wtf? borrowck pls

wtf.rs

pub fn make_mut<'b>(this: &'b mut RcThunk<'a, T>) -> &'b mut T where T: Clone {
    if let Some(thunk) = Rc::get_mut(&mut this.0) {
        return &mut **thunk;
    }

    let new_rc = Rc::new(Thunk::computed((*this.0).clone()));
    this.0 = new_rc;
    RcThunk::get_mut(this).unwrap()
}

error[E0502]: cannot borrow `this.0` as immutable because it is also borrowed as mutable
   --> src/unsync.rs:194:48
    |
190 |         if let Some(thunk) = Rc::get_mut(&mut this.0) {
    |                                               ------ mutable borrow occurs here
...
194 |         let new_rc = Rc::new(Thunk::computed((*this.0).clone()));
    |                                                ^^^^^^ immutable borrow occurs here
...
197 |     }
    |     - mutable borrow ends here

error[E0506]: cannot assign to `this.0` because it is borrowed
   --> src/unsync.rs:195:9
    |
190 |         if let Some(thunk) = Rc::get_mut(&mut this.0) {
    |                                               ------ borrow of `this.0` occurs here
...
195 |         this.0 = new_rc;
    |         ^^^^^^^^^^^^^^^ assignment to borrowed `this.0` occurs here

error[E0499]: cannot borrow `*this` as mutable more than once at a time
   --> src/unsync.rs:196:26
    |
190 |         if let Some(thunk) = Rc::get_mut(&mut this.0) {
    |                                               ------ first mutable borrow occurs here
...
196 |         RcThunk::get_mut(this).unwrap()
    |                          ^^^^ second mutable borrow occurs here
197 |     }
    |     - first borrow ends here