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!)

Author: Ryman

src/librustc_borrowck/borrowck/check_loans.rs

@@ -745,6 +745,26 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                                                      use_kind, lp_base);
             }
             LpExtend(ref lp_base, _, LpInterior(InteriorField(_))) => {
+                match lp_base.to_type().sty {
+                    ty::ty_struct(def_id, _) | ty::ty_enum(def_id, _) => {
+                        if ty::has_dtor(self.tcx(), def_id) {
+                            // In the case where the owner implements drop, then
+                            // the path must be initialized to prevent a case of
+                            // partial reinitialization
+                            let loan_path = owned_ptr_base_path_rc(lp_base);
+                            self.move_data.each_move_of(id, &loan_path, |_, _| {
+                                self.bccx
+                                    .report_partial_reinitialization_of_uninitialized_structure(
+                                        span,
+                                        &*loan_path);
+                                false
+                            });
+                            return;
+                        }
+                    },
+                    _ => {},
+                }
+
                 // assigning to `P.f` is ok if assigning to `P` is ok
                 self.check_if_assigned_path_is_moved(id, span,
                                                      use_kind, lp_base);
@@ -775,10 +795,12 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                     mark_variable_as_used_mut(self, assignee_cmt);
                 }
             }
+
             return;
         }
 
-        // Initializations are OK.
+        // Initializations are OK if and only if they aren't partial
+        // reinitialization of a partially-uninitialized structure.
         if mode == euv::Init {
             return
         }

src/librustc_borrowck/borrowck/mod.rs

@@ -686,6 +686,18 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
         }
     }
 
+    pub fn report_partial_reinitialization_of_uninitialized_structure(
+            &self,
+            span: Span,
+            lp: &LoanPath<'tcx>) {
+        self.tcx
+            .sess
+            .span_err(span,
+                      (format!("partial reinitialization of uninitialized \
+                               structure `{}`",
+                               self.loan_path_to_string(lp))).as_slice());
+    }
+
     pub fn report_reassigned_immutable_variable(&self,
                                                 span: Span,
                                                 lp: &LoanPath<'tcx>,

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()
+}