rustc_trait_selection/traits/specialize/

specialization_graph.rs

use rustc_errors::ErrorGuaranteed;
use rustc_hir::def_id::DefId;
use rustc_macros::extension;
use rustc_middle::bug;
pub use rustc_middle::traits::specialization_graph::*;
use rustc_middle::ty::fast_reject::{self, SimplifiedType, TreatParams};
use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
use tracing::{debug, instrument};

use super::OverlapError;
use crate::traits;

#[derive(Copy, Clone, Debug)]
pub enum FutureCompatOverlapErrorKind {
    LeakCheck,
}

#[derive(Debug)]
pub struct FutureCompatOverlapError<'tcx> {
    pub error: OverlapError<'tcx>,
    pub kind: FutureCompatOverlapErrorKind,
}

/// The result of attempting to insert an impl into a group of children.
#[derive(Debug)]
enum Inserted<'tcx> {
    /// The impl was inserted as a new child in this group of children.
    BecameNewSibling(Option<FutureCompatOverlapError<'tcx>>),

    /// The impl should replace existing impls [X1, ..], because the impl specializes X1, X2, etc.
    ReplaceChildren(Vec<DefId>),

    /// The impl is a specialization of an existing child.
    ShouldRecurseOn(DefId),
}

#[extension(trait ChildrenExt<'tcx>)]
impl<'tcx> Children {
    /// Insert an impl into this set of children without comparing to any existing impls.
    fn insert_blindly(&mut self, tcx: TyCtxt<'tcx>, impl_def_id: DefId) {
        let trait_ref = tcx.impl_trait_ref(impl_def_id).skip_binder();
        if let Some(st) =
            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::InstantiateWithInfer)
        {
            debug!("insert_blindly: impl_def_id={:?} st={:?}", impl_def_id, st);
            self.non_blanket_impls.entry(st).or_default().push(impl_def_id)
        } else {
            debug!("insert_blindly: impl_def_id={:?} st=None", impl_def_id);
            self.blanket_impls.push(impl_def_id)
        }
    }

    /// Removes an impl from this set of children. Used when replacing
    /// an impl with a parent. The impl must be present in the list of
    /// children already.
    fn remove_existing(&mut self, tcx: TyCtxt<'tcx>, impl_def_id: DefId) {
        let trait_ref = tcx.impl_trait_ref(impl_def_id).skip_binder();
        let vec: &mut Vec<DefId>;
        if let Some(st) =
            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::InstantiateWithInfer)
        {
            debug!("remove_existing: impl_def_id={:?} st={:?}", impl_def_id, st);
            vec = self.non_blanket_impls.get_mut(&st).unwrap();
        } else {
            debug!("remove_existing: impl_def_id={:?} st=None", impl_def_id);
            vec = &mut self.blanket_impls;
        }

        let index = vec.iter().position(|d| *d == impl_def_id).unwrap();
        vec.remove(index);
    }

    /// Attempt to insert an impl into this set of children, while comparing for
    /// specialization relationships.
    #[instrument(level = "debug", skip(self, tcx), ret)]
    fn insert(
        &mut self,
        tcx: TyCtxt<'tcx>,
        impl_def_id: DefId,
        simplified_self: Option<SimplifiedType>,
        overlap_mode: OverlapMode,
    ) -> Result<Inserted<'tcx>, OverlapError<'tcx>> {
        let mut last_lint = None;
        let mut replace_children = Vec::new();

        let possible_siblings = match simplified_self {
            Some(st) => PotentialSiblings::Filtered(filtered_children(self, st)),
            None => PotentialSiblings::Unfiltered(iter_children(self)),
        };

        for possible_sibling in possible_siblings {
            debug!(?possible_sibling);

            let create_overlap_error = |overlap: traits::coherence::OverlapResult<'tcx>| {
                let trait_ref = overlap.impl_header.trait_ref.unwrap();
                let self_ty = trait_ref.self_ty();

                OverlapError {
                    with_impl: possible_sibling,
                    trait_ref,
                    // Only report the `Self` type if it has at least
                    // some outer concrete shell; otherwise, it's
                    // not adding much information.
                    self_ty: self_ty.has_concrete_skeleton().then_some(self_ty),
                    intercrate_ambiguity_causes: overlap.intercrate_ambiguity_causes,
                    involves_placeholder: overlap.involves_placeholder,
                    overflowing_predicates: overlap.overflowing_predicates,
                }
            };

            let report_overlap_error = |overlap: traits::coherence::OverlapResult<'tcx>,
                                        last_lint: &mut _| {
                // Found overlap, but no specialization; error out or report future-compat warning.

