On Wed Jun 3, 2026 at 2:10 AM BST, Danilo Krummrich wrote:
> Rename ForLt to CovariantForLt to prepare for the introduction of a new
> ForLt base trait that does not require covariance.
>
> The existing ForLt trait requires covariance, which enables the safe
> cast_ref() method. This rename preserves the same semantics under a more
> precise name, making room for a weaker ForLt trait in a subsequent
> commit.
>
> No functional change.
>
> Signed-off-by: Danilo Krummrich <[email protected]>
> ---
> drivers/gpu/nova-core/driver.rs | 4 +-
> rust/kernel/auxiliary.rs | 23 +++++-----
> rust/kernel/types.rs | 2 +-
> rust/kernel/types/for_lt.rs | 61 ++++++++++++++-------------
> rust/macros/for_lt.rs | 4 +-
> rust/macros/lib.rs | 11 +++--
> samples/rust/rust_driver_auxiliary.rs | 8 ++--
> 7 files changed, 57 insertions(+), 56 deletions(-)
>
> diff --git a/rust/kernel/types/for_lt.rs b/rust/kernel/types/for_lt.rs
> index d44323c28e8d..ef510ab6c092 100644
> --- a/rust/kernel/types/for_lt.rs
> +++ b/rust/kernel/types/for_lt.rs
> @@ -1,8 +1,8 @@
> // SPDX-License-Identifier: Apache-2.0 OR MIT
>
> -//! Provide implementation and test of the `ForLt` trait and macro.
> +//! Provide implementation and test of the `CovariantForLt` trait and macro.
> //!
> -//! This module is hidden and user should just use `ForLt!` directly.
> +//! This module is hidden and user should just use `CovariantForLt!`
> directly.
>
> use core::marker::PhantomData;
>
> @@ -15,38 +15,39 @@
> ///
> /// # Macro
> ///
> -/// It is not recommended to implement this trait directly. `ForLt!` macro
> is provided to obtain a
> -/// type that implements this trait.
> +/// It is not recommended to implement this trait directly.
> `CovariantForLt!` macro is provided to
> +/// obtain a type that implements this trait.
> ///
> /// The full syntax is
> ///
> /// ```
> -/// # use kernel::types::ForLt;
> -/// # fn expect_lt<F: ForLt>() {}
> +/// # use kernel::types::CovariantForLt;
> +/// # fn expect_lt<F: CovariantForLt>() {}
> /// # struct TypeThatUse<'a>(&'a ());
> /// # expect_lt::<
> -/// ForLt!(for<'a> TypeThatUse<'a>)
> +/// CovariantForLt!(for<'a> TypeThatUse<'a>)
> /// # >();
> /// ```
> ///
> -/// which gives a type so that `<ForLt!(for<'a> TypeThatUse<'a>) as
> ForLt>::Of<'b>`
> +/// which gives a type so that
> +/// `<CovariantForLt!(for<'a> TypeThatUse<'a>) as CovariantForLt>::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<F: ForLt>() {}
> +/// # use kernel::types::CovariantForLt;
> +/// # fn expect_lt<F: CovariantForLt>() {}
> /// # struct TypeThatUse<'a>(&'a ());
> /// # expect_lt::<
> -/// ForLt!(TypeThatUse<'_>) // Equivalent to `ForLt!(for<'a>
> TypeThatUse<'a>)`.
> +/// CovariantForLt!(TypeThatUse<'_>) // Equivalent to
> `CovariantForLt!(for<'a> TypeThatUse<'a>)`.
> /// # >();
> /// # expect_lt::<
> -/// ForLt!(&u32) // Equivalent to `ForLt!(for<'a> &'a u32)`.
> +/// CovariantForLt!(&u32) // Equivalent to `CovariantForLt!(for<'a> &'a
> u32)`.
> /// # >();
> /// # expect_lt::<
> -/// ForLt!(u32) // Equivalent to `ForLt!(for<'a> u32)`.
> +/// CovariantForLt!(u32) // Equivalent to `CovariantForLt!(for<'a> u32)`.
> /// # >();
> /// ```
> ///
> @@ -55,10 +56,10 @@
> /// it.
