We got a test working for panicking with the appropriate number fo failures. Ultimatly we want:

+ An error maessage that says what the failures were
+ Not panicking (so these are effectively error productions)

compiler/gen_llvm/src/llvm/externs.rs

@@ -42,6 +42,43 @@ pub fn add_default_roc_externs(env: &Env<'_, '_, '_>) {
         }
     }
 
+    // roc_memcpy
+    {
+        // The type of this function (but not the implementation) should have
+        // already been defined by the builtins, which rely on it.
+        let fn_val = module.get_function("roc_memcpy").unwrap();
+        let mut params = fn_val.get_param_iter();
+        let dest_arg = params.next().unwrap();
+        let dest_alignment = 1;
+        let src_arg = params.next().unwrap();
+        let src_alignment = 1;
+        let bytes_arg = params.next().unwrap();
+
+        debug_assert!(params.next().is_none());
+
+        // Add a basic block for the entry point
+        let entry = ctx.append_basic_block(fn_val, "entry");
+
+        builder.position_at_end(entry);
+
+        // Call libc memcpy()
+        let _retval = builder
+            .build_memcpy(
+                dest_arg.into_pointer_value(),
+                dest_alignment,
+                src_arg.into_pointer_value(),
+                src_alignment,
+                bytes_arg.into_int_value(),
+            )
+            .unwrap();
+
+        builder.build_return(None);
+
+        if cfg!(debug_assertions) {
+            crate::llvm::build::verify_fn(fn_val);
+        }
+    }
+
     // roc_realloc
     {
         let libc_realloc_val = {