[2kai2kai2]

• Adds class method bindings (both static and instance). Like other bridges, these copy-in self but have no copy-out so mutations are not reflected in the host value.
• Adds unbounded generics (items that rely on interface-bounded generics are currently omitted) for functions, methods, and classes, including generated enums for unions containing type args
• Adds host callback + host value handle mapping, allowing a typed rust callback to be passed into a BAML function and executed correctly. They may raise host errors which are opaquely passed through BAML as a handle and can be rehydrated when a handle is passed back to BAML.
• Codegen for SDKs now correctly provides the implicit generic parameters created by function-typed parameters with no explicit throws clause. While type-erased in Python/TS due to their type safety limitations, Rust can generate and infer them in generic positions.

Summary by CodeRabbit

• New Features
  • Expanded Rust SDK support for generics across classes, functions, methods, and unions, including supplying explicit type arguments in calls.
  • Added a Rust host-value bridge module for host callables and opaque host errors, with improved callback-throws behavior.
• Bug Fixes
  • Improved Rust throws/callback-throws inference, including correct catch/rethrow error surfaces and typed vs opaque host error decoding.
• Documentation
  • Updated generated Rust docs to consistently use an # Errors section for throwing APIs.
• Tests
  • Significantly expanded/updated Rust SDK tests and fixtures for generics, host callables, and throws/callback-throws behavior.

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Project	Deployment	Actions	Updated (UTC)
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[github-actions]

⏭️ Performance benchmarks were skipped

Perf benchmarks (CodSpeed) are opt-in on pull requests — they no longer run on every push. They always run automatically after merge to canary/main.

To run them on this PR, do any of the following, then push a commit (or re-run CI):

• Add RUN_CODSPEED=1 to the PR description, or
• Include run-perf or /perf in the PR title or any commit message.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 1f4ce597-aeab-4c99-9905-e918aeab8735

📥 Commits

Reviewing files that changed from the base of the PR and between 36786b4 and eedc99f.

📒 Files selected for processing (1)

• baml_language/sdks/rust/bridge_rust/src/host_value.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• baml_language/sdks/rust/bridge_rust/src/host_value.rs

---

📝 Walkthrough

Walkthrough

This PR extends Rust SDK code generation and runtime support for generic types, generated methods, callback effect inference, host-callable dispatch, typed and opaque errors, and explicit wire type arguments. It also updates symbol traits, generated documentation, test fixtures, and Rust test gating.

Changes

Rust SDK code generation

Layer / File(s)	Summary
Signature lowering and effect parameters baml_language/crates/baml_codegen_types/src/symbols.rs, baml_language/crates/baml_project/src/client_codegen.rs, baml_language/sdks/rust/sdkgen_rust/src/effect_rename.rs	Symbol types derive Clone; function and method signatures use elaborated HIR data, and synthesized callback effect parameters are renamed before emission.
Generic analysis and generated bindings baml_language/sdks/rust/sdkgen_rust/src/{analyze.rs,translate_ty.rs,unions.rs}, baml_language/sdks/rust/sdkgen_rust/src/emit/*	Generic classes, TypeVars, generic unions, class methods, callback parameters, type metadata, and Rust # Errors documentation are emitted.
Wire metadata and invocation arguments baml_language/sdks/rust/bridge_rust/src/{baml_value.rs,encode.rs,runtime.rs,capi.rs}	Rust values provide wire BamlTy descriptors; class encoders and runtime invocations carry explicit generic type arguments, with host-dispatch ABI entries added.
Host-callable bridge baml_language/sdks/rust/bridge_rust/src/{host_value.rs,error.rs,lib.rs}	Callable handles, sync/async dispatch, optional argument slotting, typed throws, opaque host errors, rehydration, and related public APIs are implemented.
SDK and bridge validation baml_language/sdk_tests/**, baml_language/sdks/rust/bridge_rust/tests/*, baml_language/sdk_tests/harness_setup/src/rust.rs	Tests cover generic calls, generic round trips, host-callable behavior, callback-throws inference, error documentation, and newly enabled Rust test ports.

Estimated code review effort: 5 (Critical) | ~120 minutes

Sequence Diagram(s)

sequenceDiagram
  participant GeneratedBinding
  participant RustRuntime
  participant BAML_Engine
  participant HostCallableBridge
  GeneratedBinding->>RustRuntime: invoke with kwargs and type_args
  RustRuntime->>BAML_Engine: dispatch CallFunctionArgs
  BAML_Engine->>HostCallableBridge: dispatch registered callable
  HostCallableBridge->>HostCallableBridge: decode args and run closure
  HostCallableBridge->>BAML_Engine: complete host call with result or throw

Loading

Possibly related PRs

• BoundaryML/baml#3571: The Rust bridge work builds on the host-value handle, dispatch, and HostCallable error plumbing introduced by this PR.
• BoundaryML/baml#4033: The Rust bridge runtime and call envelope changes modify the same type-argument and bridge APIs.
• BoundaryML/baml#4087: The client-codegen signature-lowering refactor modifies the same function and method lowering pipeline.

