Expressing variant lifetimes with trait objects is tricky

[aidanhs]

https://is.gd/xCZ1UF

use std::io::{Cursor, Read, Write};

trait Ser<W> {
    fn serialize<'a, 'b>(&'a self, writer: &'b mut W);
}

fn main() {
    let mut buffer = vec![0u8];
    let frameworks: Vec<&Ser<Cursor<&mut [u8]>>> = vec![];

    for framework in frameworks {
        let mut write_cursor = Cursor::new(buffer.as_mut_slice());
        framework.serialize(&mut write_cursor);
    }
}

This does not compile with:

rustc 1.20.0-nightly (3bfc18a96 2017-06-29)
error[E0499]: cannot borrow `buffer` as mutable more than once at a time
  --> <anon>:12:44
   |
12 |         let mut write_cursor = Cursor::new(buffer.as_mut_slice());
   |                                            ^^^^^^
   |                                            |
   |                                            second mutable borrow occurs here
   |                                            first mutable borrow occurs here
...
15 | }
   | - first borrow ends here

error: aborting due to previous error(s)

But changing it to a struct with an impl works - https://is.gd/2OM3ud

Is there a way to express this? I've been fighting with it for a little while, so if there is I suspect there could be some diagnostic improvements.

[aidanhs]

Ah, changing it to let frameworks: Vec<&for<'r> Ser<Cursor<&'r mut [u8]>>> = vec![]; works (thanks to @Twey). Which makes sense.