rust error callback

Rather then throw things directly, introduce a global callback.
We will override the callback in workerd to integrate with
kj::Exception

Author: mikea

gen/src/builtin.rs

@@ -350,11 +350,11 @@ pub(super) fn write(out: &mut OutFile) {
         writeln!(out, "template <>");
         writeln!(out, "class impl<Error> final {{");
         writeln!(out, "public:");
-        writeln!(out, "  static Error error(repr::PtrLen repr) noexcept {{");
+        writeln!(out, "  static void error(const char* msg, size_t len) {{");
         writeln!(out, "    Error error;");
-        writeln!(out, "    error.msg = static_cast<char const *>(repr.ptr);");
-        writeln!(out, "    error.len = repr.len;");
-        writeln!(out, "    return error;");
+        writeln!(out, "    error.msg = msg;");
+        writeln!(out, "    error.len = len;");
+        writeln!(out, "    throw error;");
         writeln!(out, "  }}");
         writeln!(out, "}};");
     }
@@ -393,6 +393,15 @@ pub(super) fn write(out: &mut OutFile) {
     }
 
     out.end_block(Block::AnonymousNamespace);
+
+    if builtin.rust_error {
+        out.next_section();
+        writeln!(
+            out,
+            "inline void (*throw_rust_error)(const char*, size_t) = impl<Error>::error;"
+        );
+    }
+
     out.end_block(Block::InlineNamespace("cxxbridge1"));
 
     if builtin.trycatch {

gen/src/write.rs

@@ -1103,7 +1103,10 @@ fn write_rust_function_shim_impl(
     writeln!(out, ";");
     out.builtin.rust_error = true;
     writeln!(out, "  if (error$.ptr) {{");
-    writeln!(out, "    throw ::rust::impl<::rust::Error>::error(error$);");
+    writeln!(
+        out,
+        "    ::rust::throw_rust_error(static_cast<char const *>(error$.ptr), error$.len);"
+    );
     writeln!(out, "  }}");
 
     if indirect_return {

include/cxx.h

@@ -30,6 +30,10 @@ template <typename T>
 class impl;
 }
 
+// msg is 0-terminated, size counts the 0.
+// msg was new char[] allocated, ownership is transferred to throw_rust_error.
+extern void (*throw_rust_error)(const char* msg, size_t size);
+
 #ifndef CXXBRIDGE1_RUST_STRING
 #define CXXBRIDGE1_RUST_STRING
 // https://cxx.rs/binding/string.html

tests/ffi/tests.cc

@@ -2,6 +2,7 @@
 #include "tests/ffi/lib.rs.h"
 #include <cstdlib>
 #include <cstring>
+#include <exception>
 #include <iterator>
 #include <memory>
 #include <numeric>
@@ -757,6 +758,20 @@ std::unique_ptr<::F::F> c_return_ns_opaque_ptr() {
   return f;
 }
 
+struct TestException : public std::exception {
+  TestException(std::string&& msg) : message(std::move(msg)) {}
+
+  static void do_throw(const char* msg, size_t len) {
+    throw TestException(std::string(msg, len));
+  }
+
+  const char* what() const noexcept override {
+    return message.c_str();
+  }
+
+  std::string message;
+};
+
 extern "C" const char *cxx_run_test() noexcept {
 #define STRINGIFY(x) #x
 #define TOSTRING(x) STRINGIFY(x)
@@ -911,6 +926,17 @@ extern "C" const char *cxx_run_test() noexcept {
     ASSERT(std::strcmp(e.what(), "panic in cxx_test_suite::ffi::r_panic: foobar") == 0);
   }
 
+  // Test custom exception handler
+  auto prev_handler = ::rust::throw_rust_error;
+  ::rust::throw_rust_error = TestException::do_throw;
+    try {
+    r_fail_return_primitive();
+    ASSERT(false);
+  } catch (const TestException &e) {
+    ASSERT(std::strcmp(e.what(), "rust error") == 0);
+  }
+  ::rust::throw_rust_error = prev_handler;
+
   cxx_test_suite_set_correct();
   return nullptr;
 }