Warning when using `packed_bits` on a field wider than u8.

[konkers]

I'm trying to create a descriptor for a joystick with 10 buttons:

#[gen_hid_descriptor(
    (collection = APPLICATION, usage_page = GENERIC_DESKTOP, usage = JOYSTICK) = {
        (usage = X,) = {x=input;};
        (usage = Y,) = {y=input;};
        (usage_page = BUTTON, usage_min = BUTTON_1, usage_max = 10) = {
            #[packed_bits 10] #[item_settings data,variable,absolute] buttons=input;
        };
    }
)]
struct InputReport {
    x: u8,
    y: u8,
    buttons: u16,
}

This yields the following warning:

warning: borrow of packed field is unsafe and requires unsafe function or block (error E0133)
  --> targets\stm32f072-disco\src\main.rs:23:1
   |
23 | / #[gen_hid_descriptor(
24 | |     (collection = APPLICATION, usage_page = GENERIC_DESKTOP, usage = JOYSTICK) = {
25 | |         (usage = X,) = {x=input;};
26 | |         (usage = Y,) = {y=input;};
...  |
30 | |     }
31 | | )]
   | |__^
   |
   = note: `#[warn(safe_packed_borrows)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
   = note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior
   = note: this warning originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

[konkers]

A workaround seems to be to split the buttons into two u8 fields:

#[gen_hid_descriptor(
    (collection = APPLICATION, usage_page = GENERIC_DESKTOP, usage = JOYSTICK) = {
        (usage = X,) = {x=input;};
        (usage = Y,) = {y=input;};
        (usage_page = BUTTON, usage_min = BUTTON_1, usage_max = BUTTON_8) = {
            #[packed_bits 8] #[item_settings data,variable,absolute] buttons0=input;
        };
        (usage_page = BUTTON, usage_min = 9, usage_max = 10) = {
            #[packed_bits 2] #[item_settings data,variable,absolute] buttons1=input;
        };
    }
)]
pub struct InputReport {
    pub x: u8,
    pub y: u8,
    pub buttons0: u8,
    pub buttons1: u8,
}