librustc: Fix silly bug in AST conversion for const vstores. rs=bugfix

Means that we'll need another snapshot to rid the language of `[const T]`.

src/librustc/middle/typeck/astconv.rs

@@ -214,7 +214,7 @@ pub fn ast_ty_to_ty<AC:AstConv, RS:region_scope + Copy + Durable>(
                 let mut mt = ast_mt_to_mt(self, rscope, mt);
                 if a_seq_ty.mutbl == ast::m_mutbl ||
                         a_seq_ty.mutbl == ast::m_const {
-                    mt = ty::mt { ty: mt.ty, mutbl: ast::m_mutbl };
+                    mt = ty::mt { ty: mt.ty, mutbl: a_seq_ty.mutbl };
                 }
                 return ty::mk_evec(tcx, mt, vst);
             }

src/test/run-pass/const-vec-syntax.rs

@@ -0,0 +1,7 @@
+fn f(_: &const [int]) {}
+
+fn main() {
+    let v = [ 1, 2, 3 ];
+    f(v);
+}
+