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

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Message-ID: <CANiq72=4UXemR3ea5nNbqGt0Zh9q4hwiGVAn+hxfcZ-Zqa8y4w@mail.gmail.com>
Date: Mon, 3 Nov 2025 15:54:08 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Alexandre Courbot <acourbot@...dia.com>, Alice Ryhl <aliceryhl@...gle.com>, 
	Danilo Krummrich <dakr@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Joel Fernandes <joelagnelf@...dia.com>, Jesung Yang <y.j3ms.n@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/2] rust: add BitInt integer wrapping type

On Mon, Nov 3, 2025 at 3:26 PM Yury Norov <yury.norov@...il.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.

Cheers,
Miguel