Summary

• Add support for [WbgGeneric] on individual WebIDL operations (not just interface/dictionary/namespace definitions), so that specific methods can opt into typed generics without affecting all sibling methods.
• Fix return type WbgType expansion to respect wbg_generic for callback-like return types. Previously next_unstable was only set temporarily during argument expansion but not when computing the return type in create_method.
• Add [WbgGeneric] to the two createImageBitmap operations in WindowOrWorkerGlobalScope, restoring Promise<ImageBitmap> return types on Window and WorkerGlobalScope.

Motivation

When VideoFrame was stabilized in ce535e1, the createImageBitmap overloads accepting VideoFrame on Window/WorkerGlobalScope lost their typed generic return type (Promise<ImageBitmap> became bare Promise). Previously these methods were unstable (gated behind web_sys_unstable_apis) which automatically gave them generics. After stabilization, the parent WindowOrWorkerGlobalScope mixin has no [WbgGeneric], so generics were lost.

Adding [WbgGeneric] to the entire mixin was too broad — it changed setTimeout, setInterval, fetch, etc. Per-operation support allows targeting only the affected methods.

Changes

crates/webidl/src/util.rs:

• Check Signature.attrs for [WbgGeneric] and OR it with the parent definition's wbg_generic flag in two places: when setting next_unstable for argument type expansion, and when passing wbg_generic to create_method.
• In create_method, temporarily set next_unstable around the return type to_wbg_type call when wbg_generic or unstable_flag is true, so that callback-like return types also get typed expansion.

crates/web-sys/webidls/enabled/WindowOrWorkerGlobalScope.webidl: Add WbgGeneric to the two createImageBitmap operation attributes.

crates/web-sys/src/features/gen_Window.rs, gen_WorkerGlobalScope.rs: Regenerated — createImageBitmap overloads now return Promise<ImageBitmap>.