DerefMut borrow method call is too long with Deref arguments

[tirr-c]

Currently with Rust 2018, this code does not compile.

fn main() {
    let mut v = vec![1, 2, 3];
    v.swap(0, v.len() - 1);
    assert_eq!(v, &[3, 2, 1]);
}

error[E0502]: cannot borrow `v` as immutable because it is also borrowed as mutable
  |
3 |     v.swap(0, v.len() - 1);
  |     - ----    ^ immutable borrow occurs here
  |     | |
  |     | mutable borrow later used by call
  |     mutable borrow occurs here

It seems that the compiler desugars the method call like this:

let v0 = &*v;
let v1 = &mut *v;
let arg0 = 0;
let arg1 = v0.len() - 1; // v1 is still alive here
v1.swap(arg0, arg1);

But I think the DerefMut should be deferred, or use only DerefMut reference without any Deref.

let v0 = &*v;
let arg0 = 0;
let arg1 = v0.len() - 1; // no mutable borrows here

let v1 = &mut *v;
v1.swap(arg0, arg1);

let v0 = &mut *v;
let arg0 = 0;
let arg1 = v0.len() - 1;
v0.swap(arg0, arg1);

[jonas-schievink]

cc #49434

(specifically this comment seems to have found the same problem already: #49434 (comment))

[pnkfelix]

NLL triage. Tagging as P-medium, just like #49434.

[ChayimFriedman2]

The DerefMut cannot be deferred because we guarantee that the receiver is evaluated before the other operands.

What we can do is to generalize two phase borrows (or just defer the deref) when the deref is pure. I know project-deref-patterns are considering introducing a DerefPure trait, so it may be useful here too.