librustc: Allow mutable box substructure to be borrowed without requiring purity. Closes #4825.

Author: pcwalton

src/librustc/middle/borrowck/gather_loans.rs

@@ -455,7 +455,7 @@ impl gather_loan_ctxt {
             if req_mutbl == m_imm {
                 // if this is an @mut box, then it's generally OK to borrow as
                 // &imm; this will result in a write guard
-                if cmt.cat.is_mutable_box() {
+                if cmt.cat.derefs_through_mutable_box() {
                     Ok(PcOk)
                 } else {
                     // you can treat mutable things as imm if you are pure

src/librustc/middle/borrowck/preserve.rs

@@ -191,6 +191,8 @@ impl PreserveCtxt {
             // live, so we must root the pointer (i.e., inc the ref
             // count) for the duration of the loan.
             debug!("base.mutbl = %?", self.bccx.mut_to_str(base.mutbl));
+            debug!("derefs through mutable box? %?",
+                   cmt.cat.derefs_through_mutable_box());
             if cmt.cat.derefs_through_mutable_box() {
                 self.attempt_root(cmt, base, derefs)
             } else if base.mutbl == m_imm {

src/test/run-pass/borrowck-wg-mut-box-substructure.rs

@@ -0,0 +1,11 @@
+struct Foo {
+    x: ~[int]
+}
+
+fn main() {
+    let x = @mut Foo { x: ~[ 1, 2, 3, 4, 5 ] };
+    for x.x.each |x| {
+        io::println(x.to_str());
+    }
+}
+