Why won't the reference to self get dropped

Question

Consider the following programm. Why isn't the reference to self dropped after the first call to foo and how can I resolve this issue without changing the mutability of the foo function parameters.

struct MyElement<'a>(&'a u32);
struct MyStruct(u32);

impl MyStruct {
    fn foo<'a>(&'a mut self, vec: &mut Vec<MyElement<'a>>) {
        vec.push(MyElement(&self.0));
    }
}

fn main() {
    let mut mystruct = MyStruct(42);
    let mut vec = Vec::new();
    mystruct.foo(&mut vec);
    mystruct.foo(&mut vec);
}