compiler/rustc_typeck/src/collect/item_bounds.rs

use super::ItemCtxt;
use crate::astconv::{AstConv, SizedByDefault};
use rustc_hir as hir;
use rustc_infer::traits::util;
use rustc_middle::ty::subst::InternalSubsts;
use rustc_middle::ty::{self, TyCtxt};
use rustc_span::def_id::DefId;
use rustc_span::Span;

/// For associated types we include both bounds written on the type
/// (`type X: Trait`) and predicates from the trait: `where Self::X: Trait`.
///
/// Note that this filtering is done with the items identity substs to
/// simplify checking that these bounds are met in impls. This means that
/// a bound such as `for<'b> <Self as X<'b>>::U: Clone` can't be used, as in
/// `hr-associated-type-bound-1.rs`.
fn associated_type_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    assoc_item_def_id: DefId,
    bounds: &'tcx [hir::GenericBound<'tcx>],
    span: Span,
) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
    let item_ty = tcx.mk_projection(
        assoc_item_def_id,
        InternalSubsts::identity_for_item(tcx, assoc_item_def_id),
    );

    let bounds = AstConv::compute_bounds(
        &ItemCtxt::new(tcx, assoc_item_def_id),
        item_ty,
        bounds,
        SizedByDefault::Yes,
        span,
    );

    let trait_def_id = tcx.associated_item(assoc_item_def_id).container.id();
    let trait_predicates = tcx.trait_explicit_predicates_and_bounds(trait_def_id.expect_local());

    let bounds_from_parent =
        trait_predicates.predicates.iter().copied().filter(|(pred, _)| match pred.skip_binders() {
            ty::PredicateAtom::Trait(tr, _) => tr.self_ty() == item_ty,
            ty::PredicateAtom::Projection(proj) => proj.projection_ty.self_ty() == item_ty,
            ty::PredicateAtom::TypeOutlives(outlives) => outlives.0 == item_ty,
            _ => false,
        });

    let all_bounds = tcx
        .arena
        .alloc_from_iter(bounds.predicates(tcx, item_ty).into_iter().chain(bounds_from_parent));
    debug!("associated_type_bounds({}) = {:?}", tcx.def_path_str(assoc_item_def_id), all_bounds);
    all_bounds
}

/// Opaque types don't inherit bounds from their parent: for return position
/// impl trait it isn't possible to write a suitable predicate on the
/// containing function and for type-alias impl trait we don't have a backwards
/// compatibility issue.
fn opaque_type_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    opaque_def_id: DefId,
    bounds: &'tcx [hir::GenericBound<'tcx>],
    span: Span,
) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
    let item_ty =
        tcx.mk_opaque(opaque_def_id, InternalSubsts::identity_for_item(tcx, opaque_def_id));

    let bounds = ty::print::with_no_queries(|| {
        AstConv::compute_bounds(
            &ItemCtxt::new(tcx, opaque_def_id),
            item_ty,
            bounds,
            SizedByDefault::Yes,
            span,
        )
    });

    let bounds = bounds.predicates(tcx, item_ty);
    debug!("opaque_type_bounds({}) = {:?}", tcx.def_path_str(opaque_def_id), bounds);

    tcx.arena.alloc_slice(&bounds)
}

pub(super) fn explicit_item_bounds(
    tcx: TyCtxt<'_>,
    def_id: DefId,
) -> &'_ [(ty::Predicate<'_>, Span)] {
    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
    match tcx.hir().get(hir_id) {
        hir::Node::TraitItem(hir::TraitItem {
            kind: hir::TraitItemKind::Type(bounds, _),
            span,
            ..
        }) => associated_type_bounds(tcx, def_id, bounds, *span),
        hir::Node::Item(hir::Item {
            kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }),
            span,
            ..
        }) => opaque_type_bounds(tcx, def_id, bounds, *span),
        _ => bug!("item_bounds called on {:?}", def_id),
    }
}

pub(super) fn item_bounds(tcx: TyCtxt<'_>, def_id: DefId) -> &'_ ty::List<ty::Predicate<'_>> {
    tcx.mk_predicates(
        util::elaborate_predicates(
            tcx,
            tcx.explicit_item_bounds(def_id).iter().map(|&(bound, _span)| bound),
        )
        .map(|obligation| obligation.predicate),
    )
}