librustc: Forbid partial reinitialization of uninitialized structures or

enumerations that implement the `Drop` trait.

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]

----

(Joint authorship by pcwalton and Ryman; thanks all!)

src/test/compile-fail/borrowck-partial-reinit-1.rs

@@ -0,0 +1,49 @@
+// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct Test;
+
+struct Test2 {
+    b: Option<Test>,
+}
+
+struct Test3(Option<Test>);
+
+impl Drop for Test {
+    fn drop(&mut self) {
+        println!("dropping!");
+    }
+}
+
+impl Drop for Test2 {
+    fn drop(&mut self) {}
+}
+
+impl Drop for Test3 {
+    fn drop(&mut self) {}
+}
+
+fn stuff() {
+    let mut t = Test2 { b: None };
+    let u = Test;
+    drop(t);
+    t.b = Some(u);
+    //~^ ERROR partial reinitialization of uninitialized structure `t`
+
+    let mut t = Test3(None);
+    let u = Test;
+    drop(t);
+    t.0 = Some(u);
+    //~^ ERROR partial reinitialization of uninitialized structure `t`
+}
+
+fn main() {
+    stuff()
+}

src/test/compile-fail/borrowck-partial-reinit-2.rs

@@ -0,0 +1,34 @@
+// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct Test {
+    a: isize,
+    b: Option<Box<Test>>,
+}
+
+impl Drop for Test {
+    fn drop(&mut self) {
+        println!("Dropping {}", self.a);
+    }
+}
+
+fn stuff() {
+    let mut t = Test { a: 1, b: None};
+    let mut u = Test { a: 2, b: Some(Box::new(t))};
+    t.b = Some(Box::new(u));
+    //~^ ERROR partial reinitialization of uninitialized structure `t`
+    println!("done");
+}
+
+fn main() {
+    stuff();
+    println!("Hello, world!")
+}
+

src/test/compile-fail/borrowck-partial-reinit-3.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+use std::mem;
+
+struct Test { f: usize }
+impl Drop for Test {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let mut x = (Test { f: 2 }, Test { f: 4 });
+    mem::drop(x.0);
+    x.0.f = 3;
+    //~^ ERROR partial reinitialization of uninitialized structure `x.0`
+}

src/test/compile-fail/borrowck-partial-reinit-4.rs

@@ -0,0 +1,33 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct Test;
+
+struct Test2(Option<Test>);
+
+impl Drop for Test {
+    fn drop(&mut self) {
+        println!("dropping!");
+    }
+}
+
+impl Drop for Test2 {
+    fn drop(&mut self) {}
+}
+
+fn stuff() {
+    let mut x : (Test2, Test2);
+    (x.0).0 = Some(Test);
+    //~^ ERROR partial reinitialization of uninitialized structure `x.0`
+}
+
+fn main() {
+    stuff()
+}