rustc_infer/infer/canonical/

query_response.rs

//! This module contains the code to instantiate a "query result", and
//! in particular to extract out the resulting region obligations and
//! encode them therein.
//!
//! For an overview of what canonicalization is and how it fits into
//! rustc, check out the [chapter in the rustc dev guide][c].
//!
//! [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html

use std::fmt::Debug;
use std::iter;

use rustc_index::{Idx, IndexVec};
use rustc_middle::arena::ArenaAllocatable;
use rustc_middle::bug;
use rustc_middle::infer::canonical::CanonicalVarKind;
use rustc_middle::ty::{self, BoundVar, GenericArg, GenericArgKind, Ty, TyCtxt, TypeFoldable};
use tracing::{debug, instrument};

use crate::infer::canonical::instantiate::{CanonicalExt, instantiate_value};
use crate::infer::canonical::{
    Canonical, CanonicalQueryResponse, CanonicalVarValues, Certainty, OriginalQueryValues,
    QueryRegionConstraints, QueryResponse,
};
use crate::infer::region_constraints::RegionConstraintData;
use crate::infer::{
    DefineOpaqueTypes, InferCtxt, InferOk, InferResult, SubregionOrigin, TypeOutlivesConstraint,
};
use crate::traits::query::NoSolution;
use crate::traits::{ObligationCause, PredicateObligations, ScrubbedTraitError, TraitEngine};

impl<'tcx> InferCtxt<'tcx> {
    /// This method is meant to be invoked as the final step of a canonical query
    /// implementation. It is given:
    ///
    /// - the instantiated variables `inference_vars` created from the query key
    /// - the result `answer` of the query
    /// - a fulfillment context `fulfill_cx` that may contain various obligations which
    ///   have yet to be proven.
    ///
    /// Given this, the function will process the obligations pending
    /// in `fulfill_cx`:
    ///
    /// - If all the obligations can be proven successfully, it will
    ///   package up any resulting region obligations (extracted from
    ///   `infcx`) along with the fully resolved value `answer` into a
    ///   query result (which is then itself canonicalized).
    /// - If some obligations can be neither proven nor disproven, then
    ///   the same thing happens, but the resulting query is marked as ambiguous.
    /// - Finally, if any of the obligations result in a hard error,
    ///   then `Err(NoSolution)` is returned.
    #[instrument(skip(self, inference_vars, answer, fulfill_cx), level = "trace")]
    pub fn make_canonicalized_query_response<T>(
        &self,
        inference_vars: CanonicalVarValues<'tcx>,
        answer: T,
        fulfill_cx: &mut dyn TraitEngine<'tcx, ScrubbedTraitError<'tcx>>,
    ) -> Result<CanonicalQueryResponse<'tcx, T>, NoSolution>
    where
        T: Debug + TypeFoldable<TyCtxt<'tcx>>,
        Canonical<'tcx, QueryResponse<'tcx, T>>: ArenaAllocatable<'tcx>,
    {
        let query_response = self.make_query_response(inference_vars, answer, fulfill_cx)?;
        debug!("query_response = {:#?}", query_response);
        let canonical_result = self.canonicalize_response(query_response);
        debug!("canonical_result = {:#?}", canonical_result);

        Ok(self.tcx.arena.alloc(canonical_result))
    }

    /// A version of `make_canonicalized_query_response` that does
    /// not pack in obligations, for contexts that want to drop
    /// pending obligations instead of treating them as an ambiguity (e.g.
    /// typeck "probing" contexts).
    ///
    /// If you DO want to keep track of pending obligations (which
    /// include all region obligations, so this includes all cases
    /// that care about regions) with this function, you have to
    /// do it yourself, by e.g., having them be a part of the answer.
    pub fn make_query_response_ignoring_pending_obligations<T>(
        &self,
        inference_vars: CanonicalVarValues<'tcx>,
        answer: T,
    ) -> Canonical<'tcx, QueryResponse<'tcx, T>>
    where
        T: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        // While we ignore region constraints and pending obligations,
        // we do return constrained opaque types to avoid unconstrained
        // inference variables in the response. This is important as we want
        // to check that opaques in deref steps stay unconstrained.
        //
        // This doesn't handle the more general case for non-opaques as
        // ambiguous `Projection` obligations have same the issue.
        let opaque_types = if self.next_trait_solver() {
            self.inner
                .borrow_mut()
                .opaque_type_storage
                .iter_opaque_types()
                .map(|(k, v)| (k, v.ty))
                .collect()
        } else {
            vec![]
        };

