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use rustc_hir::lang_items::LangItem;
use rustc_middle::query::Providers;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_trait_selection::traits::{ObligationCause, ObligationCtxt};
/// This method returns true if and only if `adt_ty` itself has been marked as
/// eligible for structural-match: namely, if it implements both
/// `StructuralPartialEq` and `StructuralEq` (which are respectively injected by
/// `#[derive(PartialEq)]` and `#[derive(Eq)]`).
///
/// Note that this does *not* recursively check if the substructure of `adt_ty`
/// implements the traits.
fn has_structural_eq_impls<'tcx>(tcx: TyCtxt<'tcx>, adt_ty: Ty<'tcx>) -> bool {
let infcx = &tcx.infer_ctxt().build();
let cause = ObligationCause::dummy();
let ocx = ObligationCtxt::new(infcx);
// require `#[derive(PartialEq)]`
let structural_peq_def_id =
infcx.tcx.require_lang_item(LangItem::StructuralPeq, Some(cause.span));
ocx.register_bound(cause.clone(), ty::ParamEnv::empty(), adt_ty, structural_peq_def_id);
// for now, require `#[derive(Eq)]`. (Doing so is a hack to work around
// the type `for<'a> fn(&'a ())` failing to implement `Eq` itself.)
let structural_teq_def_id =
infcx.tcx.require_lang_item(LangItem::StructuralTeq, Some(cause.span));
ocx.register_bound(cause, ty::ParamEnv::empty(), adt_ty, structural_teq_def_id);
// We deliberately skip *reporting* fulfillment errors (via
// `report_fulfillment_errors`), for two reasons:
//
// 1. The error messages would mention `std::marker::StructuralPartialEq`
// (a trait which is solely meant as an implementation detail
// for now), and
//
// 2. We are sometimes doing future-incompatibility lints for
// now, so we do not want unconditional errors here.
ocx.select_all_or_error().is_empty()
}
pub fn provide(providers: &mut Providers) {
providers.has_structural_eq_impls = has_structural_eq_impls;
}
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