Prohibit assiginments to &mut pointers in borrowed locations

src/librustc/middle/borrowck/check_loans.rs

@@ -496,6 +496,12 @@ impl<'self> CheckLoanCtxt<'self> {
             // path, and check that the super path was not lent out as
             // mutable or immutable (a const loan is ok).
             //
+            // Mutability of a path can be dependent on the super path
+            // in two ways. First, it might be inherited mutability.
+            // Second, the pointee of an `&mut` pointer can only be
+            // mutated if it is found in an unaliased location, so we
+            // have to check that the owner location is not borrowed.
+            //
             // Note that we are *not* checking for any and all
             // restrictions.  We are only interested in the pointers
             // that the user created, whereas we add restrictions for
@@ -513,9 +519,12 @@ impl<'self> CheckLoanCtxt<'self> {
             let mut loan_path = loan_path;
             loop {
                 match *loan_path {
-                    // Peel back one layer if `loan_path` has
-                    // inherited mutability
-                    LpExtend(lp_base, mc::McInherited, _) => {
+                    // Peel back one layer if, for `loan_path` to be
+                    // mutable, `lp_base` must be mutable. This occurs
+                    // with inherited mutability and with `&mut`
+                    // pointers.
+                    LpExtend(lp_base, mc::McInherited, _) |
+                    LpExtend(lp_base, _, LpDeref(mc::region_ptr(ast::m_mutbl, _))) => {
                         loan_path = lp_base;
                     }
 

src/librustc/middle/borrowck/gather_loans/restrictions.rs

@@ -101,7 +101,7 @@ impl RestrictionsContext {
                 self.extend(result, cmt.mutbl, LpInterior(i), restrictions)
             }
 
-            mc::cat_deref(cmt_base, _, mc::uniq_ptr) => {
+            mc::cat_deref(cmt_base, _, pk @ mc::uniq_ptr) => {
                 // R-Deref-Send-Pointer
                 //
                 // When we borrow the interior of an owned pointer, we
@@ -110,7 +110,7 @@ impl RestrictionsContext {
                 let result = self.restrict(
                     cmt_base,
                     restrictions | RESTR_MUTATE | RESTR_CLAIM);
-                self.extend(result, cmt.mutbl, LpDeref, restrictions)
+                self.extend(result, cmt.mutbl, LpDeref(pk), restrictions)
             }
 
             mc::cat_copied_upvar(*) | // FIXME(#2152) allow mutation of upvars
@@ -129,7 +129,7 @@ impl RestrictionsContext {
                 Safe
             }
 
-            mc::cat_deref(cmt_base, _, mc::gc_ptr(m_mutbl)) => {
+            mc::cat_deref(cmt_base, _, pk @ mc::gc_ptr(m_mutbl)) => {
                 // R-Deref-Managed-Borrowed
                 //
                 // Technically, no restrictions are *necessary* here.
@@ -170,14 +170,14 @@ impl RestrictionsContext {
                 match opt_loan_path(cmt_base) {
                     None => Safe,
                     Some(lp_base) => {
-                        let lp = @LpExtend(lp_base, cmt.mutbl, LpDeref);
+                        let lp = @LpExtend(lp_base, cmt.mutbl, LpDeref(pk));
                         SafeIf(lp, ~[Restriction {loan_path: lp,
                                                   set: restrictions}])
                     }
                 }
             }
 
-            mc::cat_deref(cmt_base, _, mc::region_ptr(m_mutbl, _)) => {
+            mc::cat_deref(cmt_base, _, pk @ mc::region_ptr(m_mutbl, _)) => {
                 // Because an `&mut` pointer does not inherit its
                 // mutability, we can only prevent mutation or prevent
                 // freezing if it is not aliased. Therefore, in such
@@ -187,7 +187,7 @@ impl RestrictionsContext {
                     let result = self.restrict(
                         cmt_base,
                         RESTR_ALIAS | RESTR_MUTATE | RESTR_CLAIM);
-                    self.extend(result, cmt.mutbl, LpDeref, restrictions)
+                    self.extend(result, cmt.mutbl, LpDeref(pk), restrictions)
                 } else {
                     // R-Deref-Mut-Borrowed-2
                     Safe

src/librustc/middle/borrowck/mod.rs

@@ -261,7 +261,7 @@ pub enum LoanPath {
 
 #[deriving(Eq, IterBytes)]
 pub enum LoanPathElem {
-    LpDeref,                     // `*LV` in doc.rs
+    LpDeref(mc::PointerKind),    // `*LV` in doc.rs
     LpInterior(mc::InteriorKind) // `LV.f` in doc.rs
 }
 
@@ -295,9 +295,9 @@ pub fn opt_loan_path(cmt: mc::cmt) -> Option<@LoanPath> {
             Some(@LpVar(id))
         }
 
