Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 14 Nov 2022 14:30:10 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Miguel Ojeda <ojeda@kernel.org>
Cc:     Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Wei Liu <wei.liu@kernel.org>
Subject: Re: [PATCH v1 25/28] rust: add `build_error` crate
Message-ID: <Y3JQ8qsaV/DN3OJ7@liuwe-devbox-debian-v2>
References: <20221110164152.26136-1-ojeda@kernel.org>
 <20221110164152.26136-26-ojeda@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20221110164152.26136-26-ojeda@kernel.org>
Precedence: bulk

On Thu, Nov 10, 2022 at 05:41:37PM +0100, Miguel Ojeda wrote:
> From: Gary Guo <gary@garyguo.net>
[...]
> diff --git a/rust/build_error.rs b/rust/build_error.rs
> new file mode 100644
> index 000000000000..0ff6b33059aa
> --- /dev/null
> +++ b/rust/build_error.rs
> @@ -0,0 +1,24 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Build-time error.
> +//!
> +//! This crate provides a function `build_error`, which will panic in

a const function `build_error`

Without this I read it as a "normal non-const function".

> +//! compile-time if executed in const context, and will cause a build error
> +//! if not executed at compile time and the optimizer does not optimise away the
> +//! call.
> +//!

I can work out what the code does, but I also happen to know what Rust's
const means and its behaviour in non-const context. Other kernel
developers may not. Even so the description is a bit difficult for me to
parse.

Maybe a few sentences about const and its behaviours can help?

Thanks,
Wei.

> +//! It is used by `build_assert!` in the kernel crate, allowing checking of
> +//! conditions that could be checked statically, but could not be enforced in
> +//! Rust yet (e.g. perform some checks in const functions, but those
> +//! functions could still be called in the runtime).
> +
> +#![no_std]
> +
> +/// Panics if executed in const context, or triggers a build error if not.
> +#[inline(never)]
> +#[cold]
> +#[export_name = "rust_build_error"]
> +#[track_caller]
> +pub const fn build_error(msg: &'static str) -> ! {
> +    panic!("{}", msg);
> +}