rustc_trait_selection/traits/specialize/

mod.rs

//! Logic and data structures related to impl specialization, explained in
//! greater detail below.
//!
//! At the moment, this implementation support only the simple "chain" rule:
//! If any two impls overlap, one must be a strict subset of the other.
//!
//! See the [rustc dev guide] for a bit more detail on how specialization
//! fits together with the rest of the trait machinery.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html

pub mod specialization_graph;

use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::codes::*;
use rustc_errors::{Diag, EmissionGuarantee};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_infer::traits::Obligation;
use rustc_middle::bug;
use rustc_middle::query::LocalCrate;
use rustc_middle::traits::query::NoSolution;
use rustc_middle::ty::fast_reject::{self, TreatParams};
use rustc_middle::ty::print::PrintTraitRefExt as _;
use rustc_middle::ty::{
    self, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt, TypingMode, Unnormalized,
};
use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, sym};
use specialization_graph::GraphExt;
use tracing::{debug, instrument};

use crate::diagnostics::NegativePositiveConflict;
use crate::error_reporting::traits::to_pretty_impl_header;
use crate::infer::{InferCtxt, TyCtxtInferExt};
use crate::traits::select::IntercrateAmbiguityCause;
use crate::traits::{
    FutureCompatOverlapErrorKind, ObligationCause, ObligationCtxt, coherence,
    predicates_for_generics,
};

/// Information pertinent to an overlapping impl error.
#[derive(Debug)]
pub struct OverlapError<'tcx> {
    pub with_impl: DefId,
    pub trait_ref: ty::TraitRef<'tcx>,
    pub self_ty: Option<Ty<'tcx>>,
    pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
    pub involves_placeholder: bool,
    pub overflowing_predicates: Vec<ty::Predicate<'tcx>>,
}

/// Given the generic parameters for the requested impl, translate it to the generic parameters
/// appropriate for the actual item definition (whether it be in that impl,
/// a parent impl, or the trait).
///
/// When we have selected one impl, but are actually using item definitions from
/// a parent impl providing a default, we need a way to translate between the
/// type parameters of the two impls. Here the `source_impl` is the one we've
/// selected, and `source_args` is its generic parameters.
/// And `target_node` is the impl/trait we're actually going to get the
/// definition from. The resulting instantiation will map from `target_node`'s
/// generics to `source_impl`'s generics as instantiated by `source_args`.
///
/// For example, consider the following scenario:
///
/// ```ignore (illustrative)
/// trait Foo { ... }
/// impl<T, U> Foo for (T, U) { ... }  // target impl
/// impl<V> Foo for (V, V) { ... }     // source impl
/// ```
///
/// Suppose we have selected "source impl" with `V` instantiated with `u32`.
/// This function will produce an instantiation with `T` and `U` both mapping to `u32`.
///
/// where-clauses add some trickiness here, because they can be used to "define"
/// an argument indirectly:
///
/// ```ignore (illustrative)
/// impl<'a, I, T: 'a> Iterator for Cloned<I>
///    where I: Iterator<Item = &'a T>, T: Clone
/// ```
///
/// In a case like this, the instantiation for `T` is determined indirectly,
/// through associated type projection. We deal with such cases by using
/// *fulfillment* to relate the two impls, requiring that all projections are
/// resolved.
pub fn translate_args<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    source_impl: DefId,
    source_args: GenericArgsRef<'tcx>,
    target_node: specialization_graph::Node,
) -> GenericArgsRef<'tcx> {
    translate_args_with_cause(
        infcx,
        param_env,
        source_impl,
        source_args,
        target_node,
        &ObligationCause::dummy(),
    )
}

/// Like [translate_args], but obligations from the parent implementation
/// are registered with the provided `ObligationCause`.
///
/// This is for reporting *region* errors from those bounds. Type errors should
/// not happen because the specialization graph already checks for those, and
/// will result in an ICE.
pub fn translate_args_with_cause<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    source_impl: DefId,
    source_args: GenericArgsRef<'tcx>,
    target_node: specialization_graph::Node,
    cause: &ObligationCause<'tcx>,
) -> GenericArgsRef<'tcx> {
    debug!(
        "translate_args({:?}, {:?}, {:?}, {:?})",
        param_env, source_impl, source_args, target_node
    );
    let source_trait_ref =
        infcx.tcx.impl_trait_ref(source_impl).instantiate(infcx.tcx, source_args).skip_norm_wip();

