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charset=UTF-8 Date: Thu, 17 Sep 2026 21:26:29 +0900 Message-Id: Cc: "Miguel Ojeda" , "Boqun Feng" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Alice Ryhl" , "Trevor Gross" , "Danilo Krummrich" , "Daniel Almeida" , "Tamir Duberstein" , "Alexandre Courbot" , =?utf-8?q?Onur_=C3=96zkan?= , , Subject: Re: [PATCH v2 0/3] rust: const_eval: add a mechanism to do const trait calls From: "Eliot Courtney" To: "Gary Guo" , "Eliot Courtney" , "Miguel Ojeda" X-Mailer: aerc 0.22.0-0-gc2f86b7abde3 References: <20260903-cv-v2-0-e93b1613e40c@garyguo.net> In-Reply-To: X-ClientProxiedBy: TY4P286CA0109.JPNP286.PROD.OUTLOOK.COM (2603:1096:405:380::15) To DS0PR12MB6413.namprd12.prod.outlook.com (2603:10b6:8:ce::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6413:EE_|SA3PR12MB176418:EE_ X-MS-Office365-Filtering-Correlation-Id: 822f72bd-c4f2-45e6-6979-08df14b6e890 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|366016|376014|7416014|1800799024|10070799003|6133799003|10067099003|11063799006|4143699003|56012099006|22082099003|18002099003; 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This would only work for concrete types (not = inside >>>>> generic functions). >>>>> >>>>> As an example, convert `as_char_ptr_in_const_context`, so instead of >>>>> >>>>> kernel::str::as_char_ptr_in_const_context(c_str) >>>>> >>>>> one may write >>>>> >>>>> const_call!(c_str.as_char_ptr()) >>>>> >>>>> instead. >>>>> >>>>> Signed-off-by: Gary Guo >>>> >>>> Thanks for resending this! >>>> >>>> So I definitely like having `#[const_eval_only]`, because we do want >>>> to limit those calls sometimes and it is nice to have C++'s equivalent >>>> to `consteval` (perhaps this could be called just `#[consteval]` to >>>> match?); i.e. what we talked about in the call. > > I'm okay with either. I put in `_only` because I think it makes it very c= lear > that this is not just const-evalutable (that's provided by `const`) and i= s > *always* const evaled. > >>>> >>>> On `Const` and `const_call!`, i.e. the other two patches: I am not >>>> sure if we care that much to introduce a macro for it, but it doesn't >>>> look bad either. I guess perhaps it helps to have a consistent way to >>>> do it and to easily remove it later when the actual feature lands in >>>> Rust. I would like to hear some thoughts on it from others. > >>> >>> I think it's good to have such macro compared to our current approach; = it can be >>> made into a identity macro once we have const trait impl. >>> >>> FWIW, I even considered an extra macro that allow you to write >>> >>> #[const_trait_impl] // #[const_trait_impl(foreign)] for core types >>> impl MyTrait for MyType {} >>> >>> and generate the `impl Const` part too (and also add a line to the docu= mentation >>> that the method is const-callable with `const_call!()`). >>> >>> Best, >>> Gary >> >> I agree with the consteval macro, that sounds very nice. >> >> On the const_call! I have similar thoughts to Miguel. I reckon if we >> have a bunch of prospective users of this right now, and some thoughts >> on what would use this in the future then it's good. If it's just one >> use case right now, then not so sure. > > Some other candidates include `Alignable` (we currently have a "const_ali= gn_up") > and maybe `IntoSafeCast`. > > Best, > Gary I'm definitely not opposed to a const trait polyfill, it's pretty cool actually IMO. I also do think there is a bit of survivorship bias on the argument (which I made) that there are not many users. And it can be removed once const traits become available in MSRV. I am keen to hear from others on this.