rustc: Don't try to load dynamically-sized types when translating tag variants. Add a test case for this.

src/comp/middle/trans.rs

@@ -4920,7 +4920,8 @@ fn trans_tag_variant(@crate_ctxt cx, ast.def_id tag_id,
 
         auto arg_ty = arg_tys.(i).ty;
         auto llargval;
-        if (ty.type_is_structural(arg_ty)) {
+        if (ty.type_is_structural(arg_ty) ||
+                ty.type_has_dynamic_size(arg_ty)) {
             llargval = llargptr;
         } else {
             llargval = bcx.build.Load(llargptr);

src/test/run-pass/simple-generic-tag.rs

@@ -0,0 +1,7 @@
+tag clam[T] {
+    a(T);
+}
+
+fn main() {
+}
+