    // translate the Self and Param parts of the generic parameters, since those
    // vary across impls
    let target_args = match target_node {
        specialization_graph::Node::Impl(target_impl) => {
            // no need to translate if we're targeting the impl we started with
            if source_impl == target_impl {
                return source_args;
            }

            fulfill_implication(infcx, param_env, source_trait_ref, source_impl, target_impl, cause)
                .unwrap_or_else(|_| {
                    bug!(
                        "When translating generic parameters from {source_impl:?} to \
                        {target_impl:?}, the expected specialization failed to hold"
                    )
                })
        }
        specialization_graph::Node::Trait(..) => source_trait_ref.args,
    };

    // directly inherent the method generics, since those do not vary across impls
    source_args.rebase_onto(infcx.tcx, source_impl, target_args)
}

/// Attempt to fulfill all obligations of `target_impl` after unification with
/// `source_trait_ref`. If successful, returns the generic parameters for *all* the
/// generics of `target_impl`, including both those needed to unify with
/// `source_trait_ref` and those whose identity is determined via a where
/// clause in the impl.
fn fulfill_implication<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    source_trait_ref: ty::TraitRef<'tcx>,
    source_impl: DefId,
    target_impl: DefId,
    cause: &ObligationCause<'tcx>,
) -> Result<GenericArgsRef<'tcx>, NoSolution> {
    debug!(
        "fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
        param_env, source_trait_ref, target_impl
    );

    let ocx = ObligationCtxt::new(infcx);
    let source_trait_ref = ocx.normalize(cause, param_env, Unnormalized::new_wip(source_trait_ref));

    if !ocx.evaluate_obligations_error_on_ambiguity().no_errors() {
        infcx.dcx().span_delayed_bug(
            infcx.tcx.def_span(source_impl),
            format!("failed to fully normalize {source_trait_ref}"),
        );
        return Err(NoSolution);
    }

    let target_args = infcx.fresh_args_for_item(DUMMY_SP, target_impl);
    let target_trait_ref = ocx.normalize(
        cause,
        param_env,
        infcx.tcx.impl_trait_ref(target_impl).instantiate(infcx.tcx, target_args),
    );

    // do the impls unify? If not, no specialization.
    ocx.eq(cause, param_env, source_trait_ref, target_trait_ref)?;

    // Now check that the source trait ref satisfies all the where clauses of the target impl.
    // This is not just for correctness; we also need this to constrain any params that may
    // only be referenced via projection predicates.
    let clauses = infcx.tcx.clauses_of(target_impl).instantiate(infcx.tcx, target_args);
    let obligations = predicates_for_generics(
        |_, _| cause.clone(),
        |clause| ocx.normalize(cause, param_env, clause),
        param_env,
        clauses,
    );
    ocx.register_obligations(obligations);

    let errors = ocx.evaluate_obligations_error_on_ambiguity();
    if !errors.no_errors() {
        // no dice!
        debug!(
            "fulfill_implication: for impls on {:?} and {:?}, \
                 could not fulfill: {:?} given {:?}",
            source_trait_ref,
            target_trait_ref,
            errors,
            param_env.caller_bounds()
        );
        return Err(NoSolution);
    }

    debug!(
        "fulfill_implication: an impl for {:?} specializes {:?}",
        source_trait_ref, target_trait_ref
    );

    // Now resolve the *generic parameters* we built for the target earlier, replacing
    // the inference variables inside with whatever we got from fulfillment.
    Ok(infcx.resolve_vars_if_possible(target_args))
}

pub(super) fn specialization_enabled_in(tcx: TyCtxt<'_>, _: LocalCrate) -> bool {
    tcx.features().specialization() || tcx.features().min_specialization()
}

/// Is `specializing_impl_def_id` a specialization of `parent_impl_def_id`?
///
/// For every type that could apply to `specializing_impl_def_id`, we prove that
/// the `parent_impl_def_id` also applies (i.e. it has a valid impl header and
/// its where-clauses hold).
///
/// For the purposes of const traits, we also check that the specializing
/// impl is not more restrictive than the parent impl. That is, if the
/// `parent_impl_def_id` is a const impl (conditionally based off of some `[const]`
/// bounds), then `specializing_impl_def_id` must also be const for the same
/// set of types.
#[instrument(skip(tcx), level = "debug")]
pub(super) fn specializes(
    tcx: TyCtxt<'_>,
    (specializing_impl_def_id, parent_impl_def_id): (DefId, DefId),
) -> bool {
    // We check that the specializing impl comes from a crate that has specialization enabled,
    // or if the specializing impl is marked with `allow_internal_unstable`.
    //
    // We don't really care if the specialized impl (the parent) is in a crate that has
    // specialization enabled, since it's not being specialized, and it's already been checked
    // for coherence.
    if !tcx.specialization_enabled_in(specializing_impl_def_id.krate) {
        let span = tcx.def_span(specializing_impl_def_id);
        if !span.allows_unstable(sym::specialization)
            && !span.allows_unstable(sym::min_specialization)
        {
            return false;
        }
    }

    let specializing_impl_trait_header = tcx.impl_trait_header(specializing_impl_def_id);

