compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

use rustc_data_structures::fx::FxHashMap;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{GenericArg, GenericArgKind};
use rustc_span::Span;

use super::explicit::ExplicitPredicatesMap;
use super::utils::*;

/// Infer predicates for the items in the crate.
///
/// `global_inferred_outlives`: this is initially the empty map that
///     was generated by walking the items in the crate. This will
///     now be filled with inferred predicates.
pub(super) fn infer_predicates(
    tcx: TyCtxt<'_>,
) -> FxHashMap<DefId, ty::EarlyBinder<RequiredPredicates<'_>>> {
    debug!("infer_predicates");

    let mut explicit_map = ExplicitPredicatesMap::new();

    let mut global_inferred_outlives = FxHashMap::default();

    // If new predicates were added then we need to re-calculate
    // all crates since there could be new implied predicates.
    'outer: loop {
        let mut predicates_added = false;

        // Visit all the crates and infer predicates
        for id in tcx.hir().items() {
            let item_did = id.owner_id;

            debug!("InferVisitor::visit_item(item={:?})", item_did);

            let mut item_required_predicates = RequiredPredicates::default();
            match tcx.def_kind(item_did) {
                DefKind::Union | DefKind::Enum | DefKind::Struct => {
                    let adt_def = tcx.adt_def(item_did.to_def_id());

                    // Iterate over all fields in item_did
                    for field_def in adt_def.all_fields() {
                        // Calculating the predicate requirements necessary
                        // for item_did.
                        //
                        // For field of type &'a T (reference) or Adt
                        // (struct/enum/union) there will be outlive
                        // requirements for adt_def.
                        let field_ty = tcx.type_of(field_def.did).subst_identity();
                        let field_span = tcx.def_span(field_def.did);
                        insert_required_predicates_to_be_wf(
                            tcx,
                            field_ty,
                            field_span,
                            &global_inferred_outlives,
                            &mut item_required_predicates,
                            &mut explicit_map,
                        );
                    }
                }

                _ => {}
            };

            // If new predicates were added (`local_predicate_map` has more
            // predicates than the `global_inferred_outlives`), the new predicates
            // might result in implied predicates for their parent types.
            // Therefore mark `predicates_added` as true and which will ensure
            // we walk the crates again and re-calculate predicates for all
            // items.
            let item_predicates_len: usize =
                global_inferred_outlives.get(&item_did.to_def_id()).map_or(0, |p| p.0.len());
            if item_required_predicates.len() > item_predicates_len {
                predicates_added = true;
                global_inferred_outlives
                    .insert(item_did.to_def_id(), ty::EarlyBinder::new(item_required_predicates));
            }
        }

        if !predicates_added {
            break 'outer;
        }
    }

    global_inferred_outlives
}

fn insert_required_predicates_to_be_wf<'tcx>(
    tcx: TyCtxt<'tcx>,
    field_ty: Ty<'tcx>,
    field_span: Span,
    global_inferred_outlives: &FxHashMap<DefId, ty::EarlyBinder<RequiredPredicates<'tcx>>>,
    required_predicates: &mut RequiredPredicates<'tcx>,
    explicit_map: &mut ExplicitPredicatesMap<'tcx>,
) {
    for arg in field_ty.walk() {
        let ty = match arg.unpack() {
            GenericArgKind::Type(ty) => ty,

            // No predicates from lifetimes or constants, except potentially
            // constants' types, but `walk` will get to them as well.
            GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
        };

        match *ty.kind() {
            // The field is of type &'a T which means that we will have
            // a predicate requirement of T: 'a (T outlives 'a).
            //
            // We also want to calculate potential predicates for the T
            ty::Ref(region, rty, _) => {
                debug!("Ref");
                insert_outlives_predicate(tcx, rty.into(), region, field_span, required_predicates);
            }

            // For each Adt (struct/enum/union) type `Foo<'a, T>`, we
            // can load the current set of inferred and explicit
            // predicates from `global_inferred_outlives` and filter the
            // ones that are TypeOutlives.
            ty::Adt(def, substs) => {
                // First check the inferred predicates
                //
                // Example 1:
                //
                //     struct Foo<'a, T> {
                //         field1: Bar<'a, T>
                //     }
                //
                //     struct Bar<'b, U> {
                //         field2: &'b U
                //     }
                //
                // Here, when processing the type of `field1`, we would
                // request the set of implicit predicates computed for `Bar`
                // thus far. This will initially come back empty, but in next
                // round we will get `U: 'b`. We then apply the substitution
                // `['b => 'a, U => T]` and thus get the requirement that `T:
                // 'a` holds for `Foo`.
                debug!("Adt");
                if let Some(unsubstituted_predicates) = global_inferred_outlives.get(&def.did()) {
                    for (unsubstituted_predicate, &span) in &unsubstituted_predicates.0 {
                        // `unsubstituted_predicate` is `U: 'b` in the
                        // example above. So apply the substitution to
                        // get `T: 'a` (or `predicate`):
                        let predicate = unsubstituted_predicates
                            .rebind(*unsubstituted_predicate)
                            .subst(tcx, substs);
                        insert_outlives_predicate(
                            tcx,
                            predicate.0,
                            predicate.1,
                            span,
                            required_predicates,
                        );
                    }
                }