        self.canonicalize_response(QueryResponse {
            var_values: inference_vars,
            region_constraints: QueryRegionConstraints::default(),
            certainty: Certainty::Proven, // Ambiguities are OK!
            opaque_types,
            value: answer,
        })
    }

    /// Helper for `make_canonicalized_query_response` that does
    /// everything up until the final canonicalization.
    #[instrument(skip(self, fulfill_cx), level = "debug")]
    fn make_query_response<T>(
        &self,
        inference_vars: CanonicalVarValues<'tcx>,
        answer: T,
        fulfill_cx: &mut dyn TraitEngine<'tcx, ScrubbedTraitError<'tcx>>,
    ) -> Result<QueryResponse<'tcx, T>, NoSolution>
    where
        T: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        // Select everything, returning errors.
        let errors = fulfill_cx.evaluate_obligations_error_on_ambiguity(self);

        // True error!
        if errors.iter().any(|e| e.is_true_error()) {
            return Err(NoSolution);
        }

        let region_obligations = self.take_registered_region_obligations();
        let region_assumptions = self.take_registered_region_assumptions();
        debug!(?region_obligations);
        let region_constraints = self.with_region_constraints(|region_constraints| {
            make_query_region_constraints(
                region_obligations,
                region_constraints,
                region_assumptions,
            )
        });
        debug!(?region_constraints);

        let certainty = if errors.is_empty() { Certainty::Proven } else { Certainty::Ambiguous };

        let opaque_types = self
            .inner
            .borrow_mut()
            .opaque_type_storage
            .take_opaque_types()
            .map(|(k, v)| (k, v.ty))
            .collect();

        Ok(QueryResponse {
            var_values: inference_vars,
            region_constraints,
            certainty,
            value: answer,
            opaque_types,
        })
    }

    /// Given the (canonicalized) result to a canonical query,
    /// instantiates the result so it can be used, plugging in the
    /// values from the canonical query. (Note that the result may
    /// have been ambiguous; you should check the certainty level of
    /// the query before applying this function.)
    ///
    /// To get a good understanding of what is happening here, check
    /// out the [chapter in the rustc dev guide][c].
    ///
    /// [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html#processing-the-canonicalized-query-result
    pub fn instantiate_query_response_and_region_obligations<R>(
        &self,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        original_values: &OriginalQueryValues<'tcx>,
        query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
    ) -> InferResult<'tcx, R>
    where
        R: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        let InferOk { value: result_args, obligations } =
            self.query_response_instantiation(cause, param_env, original_values, query_response)?;

        for (predicate, _category) in &query_response.value.region_constraints.outlives {
            let predicate = instantiate_value(self.tcx, &result_args, *predicate);
            self.register_outlives_constraint(predicate, cause);
        }

        for assumption in &query_response.value.region_constraints.assumptions {
            let assumption = instantiate_value(self.tcx, &result_args, *assumption);
            self.register_region_assumption(assumption);
        }

        let user_result: R =
            query_response.instantiate_projected(self.tcx, &result_args, |q_r| q_r.value.clone());