    // We determine whether there's a subset relationship by:
    //
    // - replacing bound vars with placeholders in impl1,
    // - assuming the where clauses for impl1,
    // - instantiating impl2 with fresh inference variables,
    // - unifying,
    // - attempting to prove the where clauses for impl2
    //
    // The last three steps are encapsulated in `fulfill_implication`.
    //
    // See RFC 1210 for more details and justification.

    // Currently we do not allow e.g., a negative impl to specialize a positive one
    if specializing_impl_trait_header.polarity != tcx.impl_polarity(parent_impl_def_id) {
        return false;
    }

    // create a parameter environment corresponding to an identity instantiation of the specializing impl,
    // i.e. the most generic instantiation of the specializing impl.
    let param_env = tcx.param_env(specializing_impl_def_id);

    // Create an infcx, taking the predicates of the specializing impl as assumptions:
    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());

    let specializing_impl_trait_ref =
        specializing_impl_trait_header.trait_ref.instantiate_identity();
    let cause = &ObligationCause::dummy();
    debug!(
        "fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
        param_env, specializing_impl_trait_ref, parent_impl_def_id
    );

    // Attempt to prove that the parent impl applies, given all of the above.

    let ocx = ObligationCtxt::new(&infcx);
    let specializing_impl_trait_ref = ocx.normalize(cause, param_env, specializing_impl_trait_ref);

    if !ocx.evaluate_obligations_error_on_ambiguity().no_errors() {
        infcx.dcx().span_delayed_bug(
            infcx.tcx.def_span(specializing_impl_def_id),
            format!("failed to fully normalize {specializing_impl_trait_ref}"),
        );
        return false;
    }

    let parent_args = infcx.fresh_args_for_item(DUMMY_SP, parent_impl_def_id);
    let parent_impl_trait_ref = ocx.normalize(
        cause,
        param_env,
        infcx.tcx.impl_trait_ref(parent_impl_def_id).instantiate(infcx.tcx, parent_args),
    );

    // do the impls unify? If not, no specialization.
    let Ok(()) = ocx.eq(cause, param_env, specializing_impl_trait_ref, parent_impl_trait_ref)
    else {
        return false;
    };

    // Now check that the source trait ref satisfies all the where clauses of the target impl.
    // This is not just for correctness; we also need this to constrain any params that may
    // only be referenced via projection predicates.
    let clauses = infcx.tcx.clauses_of(parent_impl_def_id).instantiate(infcx.tcx, parent_args);
    let obligations = predicates_for_generics(
        |_, _| cause.clone(),
        |clause| ocx.normalize(cause, param_env, clause),
        param_env,
        clauses,
    );
    ocx.register_obligations(obligations);

    let errors = ocx.evaluate_obligations_error_on_ambiguity();
    if !errors.no_errors() {
        // no dice!
        debug!(
            "fulfill_implication: for impls on {:?} and {:?}, \
                 could not fulfill: {:?} given {:?}",
            specializing_impl_trait_ref,
            parent_impl_trait_ref,
            errors,
            param_env.caller_bounds()
        );
        return false;
    }

    // If the parent impl is const, then the specializing impl must be const,
    // and it must not be *more restrictive* than the parent impl (that is,
    // it cannot be const in fewer cases than the parent impl).
    if tcx.is_conditionally_const(parent_impl_def_id) {
        if !tcx.is_conditionally_const(specializing_impl_def_id) {
            return false;
        }

        let const_conditions =
            infcx.tcx.const_conditions(parent_impl_def_id).instantiate(infcx.tcx, parent_args);
        let const_conditions = const_conditions
            .into_iter()
            .map(|(trait_ref, span)| (ocx.normalize(cause, param_env, trait_ref), span));
        ocx.register_obligations(const_conditions.into_iter().map(|(trait_ref, _)| {
            Obligation::new(
                infcx.tcx,
                cause.clone(),
                param_env,
                trait_ref.to_host_effect_clause(infcx.tcx, ty::BoundConstness::Maybe),
            )
        }));

