c2rust-bitfields-derive: Support const initialization

[kkysen]

In rav1d, we need const initialization of bitfield structs to initialization statics. Otherwise, the static has to be mut, which is unsafe to access (or else you have to pay a runtime cost). For example, see this code: https://github.com/memorysafety/rav1d/blob/4029b17b6762d3dcf75c8ac7a592666d483ba78d/src/ipred_prepare_tmpl_8.rs#L273

Since const fns can't have &muts, the setter fns generated can't be made const, but we can add with setters that return an updated struct, and these can be made const. And we might as well make all of the getters const fns, too.

It seems like the relevant code is around here:

c2rust/c2rust-bitfields-derive/src/lib.rs

Lines 233 to 258 in 0ba4903

	let q = quote! {
	#[automatically_derived]
	impl #struct_ident {
	#(
	/// This method allows you to write to a bitfield with a value
	pub fn #method_name_setters(&mut self, int: #field_types_setter_arg) {
	use c2rust_bitfields::FieldType;
	let field = &mut self.#field_names_setters;
	let (lhs_bit, rhs_bit) = #field_bit_info_setters;
	int.set_field(field, (lhs_bit, rhs_bit));
	}
	/// This method allows you to read from a bitfield to a value
	pub fn #method_names(&self) -> #field_types_return {
	use c2rust_bitfields::FieldType;
	type IntType = #field_types_typedef;
	let field = &self.#field_names_getters;
	let (lhs_bit, rhs_bit) = #field_bit_info_getters;
	<IntType as FieldType>::get_field(field, (lhs_bit, rhs_bit))
	}
	)*
	}
	};

.

[kkysen]

Changing the generated code to create const fns is non-trivial since it uses a lot of traits, and traits don't allow const fns. Thus, I rolled my own version in memorysafety/rav1d#497, and it was actually really simple. I'm not sure why the implementation here needs traits at all, since the logic is pretty simple.