add tests for fn pointers

tests/compile-fail/deref_fn_ptr.rs

@@ -0,0 +1,13 @@
+#![feature(custom_attribute)]
+#![allow(dead_code, unused_attributes)]
+
+fn f() {}
+
+#[miri_run]
+fn deref_fn_ptr() -> i32 {
+    unsafe {
+        *std::mem::transmute::<fn(), *const i32>(f) //~ ERROR: tried to dereference a function pointer
+    }
+}
+
+fn main() {}

tests/compile-fail/execute_memory.rs

@@ -0,0 +1,14 @@
+#![feature(custom_attribute, box_syntax)]
+#![allow(dead_code, unused_attributes)]
+
+#[miri_run]
+fn deref_fn_ptr() {
+    //FIXME: this span is wrong
+    let x = box 42; //~ ERROR: tried to treat a memory pointer as a function pointer
+    unsafe {
+        let f = std::mem::transmute::<Box<i32>, fn()>(x);
+        f()
+    }
+}
+
+fn main() {}