        let errors = ocx.evaluate_obligations_error_on_ambiguity();
        if !errors.no_errors() {
            // no dice!
            debug!(
                "fulfill_implication: for impls on {:?} and {:?}, \
                 could not fulfill: {:?} given {:?}",
                specializing_impl_trait_ref,
                parent_impl_trait_ref,
                errors,
                param_env.caller_bounds()
            );
            return false;
        }
    }

    debug!(
        "fulfill_implication: an impl for {:?} specializes {:?}",
        specializing_impl_trait_ref, parent_impl_trait_ref
    );

    true
}

/// Query provider for `specialization_graph_of`.
pub(super) fn specialization_graph_provider(
    tcx: TyCtxt<'_>,
    trait_id: DefId,
) -> Result<&'_ specialization_graph::Graph, ErrorGuaranteed> {
    let mut sg = specialization_graph::Graph::new();
    let overlap_mode = specialization_graph::OverlapMode::get(tcx, trait_id);

    // Skip foreign non-blanket impls whose simplified-self bucket holds no
    // local impl. This is sound because:
    // - foreign impls are never overlap-checked, only recorded; `Ancestors`
    //   reads their parent lazily from metadata instead (the same value).
    // - a local non-blanket impl is only compared against blanket impls and
    //   impls in its own bucket (see `filtered_children`), and instantiation
    //   preserves the simplified type, so kept buckets are complete at every
    //   level of the tree.
    // - a local blanket impl, including alias self types which simplify to
    //   `None`, is compared against every child, so then all buckets are kept;
    //   pruning them would change error recovery (see impl-unpin.rs, `tait`
    //   revision).
    let all_impls = tcx.trait_impls_of(trait_id);
    let mut trait_impls: Vec<DefId> = all_impls.blanket_impls().to_vec();
    let has_local_blanket_impl =
        all_impls.blanket_impls().iter().any(|impl_def_id| impl_def_id.is_local());
    for (&simplified_self, bucket) in all_impls.non_blanket_impls() {
        if has_local_blanket_impl || bucket.iter().any(|impl_def_id| impl_def_id.is_local()) {
            trait_impls.extend(bucket.iter().copied());
        } else if cfg!(debug_assertions) {
            // Assert metadata-derived key matches what the overlap checker recomputes.
            for &impl_def_id in bucket {
                let self_ty = tcx.impl_trait_ref(impl_def_id).skip_binder().self_ty();
                debug_assert_eq!(
                    fast_reject::simplify_type(tcx, self_ty, TreatParams::InstantiateWithInfer),
                    Some(simplified_self),
                    "trait_impls_of bucket key disagrees with overlap-check \
                     simplification for foreign impl {impl_def_id:?}",
                );
            }
        }
    }

    // The coherence checking implementation seems to rely on impls being
    // iterated over (roughly) in definition order, so we are sorting by
    // negated `CrateNum` (so remote definitions are visited first) and then
    // by a flattened version of the `DefIndex`.
    trait_impls
        .sort_unstable_by_key(|def_id| (-(def_id.krate.as_u32() as i64), def_id.index.index()));

    let mut errored = Ok(());

    for impl_def_id in trait_impls {
        if let Some(impl_def_id) = impl_def_id.as_local() {
            // This is where impl overlap checking happens:
            let insert_result = sg.insert(tcx, impl_def_id.to_def_id(), overlap_mode);
            // Report error if there was one.
            let (overlap, used_to_be_allowed) = match insert_result {
                Err(overlap) => (Some(overlap), None),
                Ok(Some(overlap)) => (Some(overlap.error), Some(overlap.kind)),
                Ok(None) => (None, None),
            };

            if let Some(overlap) = overlap {
                errored = errored.and(report_overlap_conflict(
                    tcx,
                    overlap,
                    impl_def_id,
                    used_to_be_allowed,
                ));
            }
        } else {
            let parent = tcx.impl_parent(impl_def_id).unwrap_or(trait_id);
            sg.record_impl_from_cstore(tcx, parent, impl_def_id)
        }
    }
    errored?;

    Ok(tcx.arena.alloc(sg))
}

// This function is only used when
// encountering errors and inlining
// it negatively impacts perf.
#[cold]
#[inline(never)]
fn report_overlap_conflict<'tcx>(
    tcx: TyCtxt<'tcx>,
    overlap: OverlapError<'tcx>,
    impl_def_id: LocalDefId,
    used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
) -> Result<(), ErrorGuaranteed> {
    let impl_polarity = tcx.impl_polarity(impl_def_id.to_def_id());
    let other_polarity = tcx.impl_polarity(overlap.with_impl);
    match (impl_polarity, other_polarity) {
        (ty::ImplPolarity::Negative, ty::ImplPolarity::Positive) => {
            Err(report_negative_positive_conflict(
                tcx,
                &overlap,
                impl_def_id,
                impl_def_id.to_def_id(),
                overlap.with_impl,
            ))
        }

