Might have found a solution

[bvssvni]

The following code gives good error messages for a single back-end, and infers the types within same family:

struct VertexBuffer;
struct Texture;

trait Member { type Family; }
trait Resource<T> {}

struct Gl;
struct GlVertexBuffer;
struct GlTexture;

impl Member for GlVertexBuffer { type Family = Gl; }
impl Member for GlTexture { type Family = Gl; }

impl Resource<GlVertexBuffer> for VertexBuffer {}
impl Resource<GlTexture> for Texture {}

trait VB: Member {
    fn say_hello(&mut self);
}

impl VB for GlVertexBuffer {
    fn say_hello(&mut self) {
        println!("hello");
    }
}

fn foo<Vb, Tx>(vb: &mut Vb, _tx: Tx)
    where VertexBuffer: Resource<Vb>,
          Texture: Resource<Tx>,
          Vb: VB,
          Tx: Member<Family = Vb::Family>,
{
    vb.say_hello();
}

fn main() {
    foo(&mut GlVertexBuffer, GlTexture);
}

play.rust-lang.org

[bvssvni]

This idea was taken from the discrete library.