librustc: Forbid partial reinitialization of uninitialized structures or enumerations that implement the Drop trait.

[pcwalton]

This breaks code like:

struct Struct {
    f: String,
    g: String,
}

impl Drop for Struct { ... }

fn main() {
    let x = Struct { ... };
    drop(x);
    x.f = ...;
}

Change this code to not create partially-initialized structures. For
example:

struct Struct {
    f: String,
    g: String,
}

impl Drop for Struct { ... }

fn main() {
    let x = Struct { ... };
    drop(x);
    x = Struct {
        f: ...,
        g: ...,
    }
}

Closes #18571.

[breaking-change]

I suspect I may have missed some cases that I should have checked.

r? @nikomatsakis

[nikomatsakis]

@pcwalton pnkfelix's fragments PR landed, so I think that loan-paths have easily accessible types now.

[alexcrichton]

Closing out of inactivity (and to help clear out @bors's queue).