From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1C20F304976; Wed, 3 Jun 2026 01:10:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780449039; cv=none; b=N52kL9Uj4K3CzT0Me+7p5irlFtQ+y4+xIr1zpFymuTaEixxj4Jm/88irtAhP4Ma9qk8iWluxP1z8jYNqTX2CMyd99jRzIbavfbT/+eUpSsuZVmSDAt+s7/klnQHXySYbNhPQxFl6bfI1wI+GZsrjY53euuIuRKWshGTu/gcV3yo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780449039; c=relaxed/simple; bh=cagk3pulaq5ShsvO9EBtr9mU7mNbZy2WBoDZgl7zAfE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=pMexB7ZcHlJIBES8opsexyyXoReB3Nx+HFq8XgSvRJVM6sLMis0TYlVsuxuBqhEm6uvvtWGn766mFzpzzwaKr3Tl1jzOY6/qWQPbtKhsGz6jXpsA7+lQ3OJ/dnZOGWf9PVxww0F6xAztqV/DV5xCLOe9Li8R5rxmfY2PDG/I88A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bvjpGwio; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="bvjpGwio" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EBCD41F00893; Wed, 3 Jun 2026 01:10:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780449037; bh=1KEoJo7IVklqGes8vOy2JYJl+S37vQieuYALZbqcPNc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=bvjpGwiosFQPvp1zeoC4lFF+SJCs7y4g9VOCmLitoGujV6stCGF63t5PxAewMNBjZ XTQIALLq7SbSSZ6ZzVmQK+LZO1dyULyF2e5wtE/UrfcMncBOKPUtogEkGtG+u3orBk wbi/LwseSAacUWCq6G6vxKti8whTyuSwGwmJI4bT8zFy97xQjJwHSuuJFtceWRsfrP y5fa1YTU8zbnhqRzWkOV7r3Vzo4kZ+zYPUNEgGn9bmrNVs450pTI9T5pN6j5aShZjy JRUZwrw7UiKTUK1BfzJfApIQ2oqsEvwNR4h8kx8OFsBEbSV5YmUgM0uDDRK68m3q9v nRa09YdSvhxQA== From: Danilo Krummrich To: gregkh@linuxfoundation.org, rafael@kernel.org, dakr@kernel.org, ojeda@kernel.org, boqun@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, a.hindborg@kernel.org, aliceryhl@google.com, tmgross@umich.edu, acourbot@nvidia.com, ecourtney@nvidia.com, m.wilczynski@samsung.com, david.m.ertman@intel.com, ira.weiny@intel.com, leon@kernel.org, daniel.almeida@collabora.com, bhelgaas@google.com, kwilczynski@kernel.org Cc: driver-core@lists.linux.dev, linux-kernel@vger.kernel.org, nova-gpu@lists.linux.dev, dri-devel@lists.freedesktop.org, linux-pwm@vger.kernel.org, rust-for-linux@vger.kernel.org Subject: [PATCH v2 2/7] rust: types: introduce ForLt base trait for CovariantForLt Date: Wed, 3 Jun 2026 03:10:13 +0200 Message-ID: <20260603011020.2073650-3-dakr@kernel.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260603011020.2073650-1-dakr@kernel.org> References: <20260603011020.2073650-1-dakr@kernel.org> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Add a new ForLt trait as a base for CovariantForLt: - ForLt (non-unsafe): represents a type generic over a lifetime, with no covariance guarantee. Provides unsafe fn cast_ref_unchecked() for callers that can justify lifetime shortening via a round-trip safety argument. - CovariantForLt (unsafe): becomes a subtrait of ForLt that additionally proves the type is covariant over its lifetime parameter, providing a safe cast_ref() method. This split allows non-covariant types (e.g. types behind a Mutex) to implement ForLt and participate in DevresLt / registration data patterns where the round-trip argument suffices, without requiring a covariance proof that would fail to compile. The internal macro backend is split accordingly: ForLt! emits ForLtImpl (no covariance proof), CovariantForLt! emits CovariantForLtImpl (with compile-time covariance proof). No existing callers change; they all use CovariantForLt. Signed-off-by: Danilo Krummrich --- rust/kernel/types.rs | 1 + rust/kernel/types/for_lt.rs | 101 ++++++++++++++++++++++++++++++------ rust/macros/for_lt.rs | 50 ++++++++++++++---- rust/macros/lib.rs | 19 ++++++- 4 files changed, 145 insertions(+), 26 deletions(-) diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs index cbe6907042d3..c1ed05d1046c 100644 --- a/rust/kernel/types.rs +++ b/rust/kernel/types.rs @@ -14,6 +14,7 @@ #[doc(hidden)] pub mod for_lt; pub use for_lt::CovariantForLt; +pub use for_lt::ForLt; /// Used to transfer ownership to and from foreign (non-Rust) languages. /// diff --git a/rust/kernel/types/for_lt.rs b/rust/kernel/types/for_lt.rs index ef510ab6c092..e1774b03dd1f 100644 --- a/rust/kernel/types/for_lt.rs +++ b/rust/kernel/types/for_lt.rs @@ -1,17 +1,74 @@ // SPDX-License-Identifier: Apache-2.0 OR MIT -//! Provide implementation and test of the `CovariantForLt` trait and macro. +//! Provide implementation and test of the `ForLt` and `CovariantForLt` traits and macros. //! -//! This module is hidden and user should just use `CovariantForLt!