Inherit mutability through fixed-length vectors.

Fixes #3226.  No review (one line bug fix).

Author: nikomatsakis

src/librustc/middle/mem_categorization.rs

@@ -797,7 +797,8 @@ impl &mem_categorization_ctxt {
 
           deref_comp(_) => {
             // fixed-length vectors have no deref
-            comp(expr, base_cmt, base_cmt.ty, mt.mutbl, mt.ty)
+            let m = self.inherited_mutability(base_cmt.mutbl, mt.mutbl);
+            comp(expr, base_cmt, base_cmt.ty, m, mt.ty)
           }
         };
 

src/test/run-pass/mutability-inherits-through-fixed-length-vec.rs

@@ -0,0 +1,16 @@
+fn test1() {
+    let mut ints = [0, ..32];
+    ints[0] += 1;
+    assert ints[0] == 1;
+}
+
+fn test2() {
+    let mut ints = [0, ..32];
+    for vec::each_mut(ints) |i| { *i += 22; }
+    for ints.each |i| { assert *i == 22; }
+}
+
+fn main() {
+    test1();
+    test2();
+}