Re: [PATCH 1/2] rust: add BitInt integer wrapping type

[Yury Norov <yury.norov@gmail.com>]

On Mon, Nov 03, 2025 at 03:54:08PM +0100, Miguel Ojeda wrote:
> On Mon, Nov 3, 2025 at 3:26 PM Yury Norov <yury.norov@gmail.com> wrote:
> >
> > This is exactly what the patch does:
> 
> No, there are no arithmetic conversions going on here in the sense of
> C. It defines a particular operation for a set of types.
> 
> What you are seeing there is that literals, in Rust, do type
> inference, and so the compiler picks a type:
> 
>     https://doc.rust-lang.org/reference/expressions/literal-expr.html#r-expr.literal.int.infer
> 
> Thus if you do:
> 
>     let v1 = BitInt::<u8, 4>::from_expr(15);
>     let v2 = BitInt::<u16, 4>::from_expr(15);
>     let i = 5;
>     assert_eq!(v1 + i, 20);
>     assert_eq!(v2 + i, 20);
> 
> That will not build, because `i` cannot have two types. But it will if
> you comment one of the two asserts.
> 
> And if you do:
> 
>     let v = BitInt::<u16, 4>::from_expr(15);
>     assert_eq!(v + 5u8, 20);
> 
> It will not build either -- there is not even "widening" going on from
> `u8` to `u16` in this last example.

The current BitInt() allows this:

  let v = BitInt::<u32, 4>::new::<15>();
  assert_eq!(v * 10, 150);

It looks and feels like C integer promotion. If Rust doesn't like it,
we shouldn't allow such things with BitInt()s.