Add test for deriving Debug for const generics

src/test/ui/const-generics/derive-debug-array-wrapper.rs

@@ -0,0 +1,9 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+#[derive(Debug)]
+struct X<const N: usize> {
+    a: [u32; N], //~ ERROR `[u32; _]` doesn't implement `std::fmt::Debug`
+}
+
+fn main() {}

src/test/ui/const-generics/derive-debug-array-wrapper.stderr

@@ -0,0 +1,19 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/derive-debug-array-wrapper.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error[E0277]: `[u32; _]` doesn't implement `std::fmt::Debug`
+  --> $DIR/derive-debug-array-wrapper.rs:6:5
+   |
+LL |     a: [u32; N],
+   |     ^^^^^^^^^^^ `[u32; _]` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `[u32; _]`
+   = note: required because of the requirements on the impl of `std::fmt::Debug` for `&[u32; _]`
+   = note: required for the cast to the object type `dyn std::fmt::Debug`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.