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charset=UTF-8 Date: Tue, 26 May 2026 14:44:54 +0900 Message-Id: From: "Alexandre Courbot" To: "Danilo Krummrich" Cc: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , "Eliot Courtney" Subject: Re: [PATCH v5 22/24] rust: types: add `ForLt` trait for higher-ranked lifetime support References: <20260525202921.124698-1-dakr@kernel.org> <20260525202921.124698-23-dakr@kernel.org> In-Reply-To: <20260525202921.124698-23-dakr@kernel.org> X-ClientProxiedBy: TYWPR01CA0033.jpnprd01.prod.outlook.com (2603:1096:400:aa::20) To BY5PR12MB3986.namprd12.prod.outlook.com (2603:10b6:a03:195::27) Precedence: bulk X-Mailing-List: linux-pm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MN2PR12MB3997:EE_|SJ2PR12MB8035:EE_ X-MS-Office365-Filtering-Correlation-Id: 8bac370d-42db-45ad-28c2-08debae9ef0d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|10070799003|366016|376014|7416014|56012099003|18002099003|22082099003|11063799006|6133799003|4143699003; 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For > example, if you want to take a function that takes `&'a Foo` and gives > `Bar<'a>`, you can write: > > f: impl for<'a> FnOnce(&'a Foo) -> Bar<'a>, > > However, it becomes tricky when you want that function to not have a > fixed `Bar`, but have it be generic again. In this case, one needs > something that is generic over types that are themselves generic over > lifetimes. > > `ForLt` provides such support. It provides a trait `ForLt` which > describes a type generic over a lifetime. One may use `ForLt::Of<'a>` to > get an instance of a type for a specific lifetime. > > For the case of cross referencing, one would almost always want the > lifetime to be covariant. Therefore this is also made a requirement for > the `ForLt` trait, so functions with `ForLt` trait bound can assume > covariance. > > A macro `ForLt!()` is provided to be able to obtain a type that > implements `ForLt`. For example, `ForLt!(for<'a> Bar<'a>)` would yield a > type that `::Of<'a>` is `Bar<'a>`. This also works > with lifetime elision, e.g. `ForLt!(Bar<'_>)` or for types without > lifetime at all, e.g. `ForLt!(u32)`. > > The API design draws inspiration from the higher-kinded-types [1] crate, > however a different design decision has been taken (e.g. covariance > requirement) and the implementation is independent. > > License headers use "Apache-2.0 OR MIT" because I anticipate this to be > used in pin-init crate too which is licensed as such. > > Link: https://docs.rs/higher-kinded-types/ [1] > > Reviewed-by: Greg Kroah-Hartman > Reviewed-by: Eliot Courtney > Signed-off-by: Gary Guo > [ Handle macro_rules! invocations in the ForLt! proc macro's covariance > and WF checks. Since proc macros cannot expand macro_rules!, add a > visit_macro() implementation to conservatively assume macro > invocations may contain lifetimes, forcing them through the > compiler-assisted covariance proof. > > While at it, fix a few typos in the documentation and in the commit > message. - Danilo ] > Signed-off-by: Danilo Krummrich For everything but the proc macro, Reviewed-by: Alexandre Courbot