        (ty::ImplPolarity::Positive, ty::ImplPolarity::Negative) => {
            Err(report_negative_positive_conflict(
                tcx,
                &overlap,
                impl_def_id,
                overlap.with_impl,
                impl_def_id.to_def_id(),
            ))
        }

        _ => report_conflicting_impls(tcx, overlap, impl_def_id, used_to_be_allowed),
    }
}

fn report_negative_positive_conflict<'tcx>(
    tcx: TyCtxt<'tcx>,
    overlap: &OverlapError<'tcx>,
    local_impl_def_id: LocalDefId,
    negative_impl_def_id: DefId,
    positive_impl_def_id: DefId,
) -> ErrorGuaranteed {
    let mut diag = tcx.dcx().create_err(NegativePositiveConflict {
        impl_span: tcx.def_span(local_impl_def_id),
        trait_desc: overlap.trait_ref,
        self_ty: overlap.self_ty,
        negative_impl_span: tcx.span_of_impl(negative_impl_def_id),
        positive_impl_span: tcx.span_of_impl(positive_impl_def_id),
    });

    for cause in &overlap.intercrate_ambiguity_causes {
        cause.add_intercrate_ambiguity_hint(&mut diag);
    }

    diag.emit()
}

fn report_conflicting_impls<'tcx>(
    tcx: TyCtxt<'tcx>,
    overlap: OverlapError<'tcx>,
    impl_def_id: LocalDefId,
    used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
) -> Result<(), ErrorGuaranteed> {
    let impl_span = tcx.def_span(impl_def_id);

    // Work to be done after we've built the Diag. We have to define it now
    // because the lint emit methods don't return back the Diag that's passed
    // in.
    fn decorate<'tcx, G: EmissionGuarantee>(
        tcx: TyCtxt<'tcx>,
        overlap: &OverlapError<'tcx>,
        impl_span: Span,
        err: &mut Diag<'_, G>,
    ) {
        match tcx.span_of_impl(overlap.with_impl) {
            Ok(span) => {
                err.span_label(span, "first implementation here");

                err.span_label(
                    impl_span,
                    format!(
                        "conflicting implementation{}",
                        overlap.self_ty.map_or_else(String::new, |ty| format!(" for `{ty}`"))
                    ),
                );
            }
            Err(cname) => {
                let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
                    Some(s) => {
                        format!("conflicting implementation in crate `{cname}`:\n- {s}")
                    }
                    None => format!("conflicting implementation in crate `{cname}`"),
                };
                err.note(msg);
            }
        }

        for cause in &overlap.intercrate_ambiguity_causes {
            cause.add_intercrate_ambiguity_hint(err);
        }

        if overlap.involves_placeholder {
            coherence::add_placeholder_note(err);
        }

        if !overlap.overflowing_predicates.is_empty() {
            coherence::suggest_increasing_recursion_limit(
                tcx,
                err,
                &overlap.overflowing_predicates,
            );
        }
    }

    let msg = || {
        format!(
            "conflicting implementations of trait `{}`{}",
            overlap.trait_ref.print_trait_sugared(),
            overlap.self_ty.map_or_else(String::new, |ty| format!(" for type `{ty}`")),
        )
    };

    // Don't report overlap errors if the header references error
    if let Err(err) = (overlap.trait_ref, overlap.self_ty).error_reported() {
        return Err(err);
    }

    match used_to_be_allowed {
        None => {
            let reported = if overlap.with_impl.is_local()
                || tcx.ensure_result().orphan_check_impl(impl_def_id).is_ok()
            {
                let mut err = tcx.dcx().struct_span_err(impl_span, msg());
                err.code(E0119);
                decorate(tcx, &overlap, impl_span, &mut err);
                err.emit()
            } else {
                tcx.dcx().span_delayed_bug(impl_span, "impl should have failed the orphan check")
            };
            Err(reported)
        }
        Some(kind) => {
            let lint = match kind {
                FutureCompatOverlapErrorKind::LeakCheck => COHERENCE_LEAK_CHECK,
            };
            tcx.emit_node_span_lint(
                lint,
                tcx.local_def_id_to_hir_id(impl_def_id),
                impl_span,
                rustc_errors::DiagDecorator(|err| {
                    err.primary_message(msg());
                    decorate(tcx, &overlap, impl_span, err);
                }),
            );
            Ok(())
        }
    }
}