                // Do we *still* get overlap if we disable the future-incompatible modes?
                let should_err = traits::overlapping_trait_impls(
                    tcx,
                    possible_sibling,
                    impl_def_id,
                    traits::SkipLeakCheck::default(),
                    overlap_mode,
                )
                .is_some();

                let error = create_overlap_error(overlap);

                if should_err {
                    Err(error)
                } else {
                    *last_lint = Some(FutureCompatOverlapError {
                        error,
                        kind: FutureCompatOverlapErrorKind::LeakCheck,
                    });

                    Ok((false, false))
                }
            };

            let last_lint_mut = &mut last_lint;
            let (le, ge) = traits::overlapping_trait_impls(
                tcx,
                possible_sibling,
                impl_def_id,
                traits::SkipLeakCheck::Yes,
                overlap_mode,
            )
            .map_or(Ok((false, false)), |overlap| {
                if let Some(overlap_kind) =
                    tcx.impls_are_allowed_to_overlap(impl_def_id, possible_sibling)
                {
                    match overlap_kind {
                        ty::ImplOverlapKind::Permitted { marker: _ } => {}
                    }

                    return Ok((false, false));
                }

                let le = tcx.specializes((impl_def_id, possible_sibling));
                let ge = tcx.specializes((possible_sibling, impl_def_id));

                if le == ge { report_overlap_error(overlap, last_lint_mut) } else { Ok((le, ge)) }
            })?;

            if le && !ge {
                debug!(
                    "descending as child of TraitRef {:?}",
                    tcx.impl_trait_ref(possible_sibling).instantiate_identity()
                );

                // The impl specializes `possible_sibling`.
                return Ok(Inserted::ShouldRecurseOn(possible_sibling));
            } else if ge && !le {
                debug!(
                    "placing as parent of TraitRef {:?}",
                    tcx.impl_trait_ref(possible_sibling).instantiate_identity()
                );

                replace_children.push(possible_sibling);
            } else {
                // Either there's no overlap, or the overlap was already reported by
                // `overlap_error`.
            }
        }

        if !replace_children.is_empty() {
            return Ok(Inserted::ReplaceChildren(replace_children));
        }

        // No overlap with any potential siblings, so add as a new sibling.
        debug!("placing as new sibling");
        self.insert_blindly(tcx, impl_def_id);
        Ok(Inserted::BecameNewSibling(last_lint))
    }
}

fn iter_children(children: &Children) -> impl Iterator<Item = DefId> {
    let nonblanket = children.non_blanket_impls.iter().flat_map(|(_, v)| v.iter());
    children.blanket_impls.iter().chain(nonblanket).cloned()
}

fn filtered_children(children: &mut Children, st: SimplifiedType) -> impl Iterator<Item = DefId> {
    let nonblanket = children.non_blanket_impls.entry(st).or_default().iter();
    children.blanket_impls.iter().chain(nonblanket).cloned()
}

// A custom iterator used by Children::insert
enum PotentialSiblings<I, J>
where
    I: Iterator<Item = DefId>,
    J: Iterator<Item = DefId>,
{
    Unfiltered(I),
    Filtered(J),
}

impl<I, J> Iterator for PotentialSiblings<I, J>
where
    I: Iterator<Item = DefId>,
    J: Iterator<Item = DefId>,
{
    type Item = DefId;

    fn next(&mut self) -> Option<Self::Item> {
        match *self {
            PotentialSiblings::Unfiltered(ref mut iter) => iter.next(),
            PotentialSiblings::Filtered(ref mut iter) => iter.next(),
        }
    }
}

#[extension(pub trait GraphExt<'tcx>)]
impl<'tcx> Graph {
    /// Insert a local impl into the specialization graph. If an existing impl
    /// conflicts with it (has overlap, but neither specializes the other),
    /// information about the area of overlap is returned in the `Err`.
    fn insert(
        &mut self,
        tcx: TyCtxt<'tcx>,
        impl_def_id: DefId,
        overlap_mode: OverlapMode,
    ) -> Result<Option<FutureCompatOverlapError<'tcx>>, OverlapError<'tcx>> {
        assert!(impl_def_id.is_local());

        // FIXME: use `EarlyBinder` in `self.children`
        let trait_ref = tcx.impl_trait_ref(impl_def_id).skip_binder();
        let trait_def_id = trait_ref.def_id;

        debug!(
            "insert({:?}): inserting TraitRef {:?} into specialization graph",
            impl_def_id, trait_ref
        );

        // If the reference itself contains an earlier error (e.g., due to a
        // resolution failure), then we just insert the impl at the top level of
        // the graph and claim that there's no overlap (in order to suppress
        // bogus errors).
        if trait_ref.references_error() {
            debug!(
                "insert: inserting dummy node for erroneous TraitRef {:?}, \
                 impl_def_id={:?}, trait_def_id={:?}",
                trait_ref, impl_def_id, trait_def_id
            );

            self.parent.insert(impl_def_id, trait_def_id);
            self.children.entry(trait_def_id).or_default().insert_blindly(tcx, impl_def_id);
            return Ok(None);
        }

        let mut parent = trait_def_id;
        let mut last_lint = None;
        let simplified =
            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::InstantiateWithInfer);

