Macro tock_registers::register_bitmasks

macro_rules! register_bitmasks {
    {
        // BITFIELD_NAME OFFSET(x)
        $(#[$outer:meta])*
        $valtype:ident, $reg_mod:ident, $reg_desc:ident, [
            $( $(#[$inner:meta])* $field:ident OFFSET($offset:expr)),+ $(,)?
        ]
    } => { ... };
    {
        // BITFIELD_NAME OFFSET
        // All fields are 1 bit
        $(#[$outer:meta])*
        $valtype:ident, $reg_mod:ident, $reg_desc:ident, [
            $( $(#[$inner:meta])* $field:ident $offset:expr ),+ $(,)?
        ]
    } => { ... };
    {
        // BITFIELD_NAME OFFSET(x) NUMBITS(y)
        $(#[$outer:meta])*
        $valtype:ident, $reg_mod:ident, $reg_desc:ident, [
            $( $(#[$inner:meta])* $field:ident OFFSET($offset:expr) NUMBITS($numbits:expr) ),+ $(,)?
        ]
    } => { ... };
    {
        // BITFIELD_NAME OFFSET(x) NUMBITS(y) []
        $(#[$outer:meta])*
        $valtype:ident, $reg_mod:ident, $reg_desc:ident, [
            $( $(#[$inner:meta])* $field:ident OFFSET($offset:expr) NUMBITS($numbits:expr)
               $values:tt ),+ $(,)?
        ]
    } => { ... };
    {
        $valtype:ident, $reg_desc:ident, $(#[$outer:meta])* $field:ident,
                    $offset:expr, $numbits:expr,
                    [$( $(#[$inner:meta])* $valname:ident = $value:expr ),+ $(,)?]
    } => { ... };
    {
        $valtype:ident, $reg_desc:ident, $(#[$outer:meta])* $field:ident,
                    $offset:expr, $numbits:expr,
                    []
    } => { ... };
    (
        // final implementation of the macro
        @debug $valtype:ident, $reg_mod:ident, $reg_desc:ident, [$($field:ident),*]
    ) => { ... };
    (
        @fv_enum_type_seq $valtype:ident, $enum_val:path $(, $($rest:path),+)?
    ) => { ... };
    (
        @fv_enum_type_seq $valtype:ident $(,)?
    ) => { ... };
}

Helper macro for defining register fields.