RFC: Partial Mutability

[VitWW]

This RFC proposes to add partiality to mutability to have partial mutable variables, including Partial mutable references.
This RFC is much simpler then Mixed mutable variables #3427

Rendered

let mut p1 : mut.%{x,y} Point = Point {x:1.0, y:2.0, was_x: 4.0, was_y: 5.0, state: 12.0};
	// p1 : mut.%{x,y} Point
let mut p2 : mut.%{x,was_x} Point = Point {x:1.0, y:2.0, was_x: 4.0, was_y: 5.0, state: 12.0};
	// p2 : mut.%{x,was_x} Point

let ref_p1 = &mut p1;
	// ref_p1 : &mut.%{x, y} Point
	
let refnr_p2 = &mut.%{x} p2;
	// refnr_p2 : &mut.%{x} Point

fn vz_restore (&mut p : &mut.%{was_x} Point2)  {
   *p.x = *p.was_x;
}

vz_restore(&mut p2);

pub fn mx_rstate(&mut p : &mix Point.%{mut x, state, %any}) { /* ... */ }
// mixed partiality of type and mutability

This is an extension to Partial Types Proposal #3426

[Lokathor]

Generally the "pre-rfc" process (where you attempt to turn a partial idea into a full RFC) is done on the internals forum, https://internals.rust-lang.org/, or on the zulip, https://rust-lang.zulipchat.com/, rather than posting a draft PR.

[VitWW]

Partial Mutability will be part Partial Types (v3) Proposal, so I close this draft proposal.

Thanks to everyone read it!