bindgen: surface base-class _Impl trait requirements for composable hierarchies

[Copilot]

Fix windows-bindgen to emit _Impl super-trait bounds for default interfaces of derived classes (composable hierarchies), so implementers don't need to manually list every base class.

• Promote Class::default_interface and Class::bases to pub
• Add Interface::base_class_default_interfaces that walks the owning class's bases and returns each base's default interface
• Extend Interface::write so the _Impl trait for an exclusive default interface picks up base classes' default _Impl traits as super-trait bounds
• Gate the inherited bound on config.should_implement(base, …) so configurations that emit the derived _Impl but omit a base _Impl (e.g. --implements Derived without selecting the bases) fall back to windows_core::IUnknownImpl instead of referencing an un-emitted trait name
• Add rdl fixture class_hierarchy_impl (3-level hierarchy: Base ← Middle ← Leaf) verifying generated IMiddle_Impl: IBase_Impl and ILeaf_Impl: IMiddle_Impl + IBase_Impl
• Add rdl fixture class_hierarchy_impl_partial that selects only Test.ILeaf via implements, locking in the IUnknownImpl fallback when bases are not selected
• Regenerate crates/tests/winrt/composable/src/bindings.rs so its checked-in bindings reflect the new bounds (CI's git diff --exit-code after build.rs regen)
• All 323 bindgen/rdl fixtures pass; parallel_validation clean

[Copilot]

Pull request overview

This PR updates windows-bindgen to surface base-class default-interface implementation requirements in generated _Impl traits for composable class hierarchies, reducing the need for implementers to manually enumerate every base class in implementation scaffolding.

Changes:

• Extend bindgen _Impl super-trait bounds for exclusive default interfaces to include base classes’ default _Impl traits.
• Add a new RDL bindgen fixture (class_hierarchy_impl) validating _Impl trait super-trait chaining across a 3-level class hierarchy.
• Regenerate the WinRT composable test bindings to reflect the new emitted bounds.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/libs/bindgen/src/types/interface.rs	Adds base-class default interface discovery and wires it into _Impl super-trait requirements.
crates/libs/bindgen/src/types/class.rs	Exposes default_interface and bases for use by interface codegen.
crates/tests/libs/bindgen/data/bindgen/class_hierarchy_impl/input.rdl	New fixture input defining a 3-level class hierarchy with exclusive default interfaces.
crates/tests/libs/bindgen/data/bindgen/class_hierarchy_impl/fixture.toml	New fixture configuration enabling implementation scaffolding output.
crates/tests/libs/bindgen/data/bindgen/class_hierarchy_impl/expected.rs	Golden output asserting the new _Impl super-trait bounds.
crates/tests/winrt/composable/src/bindings.rs	Updated generated bindings to include new _Impl trait super-trait bounds.

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[kennykerr]

@copilot review feedback

[Copilot]

@copilot review feedback

Good catch. Fixed in ffa2e56: the base default interfaces are only added to required_interfaces (and thus to the _Impl super-trait bound) when config.should_implement returns true for the base, so --implements Derived without the bases falls back to windows_core::IUnknownImpl instead of referencing an un-emitted _Impl trait. Added a class_hierarchy_impl_partial rdl fixture that selects only Test.ILeaf via implements, verifying the fallback (pub trait ILeaf_Impl: windows_core::IUnknownImpl).