Support as casts in abstract consts

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-2.rs

@@ -0,0 +1,13 @@
+#![feature(const_evaluatable_checked, const_generics)]
+#![allow(incomplete_features)]
+
+trait Evaluatable<const N: u128> {}
+impl<const N: u128> Evaluatable<N> for () {}
+
+struct Foo<const N: u8>([u8; N as usize])
+//~^ Error: unconstrained generic constant
+//~| help: try adding a `where` bound using this expression: `where [(); N as usize]:`
+where
+    (): Evaluatable<{N as u128}>;
+
+fn main() {}

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-2.stderr

@@ -0,0 +1,10 @@
+error: unconstrained generic constant
+  --> $DIR/abstract-const-as-cast-2.rs:7:25
+   |
+LL | struct Foo<const N: u8>([u8; N as usize])
+   |                         ^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); N as usize]:`
+
+error: aborting due to previous error
+

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast.rs

@@ -0,0 +1,9 @@
+// check-pass
+#![feature(const_evaluatable_checked, const_generics)]
+#![allow(incomplete_features)]
+
+struct Foo<const N: u8>([u8; N as usize])
+where
+    [(); N as usize]:;
+
+fn main() {}