diff --git a/lib/llvm-backend/cpp/object_loader.hh b/lib/llvm-backend/cpp/object_loader.hh
index 0d6bcc603..f1b53f156 100644
--- a/lib/llvm-backend/cpp/object_loader.hh
+++ b/lib/llvm-backend/cpp/object_loader.hh
@@ -61,15 +61,15 @@ struct UncatchableException : WasmException
 struct UserException : UncatchableException
 {
   public:
-    UserException(std::string msg) : msg(msg) {}
+    UserException(size_t data, size_t vtable) : data(data), vtable(vtable) {}
 
     virtual std::string description() const noexcept override
     {
-        return std::string("user exception: ") + msg;
+        return "user exception";
     }
 
-  private:
-    std::string msg;
+    // The parts of a `Box<dyn Any>`.
+    size_t data, vtable;
 };
 
 struct WasmTrap : UncatchableException
@@ -176,6 +176,12 @@ extern "C"
         delete module;
     }
 
+    // Throw a fat pointer that's assumed to be `*mut dyn Any` on the rust
+    // side.
+    [[noreturn]] void throw_any(size_t data, size_t vtable) {
+        throw UserException(data, vtable);
+    }
+
     bool invoke_trampoline(
         trampoline_t trampoline,
         void *ctx,
diff --git a/lib/llvm-backend/src/backend.rs b/lib/llvm-backend/src/backend.rs
index 1a82cfaf2..f40a6dfb2 100644
--- a/lib/llvm-backend/src/backend.rs
+++ b/lib/llvm-backend/src/backend.rs
@@ -75,7 +75,11 @@ extern "C" {
     fn module_delete(module: *mut LLVMModule);
     fn get_func_symbol(module: *mut LLVMModule, name: *const c_char) -> *const vm::Func;
 
-    fn throw_trap(ty: i32);
+    fn throw_trap(ty: i32) -> !;
+
+    /// This should be the same as spliting up the fat pointer into two arguments,
+    /// but this is cleaner, I think?
+    fn throw_any(data: *mut dyn Any) -> !;
 
     #[allow(improper_ctypes)]
     fn invoke_trampoline(
@@ -433,8 +437,8 @@ impl ProtectedCaller for LLVMProtectedCaller {
 }
 
 impl UserTrapper for Placeholder {
-    unsafe fn do_early_trap(&self, _data: Box<dyn Any>) -> ! {
-        unimplemented!("do early trap")
+    unsafe fn do_early_trap(&self, data: Box<dyn Any>) -> ! {
+        throw_any(Box::leak(data))
     }
 }
 
diff --git a/lib/runtime/examples/call.rs b/lib/runtime/examples/call.rs
index 104fa9692..12f149062 100644
--- a/lib/runtime/examples/call.rs
+++ b/lib/runtime/examples/call.rs
@@ -32,11 +32,11 @@ fn get_wasm() -> Vec<u8> {
     wat2wasm(WAT).unwrap()
 }
 
-fn foobar(ctx: &mut Ctx) -> i32 {
+fn foobar(_ctx: &mut Ctx) -> i32 {
     42
 }
 
-fn do_panic(ctx: &mut Ctx) -> Result<i32, String> {
+fn do_panic(_ctx: &mut Ctx) -> Result<i32, String> {
     Err("error".to_string())
 }