        Ok(InferOk { value: user_result, obligations })
    }

    /// An alternative to
    /// `instantiate_query_response_and_region_obligations` that is more
    /// efficient for NLL. NLL is a bit more advanced in the
    /// "transition to chalk" than the rest of the compiler. During
    /// the NLL type check, all of the "processing" of types and
    /// things happens in queries -- the NLL checker itself is only
    /// interested in the region obligations (`'a: 'b` or `T: 'b`)
    /// that come out of these queries, which it wants to convert into
    /// MIR-based constraints and solve. Therefore, it is most
    /// convenient for the NLL Type Checker to **directly consume**
    /// the `QueryOutlivesConstraint` values that arise from doing a
    /// query. This is contrast to other parts of the compiler, which
    /// would prefer for those `QueryOutlivesConstraint` to be converted
    /// into the older infcx-style constraints (e.g., calls to
    /// `sub_regions` or `register_region_obligation`).
    ///
    /// Therefore, `instantiate_nll_query_response_and_region_obligations` performs the same
    /// basic operations as `instantiate_query_response_and_region_obligations` but
    /// it returns its result differently:
    ///
    /// - It creates an instantiation `S` that maps from the original
    ///   query variables to the values computed in the query
    ///   result. If any errors arise, they are propagated back as an
    ///   `Err` result.
    /// - In the case of a successful instantiation, we will append
    ///   `QueryOutlivesConstraint` values onto the
    ///   `output_query_region_constraints` vector for the solver to
    ///   use (if an error arises, some values may also be pushed, but
    ///   they should be ignored).
    /// - It **can happen** (though it rarely does currently) that
    ///   equating types and things will give rise to subobligations
    ///   that must be processed. In this case, those subobligations
    ///   are propagated back in the return value.
    /// - Finally, the query result (of type `R`) is propagated back,
    ///   after applying the instantiation `S`.
    pub fn instantiate_nll_query_response_and_region_obligations<R>(
        &self,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        original_values: &OriginalQueryValues<'tcx>,
        query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
        output_query_region_constraints: &mut QueryRegionConstraints<'tcx>,
    ) -> InferResult<'tcx, R>
    where
        R: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        let InferOk { value: result_args, mut obligations } = self
            .query_response_instantiation_guess(
                cause,
                param_env,
                original_values,
                query_response,
            )?;

        // Compute `QueryOutlivesConstraint` values that unify each of
        // the original values `v_o` that was canonicalized into a
        // variable...

        let constraint_category = cause.to_constraint_category();

        for (index, original_value) in original_values.var_values.iter().enumerate() {
            // ...with the value `v_r` of that variable from the query.
            let result_value = query_response.instantiate_projected(self.tcx, &result_args, |v| {
                v.var_values[BoundVar::new(index)]
            });
            match (original_value.kind(), result_value.kind()) {
                (GenericArgKind::Lifetime(re1), GenericArgKind::Lifetime(re2))
                    if re1.is_erased() && re2.is_erased() =>
                {
                    // No action needed.
                }

                (GenericArgKind::Lifetime(v_o), GenericArgKind::Lifetime(v_r)) => {
                    // To make `v_o = v_r`, we emit `v_o: v_r` and `v_r: v_o`.
                    if v_o != v_r {
                        output_query_region_constraints
                            .outlives
                            .push((ty::OutlivesPredicate(v_o.into(), v_r), constraint_category));
                        output_query_region_constraints
                            .outlives
                            .push((ty::OutlivesPredicate(v_r.into(), v_o), constraint_category));
                    }
                }

                (GenericArgKind::Type(v1), GenericArgKind::Type(v2)) => {
                    obligations.extend(
                        self.at(&cause, param_env)
                            .eq(DefineOpaqueTypes::Yes, v1, v2)?
                            .into_obligations(),
                    );
                }

                (GenericArgKind::Const(v1), GenericArgKind::Const(v2)) => {
                    obligations.extend(
                        self.at(&cause, param_env)
                            .eq(DefineOpaqueTypes::Yes, v1, v2)?
                            .into_obligations(),
                    );
                }

                _ => {
                    bug!("kind mismatch, cannot unify {:?} and {:?}", original_value, result_value);
                }
            }
        }

        // ...also include the other query region constraints from the query.
        output_query_region_constraints.outlives.extend(
            query_response.value.region_constraints.outlives.iter().filter_map(|&r_c| {
                let r_c = instantiate_value(self.tcx, &result_args, r_c);

                // Screen out `'a: 'a` cases.
                let ty::OutlivesPredicate(k1, r2) = r_c.0;
                if k1 != r2.into() { Some(r_c) } else { None }
            }),
        );

