From 22a14dd700338ed26f78143dfad4325566a3f4ed Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Thu, 9 Aug 2012 14:18:43 -0700
Subject: [PATCH] Enforce a stricter notion of purity when borrowing.  Fixes
 #3162.

---
 src/rustc/middle/borrowck/check_loans.rs      | 26 +++++++++++++++++--
 ...k-imm-ref-to-mut-rec-field-issue-3162-b.rs | 12 +++++++++
 ...wck-imm-ref-to-mut-rec-field-issue-3162.rs | 12 +++++++++
 3 files changed, 48 insertions(+), 2 deletions(-)
 create mode 100644 src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162-b.rs
 create mode 100644 src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162.rs

diff --git a/src/rustc/middle/borrowck/check_loans.rs b/src/rustc/middle/borrowck/check_loans.rs
index 4c596a343544..301c78aa1094 100644
--- a/src/rustc/middle/borrowck/check_loans.rs
+++ b/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)));
             }
           }
         }
diff --git a/src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162-b.rs b/src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162-b.rs
new file mode 100644
index 000000000000..3c0a53130b1d
--- /dev/null
+++ b/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);
+    }
+}
diff --git a/src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162.rs b/src/test/compile-fail/borrowck-imm-ref-to-mut-rec-field-issue-3162.rs
new file mode 100644
index 000000000000..fa31ca4340be
--- /dev/null
+++ b/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);
+    }
+}