> ///
> /// ```ignore,compile_fail
> -/// # use kernel::types::ForLt;
> -/// # fn expect_lt<F: ForLt>() {}
> +/// # use kernel::types::CovariantForLt;
> +/// # fn expect_lt<F: CovariantForLt>() {}
> /// # expect_lt::<
> -/// ForLt!(fn(&u32)) // Contravariant, will fail compilation.
> +/// CovariantForLt!(fn(&u32)) // Contravariant, will fail compilation.
> /// # >();
> /// ```
> ///
> @@ -67,23 +68,23 @@
> /// the generic parameter but is in a separate item.
> ///
> /// ```
> -/// # use kernel::types::ForLt;
> -/// fn expect_lt<F: ForLt>() {}
> +/// # use kernel::types::CovariantForLt;
> +/// fn expect_lt<F: CovariantForLt>() {}
> /// # #[allow(clippy::unnecessary_safety_comment, reason = "false positive")]
> /// fn generic_fn<T: 'static>() {
> /// // Syntactically proven by the macro
> -/// expect_lt::<ForLt!(&T)>();
> +/// expect_lt::<CovariantForLt!(&T)>();
> /// // Syntactically proven by the macro
> -/// expect_lt::<ForLt!(&KBox<T>)>();
> +/// expect_lt::<CovariantForLt!(&KBox<T>)>();
> /// // Cannot be syntactically proven, need to check covariance of `KBox`
> -/// // expect_lt::<ForLt!(&KBox<&T>)>();
> +/// // expect_lt::<CovariantForLt!(&KBox<&T>)>();
> /// }
> /// ```
> ///
> /// # Safety
> ///
> /// `Self::Of<'a>` must be covariant over the lifetime `'a`.
> -pub unsafe trait ForLt {
> +pub unsafe trait CovariantForLt {
> /// The type parameterized by the lifetime.
> type Of<'a>: 'a;
>
> @@ -94,11 +95,11 @@ fn cast_ref<'r, 'short: 'r, 'long: 'short>(long: &'r
> Self::Of<'long>) -> &'r Sel
> unsafe { core::mem::transmute(long) }
> }
> }
> -pub use macros::ForLt;
> +pub use macros::CovariantForLt;
>
> -/// This is intended to be an "unsafe-to-refer-to" type.
> +/// Helper type for the `CovariantForLt!` macro.
> ///
> -/// Must only be used by the `ForLt!` macro.
> +/// Must only be used by the `CovariantForLt!` macro.
> ///
> /// `T` is the magic `dyn for<'a> WithLt<'a, TypeThatUse<'a>>` generated by
> macro.
> ///
> @@ -107,16 +108,16 @@ fn cast_ref<'r, 'short: 'r, 'long: 'short>(long: &'r
> Self::Of<'long>) -> &'r Sel
> /// `N` is to provide the macro a place to emit arbitrary items, in case it
> needs to prove
> /// additional properties.
> #[doc(hidden)]
> -pub struct UnsafeForLtImpl<T: ?Sized, WF, const N: usize>(PhantomData<(WF,
> T)>);
> +pub struct CovariantForLtImpl<T: ?Sized, WF, const N:
> usize>(PhantomData<(WF, T)>);
I think I prefer this name to be stayed the same (so it has the "Unsafe" in it
and is not too long).
You can actually use this same helper type for both `ForLt!` and
`CovariantForLt!` later in patch 2, by using different `N` based on whether
covariance check has been conducted. So
impl<T: ?Sized + for<'a> WithLt<'a>, WF, const N: usize> ForLt for
UnsafeForLtImpl<T, WF, N> {
type Of<'a> = <T as WithLt<'a>>::Of;
}
impl<T: ?Sized + for<'a> WithLt<'a>, WF> CovariantForLt for UnsafeForLtImpl<T,
WF, 1> {}
Best,
Gary
>
> -// This is a helper trait for implementation `ForLt` to be able to use HRTB.
> +// This is a helper trait for implementation `CovariantForLt` to be able to
> use HRTB.
> #[doc(hidden)]
> pub trait WithLt<'a> {
> type Of: 'a;
> }
>
> -// SAFETY: In `ForLt!` macro, a covariance proof is generated when naming
> `UnsafeForLtImpl`
> -// and it will fail to evaluate if the type is not covariant.
