Implement RFC #43

Remove the ability of the borrow checker to determine that repeated
dereferences of a Box<T> refer to the same memory object. This will
usually require one of two workarounds:

1) The interior of a Box<T> will sometimes need to be moved / borrowed
into a temporary before moving / borrowing individual derived paths.

2) A `ref x` pattern will have to be replaced with a `box ref x`
pattern.

Fixes #16094.

[breaking-change]

src/librustc/middle/borrowck/check_loans.rs

@@ -261,6 +261,7 @@ impl<'a> CheckLoanCtxt<'a> {
         //     let x = &mut a.b.c; // Restricts a, a.b, and a.b.c
         //     let y = a;          // Conflicts with restriction
 
+        let loan_path = owned_ptr_base_path(loan_path);
         let cont = self.each_in_scope_loan(scope_id, |loan| {
             let mut ret = true;
             for restr_path in loan.restricted_paths.iter() {
@@ -395,8 +396,9 @@ impl<'a> CheckLoanCtxt<'a> {
             return true;
         }
 
+        let loan2_base_path = owned_ptr_base_path_rc(&loan2.loan_path);
         for restr_path in loan1.restricted_paths.iter() {
-            if *restr_path != loan2.loan_path { continue; }
+            if *restr_path != loan2_base_path { continue; }
 
             let old_pronoun = if new_loan.loan_path == old_loan.loan_path {
                 "it".to_string()
@@ -648,7 +650,8 @@ impl<'a> CheckLoanCtxt<'a> {
 
         debug!("check_if_path_is_moved(id={:?}, use_kind={:?}, lp={})",
                id, use_kind, lp.repr(self.bccx.tcx));
-        self.move_data.each_move_of(id, lp, |move, moved_lp| {
+        let base_lp = owned_ptr_base_path_rc(lp);
+        self.move_data.each_move_of(id, &base_lp, |move, moved_lp| {
             self.bccx.report_use_of_moved_value(
                 span,
                 use_kind,

src/test/compile-fail/borrowck-borrow-from-owned-ptr.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -52,14 +52,15 @@ fn borrow_same_field_twice_imm_imm() {
 fn borrow_both_fields_mut() {
     let mut foo = make_foo();
     let bar1 = &mut foo.bar1;
-    let _bar2 = &mut foo.bar2;
+    let _bar2 = &mut foo.bar2; //~ ERROR cannot borrow
     *bar1;
 }
 
 fn borrow_both_mut_pattern() {
     let mut foo = make_foo();
     match *foo {
         Foo { bar1: ref mut _bar1, bar2: ref mut _bar2 } => {}
+        //~^ ERROR cannot borrow
     }
 }
 
@@ -120,7 +121,7 @@ fn borrow_imm_and_base_imm() {
 fn borrow_mut_and_imm() {
     let mut foo = make_foo();
     let bar1 = &mut foo.bar1;
-    let _foo1 = &foo.bar2;
+    let _foo1 = &foo.bar2; //~ ERROR cannot borrow
     *bar1;
 }
 
@@ -133,7 +134,7 @@ fn borrow_mut_from_imm() {
 fn borrow_long_path_both_mut() {
     let mut foo = make_foo();
     let bar1 = &mut foo.bar1.int1;
-    let foo1 = &mut foo.bar2.int2;
+    let foo1 = &mut foo.bar2.int2; //~ ERROR cannot borrow
     *bar1;
     *foo1;
 }

src/test/run-pass/match-implicit-copy-unique.rs

@@ -13,8 +13,9 @@ struct Pair { a: Box<int>, b: Box<int> }
 
 pub fn main() {
     let mut x = box Pair {a: box 10, b: box 20};
-    match x {
-      box Pair {a: ref mut a, b: ref mut _b} => {
+    let x_internal = &mut *x;
+    match *x_internal {
+      Pair {a: ref mut a, b: ref mut _b} => {
         assert!(**a == 10); *a = box 30; assert!(**a == 30);
       }
     }