Add test for issue-71381

Author: JohnTitor

src/test/ui/const-generics/issues/issue-71381.rs

@@ -0,0 +1,33 @@
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+struct Test(*const usize);
+
+type PassArg = ();
+
+unsafe extern "C" fn pass(args: PassArg) {
+    println!("Hello, world!");
+}
+
+impl Test {
+    pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "C" fn(Args)>(&self) {
+        //~^ ERROR: using function pointers as const generic parameters is forbidden
+        self.0 = Self::trampiline::<Args, IDX, FN> as _
+    }
+
+    unsafe extern "C" fn trampiline<
+        Args: Sized,
+        const IDX: usize,
+        const FN: unsafe extern "C" fn(Args),
+        //~^ ERROR: using function pointers as const generic parameters is forbidden
+    >(
+        args: Args,
+    ) {
+        FN(args)
+    }
+}
+
+fn main() {
+    let x = Test();
+    x.call_me::<PassArg, 30, pass>()
+}

src/test/ui/const-generics/issues/issue-71381.stderr

@@ -0,0 +1,14 @@
+error: using function pointers as const generic parameters is forbidden
+  --> $DIR/issue-71381.rs:13:61
+   |
+LL |     pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "C" fn(Args)>(&self) {
+   |                                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: using function pointers as const generic parameters is forbidden
+  --> $DIR/issue-71381.rs:21:19
+   |
+LL |         const FN: unsafe extern "C" fn(Args),
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+