E0623 not working for below examples

Example 1
```
struct Foo;

impl Foo {
    fn foo(&self, v: &mut Vec<&u32>, x: &u32) {
        v.push(x);
    }
}
```

Example 2
```
fn foo(v: &mut Vec<&u32>, x: &u32) {
    v.push(x);
}
```

Example 3
```
struct Foo;

impl Foo {
    fn foo(&self, x: &u32) -> &u32 {
        x
    }
}
```
cc @nikomatsakis 