Silence TooGeneric error

This error may be produced during intermediate failed attempts at evaluation of a generic const, which may nevertheless succeed later.

src/test/ui/const-generics/array-size-in-generic-struct-param.rs

@@ -0,0 +1,23 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+#[allow(dead_code)]
+struct ArithArrayLen<const N: usize>([u32; 0 + N]); // ok
+
+#[derive(PartialEq, Eq)]
+struct Config {
+    arr_size: usize,
+}
+
+struct B<const CFG: Config> {
+    arr: [u8; CFG.arr_size], // ok
+}
+
+const C: Config = Config { arr_size: 5 };
+
+fn main() {
+    let b = B::<C> { arr: [1, 2, 3, 4, 5] };
+    assert_eq!(b.arr.len(), 5);
+}