From 559444cc66c787327c49e97b08ec88d96bdc9996 Mon Sep 17 00:00:00 2001
From: Googler <no-reply@google.com>
Date: Mon, 30 Mar 2026 09:57:08 -0700
Subject: [PATCH] Relax std::is_default_constructible_v requirement for C9Co
 Crubit ABIs

Currently, `c9::internal::rust::EncodeResultInCrubitBuffer` requires the Crubit ABI type to be default constructible. This breaks for complex ABI types like `rs_std::DynCallable` which requires initialization arguments.

This change delegates the ABI instantiation to the caller (either the Rust binding generator or the `CoCrubitAbi` fallback constructor). Because some ABI types are not "structural types" in C++20 and cannot be used as template arguments directly, we pass a closure type (`AbiFactory`) to `StartCoroutineFromRust` that constructs the ABI upon invocation.

PiperOrigin-RevId: 891772056
---
 rs_bindings_from_cc/generate_bindings/lib.rs | 23 +++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/rs_bindings_from_cc/generate_bindings/lib.rs b/rs_bindings_from_cc/generate_bindings/lib.rs
index b6cbaa1c8..f3fd23c3c 100644
--- a/rs_bindings_from_cc/generate_bindings/lib.rs
+++ b/rs_bindings_from_cc/generate_bindings/lib.rs
@@ -1188,9 +1188,26 @@ fn crubit_abi_type(db: &BindingsGenerator, rs_type_kind: RsTypeKind) -> Result<C
                     }
                 };
 
-                // The default constructor knows what `start_coroutine` and `destroy_at_initial_suspend`
-                // functions to use.
-                let cpp_expr_tokens = quote! { #cpp_type_tokens() };
+                let cpp_expr_tokens = {
+                    let start_coroutine_fn_tokens = match &result_type_crubit_abi_type {
+                        None => {
+                            quote! {
+                                &::c9::internal::rust::StartCoroutineFromRust
+                            }
+                        }
+                        Some(result_type_crubit_abi_type) => {
+                            let abi_expr_tokens =
+                                CrubitAbiTypeToCppExprTokens(result_type_crubit_abi_type);
+
+                            quote! {
+                                &::c9::internal::rust::StartCoroutineFromRust<[]() { return #abi_expr_tokens; }>
+                            }
+                        }
+                    };
+                    quote! {
+                        #cpp_type_tokens(#start_coroutine_fn_tokens)
+                    }
+                };
 
                 Ok(CrubitAbiType::C9Co {
                     rust_type_tokens,