rustc_hir_analysis/outlives/

implicit_infer.rs

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

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<'_>,
) -> FxIndexMap<DefId, ty::EarlyBinder<'_, RequiredPredicates<'_>>> {
    debug!("infer_predicates");

    let mut explicit_map = ExplicitPredicatesMap::new();

    let mut global_inferred_outlives = FxIndexMap::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).instantiate_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,
                        );
                    }
                }

                DefKind::TyAlias if tcx.type_alias_is_lazy(item_did) => {
                    insert_required_predicates_to_be_wf(
                        tcx,
                        tcx.type_of(item_did).instantiate_identity(),
                        tcx.def_span(item_did),
                        &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.as_ref().skip_binder().len());
            if item_required_predicates.len() > item_predicates_len {
                predicates_added = true;
                global_inferred_outlives
                    .insert(item_did.to_def_id(), ty::EarlyBinder::bind(item_required_predicates));
            }
        }

        if !predicates_added {
            break 'outer;
        }
    }

    global_inferred_outlives
}

fn insert_required_predicates_to_be_wf<'tcx>(
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>,
    span: Span,
    global_inferred_outlives: &FxIndexMap<DefId, ty::EarlyBinder<'tcx, RequiredPredicates<'tcx>>>,
    required_predicates: &mut RequiredPredicates<'tcx>,
    explicit_map: &mut ExplicitPredicatesMap<'tcx>,
) {
    for arg in ty.walk() {
        let leaf_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 *leaf_ty.kind() {
            ty::Ref(region, rty, _) => {
                // The type is `&'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`.
                debug!("Ref");
                insert_outlives_predicate(tcx, rty.into(), region, span, required_predicates);
            }

            ty::Adt(def, args) => {
                // For ADTs (structs/enums/unions), we check inferred and explicit predicates.
                debug!("Adt");
                check_inferred_predicates(
                    tcx,
                    def.did(),
                    args,
                    global_inferred_outlives,
                    required_predicates,
                );
                check_explicit_predicates(
                    tcx,
                    def.did(),
                    args,
                    required_predicates,
                    explicit_map,
                    None,
                );
            }

            ty::Alias(ty::Weak, alias) => {
                // This corresponds to a type like `Type<'a, T>`.
                // We check inferred and explicit predicates.
                debug!("Weak");
                check_inferred_predicates(
                    tcx,
                    alias.def_id,
                    alias.args,
                    global_inferred_outlives,
                    required_predicates,
                );
                check_explicit_predicates(
                    tcx,
                    alias.def_id,
                    alias.args,
                    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");
                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 args = ex_trait_ref.with_self_ty(tcx, tcx.types.usize).skip_binder().args;
                    check_explicit_predicates(
                        tcx,
                        ex_trait_ref.skip_binder().def_id,
                        args,
                        required_predicates,
                        explicit_map,
                        Some(tcx.types.self_param),
                    );
                }
            }

            ty::Alias(ty::Projection, alias) => {
                // This corresponds to a type like `<() as Trait<'a, T>>::Type`.
                // We only use the explicit predicates of the trait but
                // not the ones of the associated type itself.
                debug!("Projection");
                check_explicit_predicates(
                    tcx,
                    tcx.parent(alias.def_id),
                    alias.args,
                    required_predicates,
                    explicit_map,
                    None,
                );
            }

            // FIXME(inherent_associated_types): Use the explicit predicates from the parent impl.
            ty::Alias(ty::Inherent, _) => {}

            _ => {}
        }
    }
}

/// Check the explicit predicates declared on the type.
///
/// ### Example
///
/// ```ignore (illustrative)
/// struct Outer<'a, T> {
///     field: Inner<T>,
/// }
///
/// struct Inner<U> where U: 'static, U: Outer {
///     // ...
/// }
/// ```
/// Here, we should fetch the explicit predicates, which
/// will give us `U: 'static` and `U: Outer`. The latter we
/// can ignore, but we will want to process `U: 'static`,
/// applying the instantiation as above.
fn check_explicit_predicates<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: DefId,
    args: &[GenericArg<'tcx>],
    required_predicates: &mut RequiredPredicates<'tcx>,
    explicit_map: &mut ExplicitPredicatesMap<'tcx>,
    ignored_self_ty: Option<Ty<'tcx>>,
) {
    debug!(
        "check_explicit_predicates(def_id={:?}, \
         args={:?}, \
         explicit_map={:?}, \
         required_predicates={:?}, \
         ignored_self_ty={:?})",
        def_id, args, 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.as_ref().skip_binder() {
        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 `args`. In the
        // case that `args` come from a `dyn Trait` type, our caller will have
        // included `Self = usize` as the value for `Self`. If we were
        // to apply the args, 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).instantiate(tcx, args);
        debug!("predicate = {predicate:?}");
        insert_outlives_predicate(tcx, predicate.0, predicate.1, span, required_predicates);
    }
}

/// Check the inferred predicates declared on the type.
///
/// ### Example
///
/// ```ignore (illustrative)
/// struct Outer<'a, T> {
///     outer: Inner<'a, T>,
/// }
///
/// struct Inner<'b, U> {
///     inner: &'b U,
/// }
/// ```
///
/// Here, when processing the type of field `outer`, we would request the
/// set of implicit predicates computed for `Inner` thus far. This will
/// initially come back empty, but in next round we will get `U: 'b`.
/// We then apply the instantiation `['b => 'a, U => T]` and thus get the
/// requirement that `T: 'a` holds for `Outer`.
fn check_inferred_predicates<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: DefId,
    args: ty::GenericArgsRef<'tcx>,
    global_inferred_outlives: &FxIndexMap<DefId, ty::EarlyBinder<'tcx, RequiredPredicates<'tcx>>>,
    required_predicates: &mut RequiredPredicates<'tcx>,
) {
    // Load the current set of inferred and explicit predicates from `global_inferred_outlives`
    // and filter the ones that are `TypeOutlives`.

    let Some(predicates) = global_inferred_outlives.get(&def_id) else {
        return;
    };

    for (&predicate, &span) in predicates.as_ref().skip_binder() {
        // `predicate` is `U: 'b` in the example above.
        // So apply the instantiation to get `T: 'a`.
        let ty::OutlivesPredicate(arg, region) =
            predicates.rebind(predicate).instantiate(tcx, args);
        insert_outlives_predicate(tcx, arg, region, span, required_predicates);
    }
}