` directly. +//! This module is hidden and users should just use `ForLt!` / `CovariantForLt!` directly. use core::marker::PhantomData; /// Representation of types generic over a lifetime. /// -/// The type must be covariant over the generic lifetime, i.e. the lifetime parameter -/// can be soundly shortened. +/// # Macro +/// +/// It is not recommended to implement this trait directly. `ForLt!` macro is provided to obtain a +/// type that implements this trait. +/// +/// The full syntax is +/// +/// ``` +/// # use kernel::types::ForLt; +/// # fn expect_lt() {} +/// # struct TypeThatUse<'a>(&'a ()); +/// # expect_lt::< +/// ForLt!(for<'a> TypeThatUse<'a>) +/// # >(); +/// ``` +/// +/// which gives a type so that ` TypeThatUse<'a>) as ForLt>::Of<'b>` +/// is `TypeThatUse<'b>`. +/// +/// You may also use a short-hand syntax which works similar to lifetime elision. +/// The macro also accepts types that do not involve a lifetime at all. +/// +/// ``` +/// # use kernel::types::ForLt; +/// # fn expect_lt() {} +/// # struct TypeThatUse<'a>(&'a ()); +/// # expect_lt::< +/// ForLt!(TypeThatUse<'_>) // Equivalent to `ForLt!(for<'a> TypeThatUse<'a>)`. +/// # >(); +/// # expect_lt::< +/// ForLt!(&u32) // Equivalent to `ForLt!(for<'a> &'a u32)`. +/// # >(); +/// # expect_lt::< +/// ForLt!(u32) // Equivalent to `ForLt!(for<'a> u32)`. +/// # >(); +/// ``` +pub trait ForLt { + /// The type parameterized by the lifetime. + type Of<'a>: 'a; + + /// Cast a reference to a shorter lifetime. + /// + /// # Safety + /// + /// The caller must ensure that the lifetime shortening is sound for their use case, + /// e.g. because the `'long` reference originated from a `'short`-to-`'static` transmute + /// and this is the reverse leg of that round-trip. + #[inline(always)] + unsafe fn cast_ref_unchecked<'r, 'short: 'r, 'long: 'short>( + long: &'r Self::Of<'long>, + ) -> &'r Self::Of<'short> { + // SAFETY: Caller guarantees the lifetime shortening is sound. + unsafe { core::mem::transmute(long) } + } +} +pub use macros::ForLt; + +/// [`trait@ForLt`] subtrait for types that are covariant over their lifetime parameter. /// -/// The lifetime involved must be covariant. +/// Provides a safe [`cast_ref`](CovariantForLt::cast_ref) method for types that are proven to be +/// covariant. The `CovariantForLt!` macro syntax is the same as `ForLt!`. /// /// # Macro /// @@ -84,10 +141,7 @@ /// # Safety /// /// `Self::Of<'a>` must be covariant over the lifetime `'a`. -pub unsafe trait CovariantForLt { - /// The type parameterized by the lifetime. - type Of<'a>: 'a; - +pub unsafe trait CovariantForLt: ForLt { /// Cast a reference to a shorter lifetime. #[inline(always)] fn cast_ref<'r, 'short: 'r, 'long: 'short>(long: &'r Self::Of<'long>) -> &'r Self::Of<'short> { @@ -97,27 +151,44 @@ fn cast_ref<'r, 'short: 'r, 'long: 'short>(long: &'r Self::Of<'long>) -> &'r Sel } pub use macros::CovariantForLt; -/// Helper type for the `CovariantForLt!` macro. +/// Helper type for the `ForLt!` macro. /// -/// Must only be used by the `CovariantForLt!` macro. +/// Must only be used by the `ForLt!` macro. /// /// `T` is the magic `dyn for<'a> WithLt<'a, TypeThatUse<'a>>` generated by macro. /// /// `WF` is a type that the macro can use to assert some specific type is well-formed. +#[doc(hidden)] +pub struct ForLtImpl(PhantomData<(WF, T)>); + +/// Helper type for the `CovariantForLt!` macro. +/// +/// Must only be used by the `CovariantForLt!` macro. +/// +/// `T` and `WF` are the same as in [`ForLtImpl`]. /// /// `N` is to provide the macro a place to emit arbitrary items, in case it needs to prove /// additional properties. #[doc(hidden)] pub struct CovariantForLtImpl(PhantomData<(WF, T)>); -// This is a helper trait for implementation `CovariantForLt` to be able to use HRTB. +// This is a helper trait for implementation `ForLt` to be able to use HRTB. #[doc(hidden)] pub trait WithLt<'a> { type Of: 'a; } -// SAFETY: In `CovariantForLt!