correct preservation of explicit borrows like "let x = &*y;"

src/rustc/middle/borrowck.rs

@@ -1481,7 +1481,7 @@ impl preserve_methods for preserve_ctxt {
             // Unsafe pointers are the user's problem
             ok(())
           }
-          cat_deref(_, derefs, gc_ptr) {
+          cat_deref(base, derefs, gc_ptr) {
             // GC'd pointers of type @MT: always stable because we can inc
             // the ref count or keep a GC root as necessary.  We need to
             // insert this id into the root_map, however.
@@ -1489,9 +1489,9 @@ impl preserve_methods for preserve_ctxt {
               some(scope_id) {
                 #debug["Inserting root map entry for %s: \
                         node %d:%u -> scope %d",
-                       self.bccx.cmt_to_repr(cmt), cmt.id,
+                       self.bccx.cmt_to_repr(cmt), base.id,
                        derefs, scope_id];
-                let rk = {id: cmt.id, derefs: derefs};
+                let rk = {id: base.id, derefs: derefs};
                 self.bccx.root_map.insert(rk, scope_id);
                 ok(())
               }

src/test/compile-fail/regions-appearance-constraint.rs

@@ -0,0 +1,35 @@
+/*
+
+Tests that borrowing always produces a pointer confined to the
+innermost scope.  In this case, the variable `a` gets inferred
+to the lifetime of the `if` statement because it is assigned
+a borrow of `y` which takes place within the `if`.
+
+Note: If this constraint were lifted (as I contemplated at one point),
+it complicates the preservation mechanics in trans, though not
+irreperably.  I'm partially including this test so that if these
+semantics do change we'll remember to test this scenario.
+
+*/
+
+fn testfn(cond: bool) {
+    let mut x = @3;
+    let mut y = @4;
+
+    let mut a = &*x;
+    //!^ ERROR reference is not valid outside of its lifetime
+
+    let mut exp = 3;
+    if cond {
+        a = &*y;
+
+        exp = 4;
+    }
+
+    x = @5;
+    y = @6;
+    assert *a == exp;
+}
+
+fn main() {
+}

src/test/run-pass/borrowck-preserve-cond-box.rs

@@ -0,0 +1,31 @@
+// compile-flags:--borrowck=err
+// exec-env:RUST_POISON_ON_FREE=1
+
+fn testfn(cond: bool) {
+    let mut x = @3;
+    let mut y = @4;
+
+    // borrow x and y
+    let mut r_x = &*x;
+    let mut r_y = &*y;
+    let mut r = r_x, exp = 3;
+
+    if cond {
+        r = r_y;
+        exp = 4;
+    }
+
+    #debug["*r = %d, exp = %d", *r, exp];
+    assert *r == exp;
+
+    x = @5;
+    y = @6;
+
+    #debug["*r = %d, exp = %d", *r, exp];
+    assert *r == exp;
+}
+
+fn main() {
+    testfn(true);
+    testfn(false);
+}