RFC: Mixed mutable variables

[VitWW]

This RFC proposes Mixed mutable variables, including Mixed mutable references.

Rendered

struct Point2 {x : f32, was_x : f32}

let mut p1 : Point2 = Point2 {x:1.0, was_x:2.0};

fn pntm_store (&^type p : & ^{^const x, ^mut was_x} Point2)  {
   *p.was_x = *p.x;
}

pntm_store(&^arg p1); // effective mutable reference

let ^type p2 : ^{^mut x, ^const was_x} Point2 = Point {x:1.0, was_x: 4.0};
	
let ref_p2 = &^max p2; // mixed mutable reference

This is an extension to Partial Types Proposal #3426

[VitWW]

As I see, Partial Mutability is a better choice then Mixed mutable variables, so I close this draft proposal.

Thanks to everyone read it!