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 <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>

0 files changed, 0 insertions, 0 deletions