Function check_is_structurally_compatible 

fn check_is_structurally_compatible<'tcx>(
    tcx: TyCtxt<'tcx>,
    external_impl: LocalDefId,
    declaration: DefId,
    eii_name: Symbol,
    eii_attr_span: Span,
) -> Result<(), ErrorGuaranteed>

Checks a bunch of different properties of the impl/trait methods for compatibility, such as asyncness, number of argument, self receiver kind, and number of early- and late-bound generics.

Corresponds to check_method_is_structurally_compatible for impl method compatibility checks.