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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 <gary@garyguo.net>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
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 <dakr@kernel.org>
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There are a few cases, e.g. when dealing with data referencing each
other, one might want to write code that is generic over lifetimes. 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 `<TheType as ForLt>::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 <gregkh@linuxfoundation.org>
Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260525202921.124698-23-dakr@kernel.org
[ 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.
Fix a few typos in the documentation and in the commit message, add
empty lines before samples, add missing periods and consistently use
markdown.
- Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
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