From 1b56056294d45162325bb491f0356fbc6b473a12 Mon Sep 17 00:00:00 2001 From: Danilo Krummrich Date: Fri, 26 Jun 2026 20:36:09 +0200 Subject: [PATCH] rust: types: introduce ForLt base trait for CovariantForLt Add a new ForLt trait as a base for CovariantForLt: - ForLt (non-unsafe): represents a type generic over a lifetime, with no covariance guarantee. - 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 that use HRTB closures for sound access, without requiring a covariance proof that would fail to compile. Both macros share the UnsafeForLtImpl helper type, distinguished by a const generic N: ForLt! emits N = 0 (no covariance proof), CovariantForLt! emits N = 1 (with compile-time covariance proof). Reviewed-by: Gary Guo Acked-by: Miguel Ojeda Link: https://patch.msgid.link/20260626183630.2585057-3-dakr@kernel.org [ Merge ForLt pub use, inline resolve_hrt/ty_static, add intra-doc links. - Danilo ] Signed-off-by: Danilo Krummrich --- rust/kernel/types.rs | 5 ++- rust/kernel/types/for_lt.rs | 80 +++++++++++++++++++++++++++++-------- rust/macros/for_lt.rs | 37 ++++++++++++----- rust/macros/lib.rs | 19 ++++++++- 4 files changed, 112 insertions(+), 29 deletions(-) diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs index cbe6907042d3..699aabe01ee5 100644 --- a/rust/kernel/types.rs +++ b/rust/kernel/types.rs @@ -13,7 +13,10 @@ #[doc(hidden)] pub mod for_lt; -pub use for_lt::CovariantForLt; +pub use for_lt::{ + CovariantForLt, + 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 a11f7509633c..b8f422c802dc 100644 --- a/rust/kernel/types/for_lt.rs +++ b/rust/kernel/types/for_lt.rs @@ -1,22 +1,67 @@ // SPDX-License-Identifier: Apache-2.0 OR MIT -//! Provide implementation and test of the `CovariantForLt` trait and macro. +//! Provide implementation and test of the [`trait@ForLt`] and [`trait@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!`](macro@ForLt) / +//! [`CovariantForLt!`](macro@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 /// -/// The lifetime involved must be covariant. +/// It is not recommended to implement this trait directly. [`ForLt!`](macro@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; +} +pub use macros::ForLt; + +/// [`trait@ForLt`] subtrait for types that are covariant over their lifetime parameter. +/// +/// 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 /// -/// It is not recommended to implement this trait directly. `CovariantForLt!` macro is provided to -/// obtain a type that implements this trait. +/// It is not recommended to implement this trait directly. +/// [`CovariantForLt!`](macro@CovariantForLt) macro is provided to obtain a type that implements +/// this trait. /// /// The full syntax is /// @@ -84,10 +129,7 @@ /// # 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> { @@ -99,25 +141,29 @@ fn cast_ref<'r, 'short: 'r, 'long: 'short>(long: &'r Self::Of<'long>) -> &'r Sel /// This is intended to be an "unsafe-to-refer-to" type. /// -/// Must only be used by the `CovariantForLt!` macro. +/// Must only be used by the [`ForLt!`](macro@ForLt) / [`CovariantForLt!`](macro@CovariantForLt) +/// macros. /// /// `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. /// /// `N` is to provide the macro a place to emit arbitrary items, in case it needs to prove -/// additional properties. +/// additional properties. [`ForLt!`](macro@ForLt) emits `N = 0`; +/// [`CovariantForLt!`](macro@CovariantForLt) emits `N = 1` after a covariance proof. #[doc(hidden)] pub struct UnsafeForLtImpl(PhantomData<(WF, T)>); -// This is a helper trait for implementation `CovariantForLt` to be able to use HRTB. +// This is a helper trait for implementation of `ForLt` / `CovariantForLt` 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 -// `UnsafeForLtImpl` and it will fail to evaluate if the type is not covariant. -unsafe impl WithLt<'a>, WF> CovariantForLt for UnsafeForLtImpl { +impl WithLt<'a>, WF, const N: usize> ForLt for UnsafeForLtImpl { 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. Only `N = 1` gets this impl. +unsafe impl WithLt<'a>, WF> CovariantForLt for UnsafeForLtImpl {} diff --git a/rust/macros/for_lt.rs b/rust/macros/for_lt.rs index 9487a9352f1c..4372cbad3ec4 100644 --- a/rust/macros/for_lt.rs +++ b/rust/macros/for_lt.rs @@ -176,7 +176,12 @@ fn prove(&mut self, ty: &'a Type) { } } -pub(crate) fn covariant_for_lt(input: HigherRankedType) -> TokenStream { +/// Shared implementation for both `ForLt!` and `CovariantForLt!`. +/// +/// Both macros run the prover and emit `ProveWf` structs to check well-formedness for all lifetime +/// instances (workaround for ). `CovariantForLt!` +/// additionally emits covariance proof functions and sets `N = 1`. +fn for_lt_inner(input: HigherRankedType, prove_covariance: bool) -> TokenStream { let (ty, lifetime) = match input { HigherRankedType::Explicit { lifetime, ty, .. } => (ty, lifetime), HigherRankedType::Implicit { ty } => { @@ -211,14 +216,16 @@ struct #wf_proof_name<#lifetime>( )); // Insert a proof that the type is covariant. - let cov_proof_name = format_ident!("prove_covariant_{idx}"); - proof.push(quote!( - fn #cov_proof_name<'__short, '__long: '__short>( - long: #wf_proof_name<'__long> - ) -> #wf_proof_name<'__short> { - long - } - )); + if prove_covariance { + let cov_proof_name = format_ident!("prove_covariant_{idx}"); + proof.push(quote!( + fn #cov_proof_name<'__short, '__long: '__short>( + long: #wf_proof_name<'__long> + ) -> #wf_proof_name<'__short> { + long + } + )); + } } // Make sure that the type is wellformed when substituting lifetime with `'static`. @@ -234,6 +241,8 @@ fn #cov_proof_name<'__short, '__long: '__short>( }, ); + let n: usize = prove_covariance.into(); + quote!( ::kernel::types::for_lt::UnsafeForLtImpl::< dyn for<#lifetime> ::kernel::types::for_lt::WithLt<#lifetime, Of = #ty>, @@ -241,8 +250,16 @@ fn #cov_proof_name<'__short, '__long: '__short>( { #(#proof)* - 0 + #n } > ) } + +pub(crate) fn for_lt(input: HigherRankedType) -> TokenStream { + for_lt_inner(input, false) +} + +pub(crate) fn covariant_for_lt(input: HigherRankedType) -> TokenStream { + for_lt_inner(input, true) +} 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 {