GenericArg::Infer may be unreachable in rustdoc-types

[fluiderson]

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.