`GenericArg::Infer` may be unreachable in `rustdoc-types`

GenericArg::Infer was introduced in #83484 as a part of #85077. AFAICT, as rustdoc-types are converted from HIR, this variant was propagated to rustdoc-types. It was even documented later in #127290. But is it possible to obtain?

GenericArg is a part of GenericArgs that is a part of AssocItemConstraint, Path, and Type::QualifiedPath. Attempts to use the _ placeholder for any of them fail with the same error E0121. I've also tried to get this variant by generating JSON docs of a few huge crates, but to no avail. I guess it's impossible to use the placeholder in any position inside any item's signature. The only place it's allowed is in statements/expressions that're not represented in rustdoc-types.

I think if there's no intention to allow the placeholder inside item signatures, this variant should be removed from rustdoc-types.

The same also may be applicable to Type::Infer, but I didn't checked it as it's not important for my use case.

Start by tracing how HIR conversion produces rustdoc-types::GenericArg and Type::Infer, then inspect generated JSON documentation for signatures containing GenericArgs. Use the linked playground cases as reproduction attempts. Done means confirming whether these variants are reachable in item signatures and, if not, removing or updating the affected rustdoc-types API with appropriate validation.