                // Check if the type has any explicit predicates that need
                // to be added to `required_predicates`
                // let _: () = substs.region_at(0);
                check_explicit_predicates(
                    tcx,
                    def.did(),
                    substs,
                    required_predicates,
                    explicit_map,
                    None,
                );
            }

            ty::Dynamic(obj, ..) => {
                // This corresponds to `dyn Trait<..>`. In this case, we should
                // use the explicit predicates as well.

                debug!("Dynamic");
                debug!("field_ty = {}", &field_ty);
                debug!("ty in field = {}", &ty);
                if let Some(ex_trait_ref) = obj.principal() {
                    // Here, we are passing the type `usize` as a
                    // placeholder value with the function
                    // `with_self_ty`, since there is no concrete type
                    // `Self` for a `dyn Trait` at this
                    // stage. Therefore when checking explicit
                    // predicates in `check_explicit_predicates` we
                    // need to ignore checking the explicit_map for
                    // Self type.
                    let substs =
                        ex_trait_ref.with_self_ty(tcx, tcx.types.usize).skip_binder().substs;
                    check_explicit_predicates(
                        tcx,
                        ex_trait_ref.skip_binder().def_id,
                        substs,
                        required_predicates,
                        explicit_map,
                        Some(tcx.types.self_param),
                    );
                }
            }

            ty::Alias(ty::Projection, obj) => {
                // This corresponds to `<T as Foo<'a>>::Bar`. In this case, we should use the
                // explicit predicates as well.
                debug!("Projection");
                check_explicit_predicates(
                    tcx,
                    tcx.parent(obj.def_id),
                    obj.substs,
                    required_predicates,
                    explicit_map,
                    None,
                );
            }

            // FIXME(inherent_associated_types): Handle this case properly.
            ty::Alias(ty::Inherent, _) => {}

            _ => {}
        }
    }
}

/// We also have to check the explicit predicates
/// declared on the type.
/// ```ignore (illustrative)
/// struct Foo<'a, T> {
///     field1: Bar<T>
/// }
///
/// struct Bar<U> where U: 'static, U: Foo {
///     ...
/// }
/// ```
/// Here, we should fetch the explicit predicates, which
/// will give us `U: 'static` and `U: Foo`. The latter we
/// can ignore, but we will want to process `U: 'static`,
/// applying the substitution as above.
fn check_explicit_predicates<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: DefId,
    substs: &[GenericArg<'tcx>],
    required_predicates: &mut RequiredPredicates<'tcx>,
    explicit_map: &mut ExplicitPredicatesMap<'tcx>,
    ignored_self_ty: Option<Ty<'tcx>>,
) {
    debug!(
        "check_explicit_predicates(def_id={:?}, \
         substs={:?}, \
         explicit_map={:?}, \
         required_predicates={:?}, \
         ignored_self_ty={:?})",
        def_id, substs, explicit_map, required_predicates, ignored_self_ty,
    );
    let explicit_predicates = explicit_map.explicit_predicates_of(tcx, def_id);

    for (outlives_predicate, &span) in &explicit_predicates.0 {
        debug!("outlives_predicate = {:?}", &outlives_predicate);

        // Careful: If we are inferring the effects of a `dyn Trait<..>`
        // type, then when we look up the predicates for `Trait`,
        // we may find some that reference `Self`. e.g., perhaps the
        // definition of `Trait` was:
        //
        // ```
        // trait Trait<'a, T> where Self: 'a  { .. }
        // ```
        //
        // we want to ignore such predicates here, because
        // there is no type parameter for them to affect. Consider
        // a struct containing `dyn Trait`:
        //
        // ```
        // struct MyStruct<'x, X> { field: Box<dyn Trait<'x, X>> }
        // ```
        //
        // The `where Self: 'a` predicate refers to the *existential, hidden type*
        // that is represented by the `dyn Trait`, not to the `X` type parameter
        // (or any other generic parameter) declared on `MyStruct`.
        //
        // Note that we do this check for self **before** applying `substs`. In the
        // case that `substs` come from a `dyn Trait` type, our caller will have
        // included `Self = usize` as the value for `Self`. If we were
        // to apply the substs, and not filter this predicate, we might then falsely
        // conclude that e.g., `X: 'x` was a reasonable inferred requirement.
        //
        // Another similar case is where we have an inferred
        // requirement like `<Self as Trait>::Foo: 'b`. We presently
        // ignore such requirements as well (cc #54467)-- though
        // conceivably it might be better if we could extract the `Foo
        // = X` binding from the object type (there must be such a
        // binding) and thus infer an outlives requirement that `X:
        // 'b`.
        if let Some(self_ty) = ignored_self_ty
            && let GenericArgKind::Type(ty) = outlives_predicate.0.unpack()
            && ty.walk().any(|arg| arg == self_ty.into())
        {
            debug!("skipping self ty = {:?}", &ty);
            continue;
        }

        let predicate = explicit_predicates.rebind(*outlives_predicate).subst(tcx, substs);
        debug!("predicate = {:?}", &predicate);
        insert_outlives_predicate(tcx, predicate.0, predicate.1, span, required_predicates);
    }
}