add test

src/test/ui/const-generics/issues/issue-90455.rs

@@ -0,0 +1,12 @@
+#![feature(generic_const_exprs, adt_const_params)]
+#![allow(incomplete_features)]
+
+struct FieldElement<const N: &'static str> {
+    n: [u64; num_limbs(N)],
+    //~^ ERROR unconstrained generic constant
+}
+const fn num_limbs(_: &str) -> usize {
+    0
+}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-90455.stderr

@@ -0,0 +1,10 @@
+error: unconstrained generic constant
+  --> $DIR/issue-90455.rs:5:8
+   |
+LL |     n: [u64; num_limbs(N)],
+   |        ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); num_limbs(N)]:`
+
+error: aborting due to previous error
+