-        mc::cat_deref(cmt_base, _, _) => {
+        mc::cat_deref(cmt_base, _, pk) => {
             do opt_loan_path(cmt_base).map_move |lp| {
-                @LpExtend(lp, cmt.mutbl, LpDeref)
+                @LpExtend(lp, cmt.mutbl, LpDeref(pk))
             }
         }
 
@@ -728,7 +728,7 @@ impl BorrowckCtxt {
                                                  loan_path: &LoanPath,
                                                  out: &mut ~str) {
         match *loan_path {
-            LpExtend(_, _, LpDeref) => {
+            LpExtend(_, _, LpDeref(_)) => {
                 out.push_char('(');
                 self.append_loan_path_to_str(loan_path, out);
                 out.push_char(')');
@@ -776,7 +776,7 @@ impl BorrowckCtxt {
                 out.push_str("[]");
             }
 
-            LpExtend(lp_base, _, LpDeref) => {
+            LpExtend(lp_base, _, LpDeref(_)) => {
                 out.push_char('*');
                 self.append_loan_path_to_str(lp_base, out);
             }
@@ -854,7 +854,7 @@ impl Repr for LoanPath {
                 fmt!("$(%?)", id)
             }
 
-            &LpExtend(lp, _, LpDeref) => {
+            &LpExtend(lp, _, LpDeref(_)) => {
                 fmt!("%s.*", lp.repr(tcx))
             }
 

src/librustc/middle/mem_categorization.rs

@@ -59,18 +59,18 @@ use syntax::print::pprust;
 
 #[deriving(Eq)]
 pub enum categorization {
-    cat_rvalue(ast::NodeId),          // temporary val, argument is its scope
+    cat_rvalue(ast::NodeId),           // temporary val, argument is its scope
     cat_static_item,
     cat_implicit_self,
     cat_copied_upvar(CopiedUpvar),     // upvar copied into @fn or ~fn env
     cat_stack_upvar(cmt),              // by ref upvar from &fn
-    cat_local(ast::NodeId),           // local variable
-    cat_arg(ast::NodeId),             // formal argument
-    cat_deref(cmt, uint, ptr_kind),    // deref of a ptr
+    cat_local(ast::NodeId),            // local variable
+    cat_arg(ast::NodeId),              // formal argument
+    cat_deref(cmt, uint, PointerKind), // deref of a ptr
     cat_interior(cmt, InteriorKind),   // something interior: field, tuple, etc
     cat_downcast(cmt),                 // selects a particular enum variant (*)
-    cat_discr(cmt, ast::NodeId),      // match discriminant (see preserve())
-    cat_self(ast::NodeId),            // explicit `self`
+    cat_discr(cmt, ast::NodeId),       // match discriminant (see preserve())
+    cat_self(ast::NodeId),             // explicit `self`
 
     // (*) downcast is only required if the enum has more than one variant
 }
@@ -82,8 +82,8 @@ pub struct CopiedUpvar {
 }
 
 // different kinds of pointers:
-#[deriving(Eq)]
-pub enum ptr_kind {
+#[deriving(Eq, IterBytes)]
+pub enum PointerKind {
     uniq_ptr,
     gc_ptr(ast::mutability),
     region_ptr(ast::mutability, ty::Region),
@@ -147,7 +147,7 @@ pub type cmt = @cmt_;
 // We pun on *T to mean both actual deref of a ptr as well
 // as accessing of components:
 pub enum deref_kind {
-    deref_ptr(ptr_kind),
+    deref_ptr(PointerKind),
     deref_interior(InteriorKind),
 }
 
@@ -1233,7 +1233,7 @@ impl Repr for categorization {
     }
 }
 
-pub fn ptr_sigil(ptr: ptr_kind) -> ~str {
+pub fn ptr_sigil(ptr: PointerKind) -> ~str {
     match ptr {
         uniq_ptr => ~"~",
         gc_ptr(_) => ~"@",

src/test/compile-fail/borrowck-assign-to-andmut-in-borrowed-loc.rs

@@ -0,0 +1,31 @@
+// Copyright 2012 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.
+
+// Test that assignments to an `&mut` pointer which is found in a
+// borrowed (but otherwise non-aliasable) location is illegal.
+
+struct S<'self> {
+    pointer: &'self mut int
+}
+
+fn copy_borrowed_ptr<'a>(p: &'a mut S<'a>) -> S<'a> {
+    S { pointer: &mut *p.pointer }
+}
+
+fn main() {
+    let mut x = 1;
+
+    {
+        let mut y = S { pointer: &mut x };
+        let z = copy_borrowed_ptr(&mut y);
+        *y.pointer += 1; //~ ERROR cannot assign
+        *z.pointer += 1;
+    }
+}