Constant MAX_DESCRIPTOR_LEN 

pub const MAX_DESCRIPTOR_LEN: u32 = 32768;

Maximum length of a configuration descriptor block (32 KiB).

Note: While the USB specification allows configuration descriptors up to 65,535 bytes (64 KiB), this limit is set to 32,768 bytes (32 KiB) to leave ample headroom (32 KiB) for future FIDL protocol evolution (new struct fields, handles, or message headers) within Zircon’s 64 KiB channel message limit (ZX_CHANNEL_MAX_MSG_BYTES, 65,536 bytes).