`__BindgenBitfieldUnit::new` isn't a `const fn` for rust-target >= 1.30

[Wallacoloo]

Input C/C++ Header

struct flags {
    int f1: 1;
    int f2: 1;
};

Bindgen Invocation

$ bindgen bitfield-testcase.c --rust-target 1.33

Actual Results

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Copy, Clone, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct __BindgenBitfieldUnit<Storage, Align>
where
    Storage: AsRef<[u8]> + AsMut<[u8]>,
{
    storage: Storage,
    align: [Align; 0],
}
impl<Storage, Align> __BindgenBitfieldUnit<Storage, Align>
where
    Storage: AsRef<[u8]> + AsMut<[u8]>,
{
    #[inline]
    pub fn new(storage: Storage) -> Self {
        Self { storage, align: [] }
    }
    #[inline]
    pub fn get_bit(&self, index: usize) -> bool {
        debug_assert!(index / 8 < self.storage.as_ref().len());
        let byte_index = index / 8;
        let byte = self.storage.as_ref()[byte_index];
        let bit_index = if cfg!(target_endian = "big") {
            7 - (index % 8)
        } else {
            index % 8
        };
        let mask = 1 << bit_index;
        byte & mask == mask
    }
    #[inline]
    pub fn set_bit(&mut self, index: usize, val: bool) {
        debug_assert!(index / 8 < self.storage.as_ref().len());
        let byte_index = index / 8;
        let byte = &mut self.storage.as_mut()[byte_index];
        let bit_index = if cfg!(target_endian = "big") {
            7 - (index % 8)
        } else {
            index % 8
        };
        let mask = 1 << bit_index;
        if val {
            *byte |= mask;
        } else {
            *byte &= !mask;
        }
    }
    #[inline]
    pub fn get(&self, bit_offset: usize, bit_width: u8) -> u64 {
        debug_assert!(bit_width <= 64);
        debug_assert!(bit_offset / 8 < self.storage.as_ref().len());
        debug_assert!(
            (bit_offset + (bit_width as usize)) / 8
                <= self.storage.as_ref().len()
        );
        let mut val = 0;
        for i in 0..(bit_width as usize) {
            if self.get_bit(i + bit_offset) {
                let index = if cfg!(target_endian = "big") {
                    bit_width as usize - 1 - i
                } else {
                    i
                };
                val |= 1 << index;
            }
        }
        val
    }
    #[inline]
    pub fn set(&mut self, bit_offset: usize, bit_width: u8, val: u64) {
        debug_assert!(bit_width <= 64);
        debug_assert!(bit_offset / 8 < self.storage.as_ref().len());
        debug_assert!(
            (bit_offset + (bit_width as usize)) / 8
                <= self.storage.as_ref().len()
        );
        for i in 0..(bit_width as usize) {
            let mask = 1 << i;
            let val_bit_is_set = val & mask == mask;
            let index = if cfg!(target_endian = "big") {
                bit_width as usize - 1 - i
            } else {
                i
            };
            self.set_bit(index + bit_offset, val_bit_is_set);
        }
    }
}
#[repr(C)]
#[repr(align(4))]
#[derive(Debug, Copy, Clone)]
pub struct flags {
    pub _bitfield_1: __BindgenBitfieldUnit<[u8; 1usize], u8>,
    pub __bindgen_padding_0: [u8; 3usize],
}

Expected Results

The min_const_fn feature was introduced in Rust 1.30. Because the --rust-target is >= 1.30, __BindgenBitfieldUnit::new should have been declared as a const fn:

impl<Storage, Align> __BindgenBitfieldUnit<Storage, Align>
where
    Storage: AsRef<[u8]> + AsMut<[u8]>,
{
    #[inline]
    pub const fn new(storage: Storage) -> Self {
        Self { storage, align: [] }
    }
    // other methods ...
}

The current behavior is undesirable because it means that it's impossible to instantiate a type containing a bitfield as a static binding using safe rust. i.e., the following is not possible:

static my_flags: flags = /* ??? */;

The same problem exists with __BindgenUnionField.

[Wallacoloo]

I'm going to take a stab at fixing this. It looks like I can add a line to src/features.rs to detect the presence of rust >= 1.30, and then modify the utils::prepend_union_types function in src/codegen/mod.rs to conditionally output new as const.