Function get_buffer_source_copy 

pub(crate) fn get_buffer_source_copy(
    source: ArrayBufferViewOrArrayBufferRef<'_>,
) -> Vec<u8> 

https://webidl.spec.whatwg.org/#dfn-get-buffer-source-copy

Spec steps and how they’re covered:

• Convert to JS value: Handled by WebIDL bindings before this function receives the typed ArrayBufferViewOrArrayBuffer union.

• ArrayBufferView offset/length: view.to_vec() (mozjs) calls GetArrayBufferViewLengthAndData, which respects the view’s [[ByteOffset]] and [[ByteLength]] — only the viewed bytes are copied.

• ArrayBuffer: buffer.to_vec() (mozjs) calls GetArrayBufferLengthAndData, copying the entire buffer contents.

• Detached buffer: When a buffer is detached, SpiderMonkey’s GetArrayBuffer(LengthAndData|ViewLengthAndData) returns a null pointer and zero length. to_vec() thus produces an empty Vec<u8>.

• SharedArrayBuffer: Not applicable — ArrayBufferViewOrArrayBuffer does not include a SharedArrayBuffer variant.