Re: [PATCH v4 2/7] rust: types: introduce ForLt base trait for CovariantForLt

[Miguel Ojeda]

On Fri, Jun 26, 2026 at 8:36 PM Danilo Krummrich <dakr@xxxxxxxxxx> 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@xxxxxxxxxx>

If this goes in at the same time as the move (as I assume), then am I
understanding it right that if someone else was using `ForLt`
(trait/macro) things would either break at compile-time (which is OK)
or, in the covariant type case with no `cast_ref()`, it would build,
but someone could in principle have relied on `ForLt` providing the
covariance guarantee in unsafe code?

Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>

We could take the chance to add some missing intra-doc links, but I
can add a good first issue.

Thanks!

Cheers,
Miguel