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charset=UTF-8 Date: Thu, 27 Aug 2026 19:42:33 +0900 Message-Id: To: "Alice Ryhl" Cc: "Eliot Courtney" , "Burak Emir" , "Yury Norov" , "Miguel Ojeda" , "Boqun Feng" , "Gary Guo" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Trevor Gross" , "Danilo Krummrich" , "Daniel Almeida" , "Tamir Duberstein" , =?utf-8?q?Onur_=C3=96zkan?= , "David Airlie" , "Simona Vetter" , "Greg Kroah-Hartman" , "John Hubbard" , "Alistair Popple" , "Timur Tabi" , "Zhi Wang" , , , , Subject: Re: [PATCH v8 03/12] rust: num: add cv! macro to create values from constant expressions From: "Alexandre Courbot" References: <20260827-chid-v8-0-bc74c77d0214@nvidia.com> <20260827-chid-v8-3-bc74c77d0214@nvidia.com> In-Reply-To: X-ClientProxiedBy: TY4PR01CA0083.jpnprd01.prod.outlook.com (2603:1096:405:36c::13) To MW4PR12MB6873.namprd12.prod.outlook.com (2603:10b6:303:20c::17) Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR12MB6873:EE_|MW4PR12MB7286:EE_ X-MS-Office365-Filtering-Correlation-Id: 06c35aed-671a-40fd-2fdc-08df0427e938 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|10070799003|1800799024|366016|23010399003|18002099003|22082099003|4143699003|56012099006|11063799006|10067099003; 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It's >> unfortunate because it disincentivizes using it in interface boundaries. >> Introduce a macro to make it nicer to use. The macro `cv!` (for constant >> value) takes a const integer expression and widens it to i128 (at build >> time only) before passing it as a const generic value to a new trait >> function `FromConst::from_const`. The trait is implemented by NonZero, >> Bounded, and Alignment and lets values of each be constructed from >> constants without a verbose turbofish syntax. For example, >> `const { NonZero::new(1).unwrap() }` can be written as `cv!(1)`. >>=20 >> Suggested-by: Gary Guo >> Signed-off-by: Eliot Courtney > > This doesn't work in const context, so I don't think this is a great > strategy. > > I would want to use it for cases like this: > > drivers/android/binder/netlink.rs > const BINDER_CMD_REPORT: u8 =3D kernel::uapi::BINDER_CMD_REPORT a= s u8; > const BINDER_A_REPORT_ERROR: c_int =3D kernel::uapi::BINDER_A_REP= ORT_ERROR as c_int; > const BINDER_A_REPORT_CONTEXT: c_int =3D kernel::uapi::BINDER_A_R= EPORT_CONTEXT as c_int; > const BINDER_A_REPORT_FROM_PID: c_int =3D kernel::uapi::BINDER_A_= REPORT_FROM_PID as c_int; > const BINDER_A_REPORT_FROM_TID: c_int =3D kernel::uapi::BINDER_A_= REPORT_FROM_TID as c_int; > const BINDER_A_REPORT_TO_PID: c_int =3D kernel::uapi::BINDER_A_RE= PORT_TO_PID as c_int; > const BINDER_A_REPORT_TO_TID: c_int =3D kernel::uapi::BINDER_A_RE= PORT_TO_TID as c_int; > const BINDER_A_REPORT_IS_REPLY: c_int =3D kernel::uapi::BINDER_A_= REPORT_IS_REPLY as c_int; > const BINDER_A_REPORT_FLAGS: c_int =3D kernel::uapi::BINDER_A_REP= ORT_FLAGS as c_int; > const BINDER_A_REPORT_CODE: c_int =3D kernel::uapi::BINDER_A_REPO= RT_CODE as c_int; > const BINDER_A_REPORT_DATA_SIZE: c_int =3D kernel::uapi::BINDER_A= _REPORT_DATA_SIZE as c_int; `const_as!` [1] should do the trick for this, provided you don't need to create a const `NonZero`. [1] https://lore.kernel.org/all/20260825-const_as-v1-1-1ce712225fe2@nvidia.= com/