Poem

A rabbit hops through types so bright,
Generic carrots take their flight.
Callbacks whisper, errors gleam,
Handles cross the BAML stream.
Rust docs bloom with # Errors true—
A fluffy bridge now carries through!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately captures the PR’s main additions: class methods, generics, and host callbacks.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch kai/rust-bridge

---

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[github-actions]

Binary size checks passed

✅ 7 passed

	Artifact	Platform	File	Gzip	Gated on	Baseline	Delta	Status
✅	baml-cli	Linux	🔒 25.5 MB	10.8 MB	file	25.3 MB	+218.5 KB (+0.9%)	OK
✅	packed-program	Linux	🔒 17.0 MB	7.0 MB	file	17.0 MB	+4.3 KB (+0.0%)	OK
✅	baml-cli	macOS	🔒 19.7 MB	9.4 MB	file	19.5 MB	+198.6 KB (+1.0%)	OK
✅	packed-program	macOS	🔒 13.2 MB	6.2 MB	file	13.2 MB	+16.7 KB (+0.1%)	OK
✅	baml-cli	Windows	🔒 21.3 MB	9.6 MB	file	21.1 MB	+206.8 KB (+1.0%)	OK
✅	packed-program	Windows	🔒 14.2 MB	6.2 MB	file	14.1 MB	+11.0 KB (+0.1%)	OK
✅	bridge_wasm	WASM	16.2 MB	🔒 4.4 MB	gzip	4.4 MB	-504 B (-0.0%)	OK

🔒 = the size this artifact is GATED on (ceiling + delta). Binaries gate on file size (installed binary); WASM gates on gzip (download size). The other size is shown for information only.

---

Generated by cargo size-gate · workflow run

[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (1)

baml_language/sdks/rust/bridge_rust/src/host_value.rs (1)

117-124: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Consider adding #[cfg(test)] unit tests for these pure helpers.

next_key/slot_args are pure, easily unit-testable (positional/named slotting, missing-required-arg error, wraparound-skip-zero). Per coding guidelines, prefer unit tests over integration tests where possible; today coverage for this logic lives only in the integration tests under bridge_rust/tests/conversions.rs.

As per coding guidelines: "Prefer writing Rust unit tests over integration tests where possible."

Also applies to: 556-588

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@baml_language/sdks/rust/bridge_rust/src/host_value.rs` around lines 117 -
124, Add a #[cfg(test)] unit-test module in host_value.rs covering the pure
helpers next_key and slot_args: verify positional and named slotting,
missing-required-argument errors, and next_key’s wraparound behavior skipping
zero. Keep the tests focused on these helpers and use existing test conventions
and symbols.

Source: Coding guidelines

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@baml_language/sdks/rust/bridge_rust/src/host_value.rs`:
- Around line 247-268: Update the closure invocation in the outer DISPATCH_RT
task to run through handle.spawn_blocking, await its JoinHandle, and preserve
the existing success, argument-decode failure, and panic/error completion paths.
Ensure the sync closure body executes off the async dispatch workers and that
any spawn_blocking JoinError calls complete_bridge_failure so every in-flight
call is completed.

---

Nitpick comments:
In `@baml_language/sdks/rust/bridge_rust/src/host_value.rs`:
- Around line 117-124: Add a #[cfg(test)] unit-test module in host_value.rs
covering the pure helpers next_key and slot_args: verify positional and named
slotting, missing-required-argument errors, and next_key’s wraparound behavior
skipping zero. Keep the tests focused on these helpers and use existing test
conventions and symbols.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: b73af529-f218-4ae1-8f14-fff813e04478

📥 Commits

Reviewing files that changed from the base of the PR and between 68da999 and 03212bd.

📒 Files selected for processing (31)

• baml_language/crates/baml_codegen_types/src/symbols.rs
• baml_language/crates/baml_project/src/client_codegen.rs
• baml_language/sdk_tests/crates/rust/function_calls/customizable/test_callback_throws.rs
• baml_language/sdk_tests/crates/rust/function_calls/customizable/test_generic_calls.rs
• baml_language/sdk_tests/crates/rust/function_calls/customizable/test_generic_inference.rs
• baml_language/sdk_tests/crates/rust/function_calls/customizable/test_host_callables.rs
• baml_language/sdk_tests/crates/rust/function_calls/customizable/test_raises.rs
• baml_language/sdk_tests/crates/rust/type_shapes/customizable/roundtrip_tests/test_forward_refs.rs
• baml_language/sdk_tests/crates/rust/type_shapes/customizable/roundtrip_tests/test_generics.rs
• baml_language/sdk_tests/crates/rust/type_shapes/customizable/roundtrip_tests/test_primitives.rs
• baml_language/sdk_tests/crates/rust/type_shapes/customizable/test_generic.rs
• baml_language/sdk_tests/fixtures/function_calls/baml_src/ns_host_callable_tests/main.baml
• baml_language/sdk_tests/harness_setup/src/rust.rs
• baml_language/sdks/rust/bridge_rust/src/baml_value.rs
• baml_language/sdks/rust/bridge_rust/src/capi.rs
• baml_language/sdks/rust/bridge_rust/src/encode.rs
• baml_language/sdks/rust/bridge_rust/src/error.rs
• baml_language/sdks/rust/bridge_rust/src/host_value.rs
• baml_language/sdks/rust/bridge_rust/src/lib.rs
• baml_language/sdks/rust/bridge_rust/src/runtime.rs
• baml_language/sdks/rust/bridge_rust/tests/conversions.rs
• baml_language/sdks/rust/bridge_rust/tests/live_engine.rs
• baml_language/sdks/rust/sdkgen_rust/src/analyze.rs
• baml_language/sdks/rust/sdkgen_rust/src/effect_rename.rs
• baml_language/sdks/rust/sdkgen_rust/src/emit/class.rs
• baml_language/sdks/rust/sdkgen_rust/src/emit/enum_.rs
• baml_language/sdks/rust/sdkgen_rust/src/emit/function.rs
• baml_language/sdks/rust/sdkgen_rust/src/emit/union.rs
• baml_language/sdks/rust/sdkgen_rust/src/lib.rs
• baml_language/sdks/rust/sdkgen_rust/src/translate_ty.rs
• baml_language/sdks/rust/sdkgen_rust/src/unions.rs