gccrs: fix ICE for empty enum variant

gcc/rust/ChangeLog:

	* rust-gcc.cc (constructor_expression):
	Ensure vec_alloc reserves at least one element.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3947.rs: New test.

Signed-off-by: lishin <lishin1008@gmail.com>

Author: Lishin1215

gcc/rust/rust-gcc.cc

@@ -1258,7 +1258,7 @@ constructor_expression (tree type_tree, bool is_variant,
     return error_mark_node;
 
   vec<constructor_elt, va_gc> *init;
-  vec_alloc (init, vals.size ());
+  vec_alloc (init, union_index != -1 ? 1 : vals.size ());
 
   tree sink = NULL_TREE;
   bool is_constant = true;

gcc/testsuite/rust/compile/issue-3947.rs

@@ -0,0 +1,10 @@
+enum _Enum {
+    A(),
+}
+
+type _E = _Enum;
+
+// { dg-warning "function is never used: '_a'" "" { target *-*-* } .+1 }
+const fn _a() -> _Enum {
+    _E::A()
+}