Re: [PATCH v3 2/7] rust: types: introduce ForLt base trait for CovariantForLt

["Gary Guo" <gary@garyguo.net>]

On Fri Jun 19, 2026 at 12:08 AM BST, Danilo Krummrich wrote:
> 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).
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/types.rs        |  1 +
>  rust/kernel/types/for_lt.rs | 72 +++++++++++++++++++++++++++++--------
>  rust/macros/for_lt.rs       | 53 +++++++++++++++++++++------
>  rust/macros/lib.rs          | 19 +++++++++-
>  4 files changed, 118 insertions(+), 27 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 a11f7509633c..0b53494080b7 100644
> --- a/rust/kernel/types/for_lt.rs
> +++ b/rust/kernel/types/for_lt.rs
> @@ -1,17 +1,59 @@
>  // 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 lifetime involved must be covariant.
> +/// The full syntax is
> +///
> +/// ```
> +/// # use kernel::types::ForLt;
> +/// # fn expect_lt<F: ForLt>() {}
> +/// # struct TypeThatUse<'a>(&'a ());
> +/// # expect_lt::<
> +/// ForLt!(for<'a> TypeThatUse<'a>)
> +/// # >();
> +/// ```
> +///
> +/// which gives a type so that `<ForLt!(for<'a> 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<F: ForLt>() {}
> +/// # 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
>  ///
> @@ -84,10 +126,7 @@
>  /// # 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 +138,28 @@ 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!` / `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!` emits `N = 0`; `CovariantForLt!` emits `N = 1` after a
> +/// covariance proof.
>  #[doc(hidden)]
>  pub struct UnsafeForLtImpl<T: ?Sized, WF, const N: usize>(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<T: ?Sized + for<'a> WithLt<'a>, WF> CovariantForLt for UnsafeForLtImpl<T, WF, 0> {
> +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;
>  }
> +
> +// 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<T: ?Sized + for<'a> WithLt<'a>, WF> CovariantForLt for UnsafeForLtImpl<T, WF, 1> {}
> diff --git a/rust/macros/for_lt.rs b/rust/macros/for_lt.rs
> index e1233701d6cc..d5f728a464ca 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 +190,42 @@ 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::UnsafeForLtImpl::<
> +            dyn for<#lifetime> ::kernel::types::for_lt::WithLt<#lifetime, Of = #ty>,
> +            #ty_static,
> +            0,
> +        >

This is still unsound, as I mentioned in the last version that the prover must
be kept.

`'static` helps with a specific case where it ensures that `ForLt!(for<'a> &'a T)`
requiress `T: 'static`, but when another lifetime (that is not 'static is
involved) the prover is still needed.

A very simple case

    fn test_for_lt<'a>(x: &'a ()) {
        let _: ForLt!(for<'b> &'a &'b ());
    }

this needs to fail the build because 'a and 'b are unrelated (otherwise from the
type you would be able to obtain `'b: 'a` from nowhere).

Best,
Gary

> +    )
> +}
> +
> +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 +263,7 @@ 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::UnsafeForLtImpl::<
> @@ -241,7 +272,7 @@ fn #cov_proof_name<'__short, '__long: '__short>(
>              {
>                  #(#proof)*
>  
> -                0
> +                1
>              }
>          >
>      )
> 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 {