Rust macro issue, "no rules expected the token `2`"

bit_struct.rs

macro_rules! field_size {
  (1) => (bool);
  (2) => (u8);
  (3) => (u8);
  (4) => (u8);
  (5) => (u8);
  (6) => (u8);
  (7) => (u8);
  (8) => (u8);
}

macro_rules! size_mask {
  (1) => (0b1);
  (2) => (0b11);
  (3) => (0b111);
  (4) => (0b1111);
  (5) => (0b1_1111);
  (6) => (0b11_1111);
  (7) => (0b111_1111);
  (8) => (0b1111_1111);
}

macro_rules! bit_struct {
    (@field $name: ident, $size: expr, $offset: expr) => {
      pub fn $name(self) -> field_size!($size) {
        let mask = (size_mask!($size)) << $offset;
        let value = ((self.0 & mask) >> $offset) as field_size!($size);
        value
      }
    };
    (@fields $offset: expr, []) => {};
    (@fields $offset: expr, [$name: ident($size: expr), $($tail_name: ident($tail_size:expr)),*]) => {
      bit_struct!(@field $name, $size, ($offset + $size));
      bit_struct!(@fields ($offset + $size), [$($tail_name($tail_size)),*]);
    };
    { struct $struct_name: ident : $_repr_: ident {
        $($name: ident($size: expr)),+
      }
    } => {
      pub struct $struct_name ($_repr_);

      impl $struct_name {
        bit_struct!(@fields 0usize, [$($name($size)),+]);
      }
    }
}

const x : field_size!(6) = size_mask!(2);

bit_struct! {
  struct Palette: u8 {
    palette_0(2),
    palette_1(2),
    palette_2(2),
    palette_3(2)
  }
}