        // Descend the specialization tree, where `parent` is the current parent node.
        loop {
            let insert_result = self.children.entry(parent).or_default().insert(
                tcx,
                impl_def_id,
                simplified,
                overlap_mode,
            )?;

            match insert_result {
                Inserted::BecameNewSibling(opt_lint) => {
                    last_lint = opt_lint;
                    break;
                }
                Inserted::ReplaceChildren(grand_children_to_be) => {
                    // We currently have
                    //
                    //     P
                    //     |
                    //     G
                    //
                    // and we are inserting the impl N. We want to make it:
                    //
                    //     P
                    //     |
                    //     N
                    //     |
                    //     G

                    // Adjust P's list of children: remove G and then add N.
                    {
                        let siblings = self.children.get_mut(&parent).unwrap();
                        for &grand_child_to_be in &grand_children_to_be {
                            siblings.remove_existing(tcx, grand_child_to_be);
                        }
                        siblings.insert_blindly(tcx, impl_def_id);
                    }

                    // Set G's parent to N and N's parent to P.
                    for &grand_child_to_be in &grand_children_to_be {
                        self.parent.insert(grand_child_to_be, impl_def_id);
                    }
                    self.parent.insert(impl_def_id, parent);

                    // Add G as N's child.
                    for &grand_child_to_be in &grand_children_to_be {
                        self.children
                            .entry(impl_def_id)
                            .or_default()
                            .insert_blindly(tcx, grand_child_to_be);
                    }
                    break;
                }
                Inserted::ShouldRecurseOn(new_parent) => {
                    parent = new_parent;
                }
            }
        }

        self.parent.insert(impl_def_id, parent);
        Ok(last_lint)
    }

    /// Insert cached metadata mapping from a child impl back to its parent.
    fn record_impl_from_cstore(&mut self, tcx: TyCtxt<'tcx>, parent: DefId, child: DefId) {
        if self.parent.insert(child, parent).is_some() {
            bug!(
                "When recording an impl from the crate store, information about its parent \
                 was already present."
            );
        }

        self.children.entry(parent).or_default().insert_blindly(tcx, child);
    }
}

/// Locate the definition of an associated type in the specialization hierarchy,
/// starting from the given impl.
pub(crate) fn assoc_def(
    tcx: TyCtxt<'_>,
    impl_def_id: DefId,
    assoc_def_id: DefId,
) -> Result<LeafDef, ErrorGuaranteed> {
    let trait_def_id = tcx.impl_trait_id(impl_def_id);
    let trait_def = tcx.trait_def(trait_def_id);

    // This function may be called while we are still building the
    // specialization graph that is queried below (via TraitDef::ancestors()),
    // so, in order to avoid unnecessary infinite recursion, we manually look
    // for the associated item at the given impl.
    // If there is no such item in that impl, this function will fail with a
    // cycle error if the specialization graph is currently being built.
    if let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&assoc_def_id) {
        // Ensure that the impl is constrained, otherwise projection may give us
        // bad unconstrained infer vars.
        if let Some(impl_def_id) = impl_def_id.as_local() {
            tcx.ensure_ok().enforce_impl_non_lifetime_params_are_constrained(impl_def_id)?;
        }

        let item = tcx.associated_item(impl_item_id);
        let impl_node = Node::Impl(impl_def_id);
        return Ok(LeafDef {
            item,
            defining_node: impl_node,
            finalizing_node: if item.defaultness(tcx).is_default() {
                None
            } else {
                Some(impl_node)
            },
        });
    }

    let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
    if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_def_id) {
        // Ensure that the impl is constrained, otherwise projection may give us
        // bad unconstrained infer vars.
        if let ty::AssocContainer::TraitImpl(_) = assoc_item.item.container
            && let Some(impl_def_id) = assoc_item.item.container_id(tcx).as_local()
        {
            tcx.ensure_ok().enforce_impl_non_lifetime_params_are_constrained(impl_def_id)?;
        }

        Ok(assoc_item)
    } else {
        // This is saying that neither the trait nor
        // the impl contain a definition for this
        // associated type. Normally this situation
        // could only arise through a compiler bug --
        // if the user wrote a bad item name, it
        // should have failed during HIR ty lowering.
        bug!(
            "No associated type `{}` for {}",
            tcx.item_name(assoc_def_id),
            tcx.def_path_str(impl_def_id)
        )
    }
}