = C::Texture>,