> -unsafe impl<T: ?Sized + for<'a> WithLt<'a>, WF> ForLt for UnsafeForLtImpl<T,
> WF, 0> {
> +// 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<T: ?Sized + for<'a> WithLt<'a>, WF> CovariantForLt for
> CovariantForLtImpl<T, WF, 0> {
> type Of<'a> = <T as WithLt<'a>>::Of;
> }
> diff --git a/rust/macros/for_lt.rs b/rust/macros/for_lt.rs
> index 364d4113cd10..3cb094d00548 100644
> --- a/rust/macros/for_lt.rs
> +++ b/rust/macros/for_lt.rs
> @@ -176,7 +176,7 @@ fn prove(&mut self, ty: &'a Type) {
> }
> }
>
> -pub(crate) fn for_lt(input: HigherRankedType) -> TokenStream {
> +pub(crate) fn covariant_for_lt(input: HigherRankedType) -> TokenStream {
> let (ty, lifetime) = match input {
> HigherRankedType::Explicit { lifetime, ty, .. } => (ty, lifetime),
> HigherRankedType::Implicit { ty } => {
> @@ -235,7 +235,7 @@ fn #cov_proof_name<'__short, '__long: '__short>(
> );
>
> quote!(
> - ::kernel::types::for_lt::UnsafeForLtImpl::<
> + ::kernel::types::for_lt::CovariantForLtImpl::<
> dyn for<#lifetime> ::kernel::types::for_lt::WithLt<#lifetime, Of
> = #ty>,
> #ty_static,
> {
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index 4a48fabbc268..2167cb270928 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -491,14 +491,13 @@ pub fn kunit_tests(attr: TokenStream, input:
> TokenStream) -> TokenStream {
> .into()
> }
>
> -/// Obtain a type that implements [`ForLt`] for the given higher-ranked type.
> +/// Obtain a type that implements [`CovariantForLt`] for the given
> higher-ranked type.
> ///
> -/// Please refer to the documentation of the [`ForLt`] trait.
> +/// Please refer to the documentation of the [`CovariantForLt`] trait.
> ///
> -/// [`ForLt`]: trait.ForLt.html
> +/// [`CovariantForLt`]: trait.CovariantForLt.html
> #[proc_macro]
> -// The macro shares the name with the trait.
> #[allow(non_snake_case)]
> -pub fn ForLt(input: TokenStream) -> TokenStream {
> - for_lt::for_lt(parse_macro_input!(input)).into()
> +pub fn CovariantForLt(input: TokenStream) -> TokenStream {
> + for_lt::covariant_for_lt(parse_macro_input!(input)).into()
> }
> diff --git a/samples/rust/rust_driver_auxiliary.rs
> b/samples/rust/rust_driver_auxiliary.rs
> index 2c1351040e45..92ee6a6d348e 100644
> --- a/samples/rust/rust_driver_auxiliary.rs
> +++ b/samples/rust/rust_driver_auxiliary.rs
> @@ -13,7 +13,7 @@
> driver,
> pci,
> prelude::*,
> - types::ForLt,
> + types::CovariantForLt,
> InPlaceModule, //
> };
>
> @@ -60,8 +60,8 @@ struct Data<'bound> {
>
> #[allow(clippy::type_complexity)]
> struct ParentData<'bound> {
> - _reg0: auxiliary::Registration<'bound, ForLt!(Data<'_>)>,
> - _reg1: auxiliary::Registration<'bound, ForLt!(Data<'_>)>,
> + _reg0: auxiliary::Registration<'bound, CovariantForLt!(Data<'_>)>,
> + _reg1: auxiliary::Registration<'bound, CovariantForLt!(Data<'_>)>,
> }
>
> kernel::pci_device_table!(
> @@ -115,7 +115,7 @@ fn probe<'bound>(
>
> impl ParentDriver {
> fn connect(adev: &auxiliary::Device<Bound>) -> Result {
> - let data = adev.registration_data::<ForLt!(Data<'_>)>()?;
> + let data = adev.registration_data::<CovariantForLt!(Data<'_>)>()?;
> let pdev = data.parent;
>
> dev_info!(
