Promising a small fixed number of generic method instantiations in a vtable

help
1

I'm trying to write a character decoder trait that supports generic implementation of the conversion algorithm so that both UTF-8 and UTF-16 instantiations of the conversion are generated at compile time. Additionally, I want the trait to be usable in a type erasure setting. I.e. it has to work as a vtable.

Even though I've added a marked trait bound on the generic part to constrain the generic instantiations of the logically private generic method to a reasonable number (2), the compiler complains. How should I promise to the compiler that, really, the decode method only gets instantiated for u8 and u16, so there's no need to worry about a large or unbounded number of instantiations?

enum DecoderResult {
   Overflow,
   Underflow,
   Malformed,
}

#[no_mangle]
pub extern fn Decoder_decode_to_utf16(decoder: &mut Decoder, src: *const u8, src_len: *mut usize, dst: *mut u16, dst_len: *mut usize, last: bool) -> DecoderResult {
    let src_slice = unsafe { std::slice::from_raw_parts(src, *src_len) };
    let dst_slice = unsafe { std::slice::from_raw_parts_mut(dst, *dst_len) };
    let (result, read, written) = decoder.decode_to_utf16(src_slice, dst_slice, last);
    unsafe {
        *src_len = read;
        *dst_len = written;
    }
    result
}

trait UtfUnit {}

impl UtfUnit for u8 {}

impl UtfUnit for u16 {}

trait Decoder {
    // public
    fn decode_to_utf16(&mut self, src: &[u8], dst: &mut [u16], last: bool) -> (DecoderResult, usize, usize) {
        self.decode(src, dst, last)
    }

    // public
    fn decode_to_utf8(&mut self, src: &[u8], dst: &mut [u8], last: bool) -> (DecoderResult, usize, usize) {
        self.decode(src, dst, last)
    }

    // private
    fn decode<T: UtfUnit>(&mut self, src: &[u8], dst: &mut [T], last: bool) -> (DecoderResult, usize, usize);
}
2

Looks like a workaround is to

  1. not provide default implementations for decode_to_utf16 and decode_to_utf8 in Decoder
  2. removed decode<T> from Decoder
  3. implement decode<T> for every FooDecoder struct
  4. have some copypasta for implementing Decoder for each FooDecoder

Like this:

pub enum DecoderResult {
   Overflow,
   Underflow,
   Malformed,
}

#[no_mangle]
pub extern fn decoder_decode_to_utf16(decoder: &mut Decoder, src: *const u8, src_len: *mut usize, dst: *mut u16, dst_len: *mut usize, last: bool) -> DecoderResult {
    let src_slice = unsafe { std::slice::from_raw_parts(src, *src_len) };
    let dst_slice = unsafe { std::slice::from_raw_parts_mut(dst, *dst_len) };
    let (result, read, written) = decoder.decode_to_utf16(src_slice, dst_slice, last);
    unsafe {
        *src_len = read;
        *dst_len = written;
    }
    result
}

trait UtfUnit {}

impl UtfUnit for u8 {}

impl UtfUnit for u16 {}

pub trait Decoder {
    fn decode_to_utf16(&mut self, src: &[u8], dst: &mut [u16], last: bool) -> (DecoderResult, usize, usize);

    fn decode_to_utf8(&mut self, src: &[u8], dst: &mut [u8], last: bool) -> (DecoderResult, usize, usize);

}

struct FooDecoder {
    // ...
}

impl FooDecoder {
    fn decode<T: UtfUnit>(&mut self, src: &[u8], dst: &mut [T], last: bool) -> (DecoderResult, usize, usize) {
        // ...
    }
}

// copypasta for each Decoder implementation
impl Decoder for FooDecoder {
    fn decode_to_utf16(&mut self, src: &[u8], dst: &mut [u16], last: bool) -> (DecoderResult, usize, usize) {
        self.decode(src, dst, last)
    }

    fn decode_to_utf8(&mut self, src: &[u8], dst: &mut [u8], last: bool) -> (DecoderResult, usize, usize) {
        self.decode(src, dst, last)
    }
}