tests: Add tests that use const fns.

Author: davidtwco

src/test/ui/consts/rfc-2203-const-array-repeat-exprs/const-fns.rs

@@ -0,0 +1,26 @@
+// ignore-tidy-linelength
+// ignore-compare-mode-nll
+#![feature(const_in_array_repeat_expressions, nll)]
+#![allow(warnings)]
+
+// Some type that is not copyable.
+struct Bar;
+
+const fn type_no_copy() -> Option<Bar> {
+    None
+}
+
+const fn type_copy() -> u32 {
+    3
+}
+
+fn no_copy() {
+    const ARR: [Option<Bar>; 2] = [type_no_copy(); 2];
+    //~^ ERROR the trait bound `std::option::Option<Bar>: std::marker::Copy` is not satisfied [E0277]
+}
+
+fn copy() {
+    const ARR: [u32; 2] = [type_copy(); 2];
+}
+
+fn main() {}

src/test/ui/consts/rfc-2203-const-array-repeat-exprs/const-fns.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `std::option::Option<Bar>: std::marker::Copy` is not satisfied
+  --> $DIR/const-fns.rs:18:35
+   |
+LL |     const ARR: [Option<Bar>; 2] = [type_no_copy(); 2];
+   |                                   ^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `std::option::Option<Bar>`
+   |
+   = help: the following implementations were found:
+             <std::option::Option<T> as std::marker::Copy>
+   = note: the `Copy` trait is required because the repeated element will be copied
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.