Should the mutable borrow of self in method call range over the arguments?

[honzasp]

When a &mut self method is called, the receiver is mutably borrowed before the arguments are evaluated, so the following program does not compile:

fn main() {
  let mut vec = vec![1, 2, 3];
  vec.push(vec.len());
  assert_eq!(vec, vec![1, 2, 3, 4]);
}

But when the argument explicitly evaluated before the call in a let statement, is compiles just fine:

fn main() {
  let mut vec = vec![1, 2, 3];
  let vec_len = vec.len();
  vec.push(vec_len);
  assert_eq!(vec, vec![1, 2, 3, 3]);
}

I am not sure if there is a good reason for the first program to be rejected, I would expect e1(e2) to be equivalent to { let x = e2; e1(x) }.

[Gankra]

I believe this sort of thing is intended to be resolved by SEME regions: rust-lang/rfcs#396

[Gankra]

In particular, motivating example 3: https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md