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Although the documentation makes it clear this should never be referenced directly, doing so is still possible, which creates a loophole for breaking covariance using only safe code. For instance (untested, and based on my limited understanding of the patch): type Bad =3D UnsafeForLtImpl< dyn for<'a> WithLt<'a, Of =3D Cell<&'a u8>>, (), 0, >; The blanket `ForLt` implementation applies, and now we can store a shorter-lived reference into the `Cell` and potentially observe it after its lifetime. This could be worked around by defining the type implementing `ForLt` locally, at the cost of ergonomics since the `ForLt` type would be declared separately: // Defines `DataLt` and implements `ForLt` on it after emitting // covariance proof. define_for_lt!(DataLt =3D for<'a> Data<'a>); ... something: Devres> I'm not claiming this is better; but imho the tradeoff is worth considering.