Don't unify from mutable? to immutable

Immutable is supposed to be a subtype of mutable-huh.

src/comp/middle/ty.rs

@@ -1952,7 +1952,6 @@ mod unify {
        option::t<ast::mutability> {
         if expected == actual { ret some(expected); }
         if expected == ast::maybe_mut { ret some(actual); }
-        if actual == ast::maybe_mut { ret some(expected); }
         ret none;
     }
     tag fn_common_res {

src/lib/vec.rs

@@ -5,7 +5,7 @@ import uint::next_power_of_two;
 import ptr::addr_of;
 
 native "rust-intrinsic" mod rusti {
-    fn vec_len<T>(&&v: [T]) -> uint;
+    fn vec_len<T>(&&v: [mutable? T]) -> uint;
 }
 
 native "rust" mod rustrt {
@@ -86,7 +86,7 @@ pure fn is_not_empty<T>(v: [mutable? T]) -> bool { ret !is_empty(v); }
 fn head<@T>(v: [mutable? T]) : is_not_empty(v) -> T { ret v[0]; }
 
 /// Returns all but the first element of a vector
-fn tail<@T>(v: [mutable? T]) : is_not_empty(v) -> [mutable? T] {
+fn tail<@T>(v: [mutable? T]) : is_not_empty(v) -> [T] {
     ret slice(v, 1u, len(v));
 }