` macro, a covariance proof is generated when naming -// `CovariantForLtImpl` and it will fail to evaluate if the type is not covariant. -unsafe impl WithLt<'a>, WF> CovariantForLt for CovariantForLtImpl { +impl WithLt<'a>, WF> ForLt for ForLtImpl { + type Of<'a> = >::Of; +} + +impl WithLt<'a>, WF, const N: usize> ForLt for CovariantForLtImpl { type Of<'a> = >::Of; } + +// SAFETY: In `CovariantForLt!` macro, a covariance proof is generated in the `N` const generic +// and it will fail to evaluate if the type is not covariant. +unsafe impl WithLt<'a>, WF, const N: usize> CovariantForLt + for CovariantForLtImpl +{ +} diff --git a/rust/macros/for_lt.rs b/rust/macros/for_lt.rs index 3cb094d00548..ad9809563fab 100644 --- a/rust/macros/for_lt.rs +++ b/rust/macros/for_lt.rs @@ -176,8 +176,10 @@ fn prove(&mut self, ty: &'a Type) { } } -pub(crate) fn covariant_for_lt(input: HigherRankedType) -> TokenStream { - let (ty, lifetime) = match input { +/// Resolve the higher-ranked type into a concrete `(ty, lifetime)` pair, expanding elided +/// lifetimes as needed. Shared by both `for_lt` and `covariant_for_lt`. +fn resolve_hrt(input: HigherRankedType) -> (Type, Lifetime) { + match input { HigherRankedType::Explicit { lifetime, ty, .. } => (ty, lifetime), HigherRankedType::Implicit { ty } => { // If there's no explicit `for<'a>` binder, inject a synthetic `'__elided` lifetime @@ -188,7 +190,41 @@ pub(crate) fn covariant_for_lt(input: HigherRankedType) -> TokenStream { }; (ty.expand_elided_lifetime(&lifetime), lifetime) } - }; + } +} + +/// Produce the `'static`-substituted type for the WF check. Shared by both macros. +fn ty_static(ty: &Type, lifetime: &Lifetime) -> Type { + ty.replace_lifetime( + lifetime, + &Lifetime { + apostrophe: Span::mixed_site(), + ident: format_ident!("static"), + }, + ) +} + +pub(crate) fn for_lt(input: HigherRankedType) -> TokenStream { + let (ty, lifetime) = resolve_hrt(input); + + // Make sure that the type is wellformed when substituting lifetime with `'static`. + // + // Currently the Rust compiler doesn't check this, see the `ProveWf` documentation in + // `covariant_for_lt` below. + // + // We prefer to use this way of proving WF-ness as it can work when generics are involved. + let ty_static = ty_static(&ty, &lifetime); + + quote!( + ::kernel::types::for_lt::ForLtImpl::< + dyn for<#lifetime> ::kernel::types::for_lt::WithLt<#lifetime, Of = #ty>, + #ty_static, + > + ) +} + +pub(crate) fn covariant_for_lt(input: HigherRankedType) -> TokenStream { + let (ty, lifetime) = resolve_hrt(input); let mut prover = Prover(&lifetime, Vec::new()); prover.prove(&ty); @@ -226,13 +262,7 @@ fn #cov_proof_name<'__short, '__long: '__short>( // Currently the Rust compiler doesn't check this, see the above `ProveWf` documentation. // // We prefer to use this way of proving WF-ness as it can work when generics are involved. - let ty_static = ty.replace_lifetime( - &lifetime, - &Lifetime { - apostrophe: Span::mixed_site(), - ident: format_ident!("static"), - }, - ); + let ty_static = ty_static(&ty, &lifetime); quote!( ::kernel::types::for_lt::CovariantForLtImpl::< diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs index 2167cb270928..e970769609f3 100644 --- a/rust/macros/lib.rs +++ b/rust/macros/lib.rs @@ -491,11 +491,28 @@ pub fn kunit_tests(attr: TokenStream, input: TokenStream) -> TokenStream { .into() } -/// Obtain a type that implements [`CovariantForLt`] for the given higher-ranked type. +/// Obtain a type that implements [`ForLt`] for the given higher-ranked type. +/// +/// Please refer to the documentation of the [`ForLt`] trait. +/// +/// [`ForLt`]: trait.ForLt.html +#[proc_macro] +#[allow(non_snake_case)] +pub fn ForLt(input: TokenStream) -> TokenStream { + for_lt::for_lt(parse_macro_input!(input)).into() +} + +/// Obtain a type that implements [`CovariantForLt`] (and [`ForLt`]) for the given higher-ranked +/// type. +/// +/// Unlike [`ForLt!`], this macro additionally proves that the type is covariant over the lifetime, +/// providing a safe [`CovariantForLt::cast_ref`] method. /// /// Please refer to the documentation of the [`CovariantForLt`] trait. /// /// [`CovariantForLt`]: trait.CovariantForLt.html +/// [`CovariantForLt::cast_ref`]: trait.CovariantForLt.html#method.cast_ref +/// [`ForLt`]: trait.ForLt.html #[proc_macro] #[allow(non_snake_case)] pub fn CovariantForLt(input: TokenStream) -> TokenStream { -- 2.54.0