Syntax sugar for mutating a rvalue

[kyouko-taiga]

Consider the following program:

public fun main() {
  var x = (1, 2)
  let p = mutable_pointer[to: &x]
  &(p.copy().unsafe[]).0 = 3
  print(x.0)
}

The parentheses around p.copy().unsafe[] are necessary because we must tell the compiler that we're mutating the result of unsafe and not trying to apply the mutating variant of copy (should it be a method bundle).

As @dabrahams suggested, we could eliminate these parentheses by allowing & to occur after a rvalue. In that case, it would apply to the result of that rvalue rather than the receiver of the first method call in case the expression start with one. For example: p.copy()&.unsafe[].0 = 3.