---

yaml
---
r: 27692
b: refs/heads/try
c: 22a14dd
h: refs/heads/master
v: v3

Author: nikomatsakis

[refs]

@@ -2,7 +2,7 @@
 refs/heads/master: cd6f24f9d14ac90d167386a56e7a6ac1f0318195
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: cd6f24f9d14ac90d167386a56e7a6ac1f0318195
-refs/heads/try: be2e4ef624acdeb260bc1ba86e9a357b30775baa
+refs/heads/try: 22a14dd700338ed26f78143dfad4325566a3f4ed
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/ndm: f3868061cd7988080c30d6d5bf352a5a5fe2460b
 refs/heads/try2: d0c6ce338884ee21843f4b40bf6bf18d222ce5df

branches/try/src/rustc/middle/borrowck/check_loans.rs

@@ -322,10 +322,32 @@ impl check_loan_ctxt {
         // is not visible from the outside
         match self.purity(ex.id) {
           none => (),
-          some(pc) => {
-            if cmt.lp.is_none() {
+          some(pc @ pc_cmt(_)) => {
+            // Subtle: Issue #3162.  If we are enforcing purity
+            // because there is a reference to aliasable, mutable data
+            // that we require to be immutable, we can't allow writes
+            // even to data owned by the current stack frame.  This is
+            // because that aliasable data might have been located on
+            // the current stack frame, we don't know.
+            match cmt.lp {
+              some(@lp_local(*)) | some(@lp_arg(*)) => {
+                // it's ok to mutate a local variable, as it is either
+                // lent our or not.  The problem arises when you have
+                // some subcomponent that might have been lent out
+                // through an alias on the condition that you ensure
+                // purity.
+              }
+              none | some(@lp_comp(*)) | some(@lp_deref(*)) => {
                 self.report_purity_error(
                     pc, ex.span, at.ing_form(self.bccx.cmt_to_str(cmt)));
+              }
+            }
+          }
+          some(pc_pure_fn) => {
+            if cmt.lp.is_none() {
+                self.report_purity_error(
+                    pc_pure_fn, ex.span,
+                    at.ing_form(self.bccx.cmt_to_str(cmt)));
             }
           }
         }

branches/try/src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162-b.rs

@@ -0,0 +1,12 @@
+fn each<T>(x: &[T], op: fn(elem: &T) -> bool) {
+    uint::range(0, x.len(), |i| op(&x[i]));
+}
+
+fn main() {
+    let x = [{mut a: 0}];
+    for each(x) |y| {
+        let z = &y.a; //~ ERROR illegal borrow unless pure
+        x[0].a = 10; //~ NOTE impure due to assigning to mutable field
+        log(error, z);
+    }
+}

branches/try/src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162.rs

@@ -0,0 +1,12 @@
+fn each<T>(x: &[T], op: fn(elem: &T) -> bool) {
+    uint::range(0, x.len(), |i| op(&x[i]));
+}
+
+fn main() {
+    let x = ~[{mut a: 0}];
+    for each(x) |y| {
+        let z = &y.a; //~ ERROR illegal borrow unless pure
+        x[0].a = 10; //~ NOTE impure due to assigning to mutable field
+        log(error, z);
+    }
+}