        // FIXME(higher_ranked_auto): Optimize this to instantiate all assumptions
        // at once, rather than calling `instantiate_value` repeatedly which may
        // create more universes.
        output_query_region_constraints.assumptions.extend(
            query_response
                .value
                .region_constraints
                .assumptions
                .iter()
                .map(|&r_c| instantiate_value(self.tcx, &result_args, r_c)),
        );

        let user_result: R =
            query_response.instantiate_projected(self.tcx, &result_args, |q_r| q_r.value.clone());

        Ok(InferOk { value: user_result, obligations })
    }

    /// Given the original values and the (canonicalized) result from
    /// computing a query, returns an instantiation that can be applied
    /// to the query result to convert the result back into the
    /// original namespace.
    ///
    /// The instantiation also comes accompanied with subobligations
    /// that arose from unification; these might occur if (for
    /// example) we are doing lazy normalization and the value
    /// assigned to a type variable is unified with an unnormalized
    /// projection.
    fn query_response_instantiation<R>(
        &self,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        original_values: &OriginalQueryValues<'tcx>,
        query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
    ) -> InferResult<'tcx, CanonicalVarValues<'tcx>>
    where
        R: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        debug!(
            "query_response_instantiation(original_values={:#?}, query_response={:#?})",
            original_values, query_response,
        );

        let mut value = self.query_response_instantiation_guess(
            cause,
            param_env,
            original_values,
            query_response,
        )?;

        value.obligations.extend(
            self.unify_query_response_instantiation_guess(
                cause,
                param_env,
                original_values,
                &value.value,
                query_response,
            )?
            .into_obligations(),
        );

        Ok(value)
    }

    /// Given the original values and the (canonicalized) result from
    /// computing a query, returns a **guess** at an instantiation that
    /// can be applied to the query result to convert the result back
    /// into the original namespace. This is called a **guess**
    /// because it uses a quick heuristic to find the values for each
    /// canonical variable; if that quick heuristic fails, then we
    /// will instantiate fresh inference variables for each canonical
    /// variable instead. Therefore, the result of this method must be
    /// properly unified
    #[instrument(level = "debug", skip(self, param_env))]
    fn query_response_instantiation_guess<R>(
        &self,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        original_values: &OriginalQueryValues<'tcx>,
        query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
    ) -> InferResult<'tcx, CanonicalVarValues<'tcx>>
    where
        R: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        // For each new universe created in the query result that did
        // not appear in the original query, create a local
        // superuniverse.
        let mut universe_map = original_values.universe_map.clone();
        let num_universes_in_query = original_values.universe_map.len();
        let num_universes_in_response = query_response.max_universe.as_usize() + 1;
        for _ in num_universes_in_query..num_universes_in_response {
            universe_map.push(self.create_next_universe());
        }
        assert!(!universe_map.is_empty()); // always have the root universe
        assert_eq!(universe_map[ty::UniverseIndex::ROOT.as_usize()], ty::UniverseIndex::ROOT);

        // Every canonical query result includes values for each of
        // the inputs to the query. Therefore, we begin by unifying
        // these values with the original inputs that were
        // canonicalized.
        let result_values = &query_response.value.var_values;
        assert_eq!(original_values.var_values.len(), result_values.len());

        // Quickly try to find initial values for the canonical
        // variables in the result in terms of the query. We do this
        // by iterating down the values that the query gave to each of
        // the canonical inputs. If we find that one of those values
        // is directly equal to one of the canonical variables in the
        // result, then we can type the corresponding value from the
        // input. See the example above.
        let mut opt_values: IndexVec<BoundVar, Option<GenericArg<'tcx>>> =
            IndexVec::from_elem_n(None, query_response.variables.len());

        for (original_value, result_value) in iter::zip(&original_values.var_values, result_values)
        {
            match result_value.kind() {
                GenericArgKind::Type(result_value) => {
                    // We disable the instantiation guess for inference variables
                    // and only use it for placeholders. We need to handle the
                    // `sub_root` of type inference variables which would make this
                    // more involved. They are also a lot rarer than region variables.
                    if let ty::Bound(index_kind, b) = *result_value.kind()
                        && !matches!(
                            query_response.variables[b.var.as_usize()],
                            CanonicalVarKind::Ty { .. }
                        )
                    {
                        // We only allow a `Canonical` index in generic parameters.
                        assert!(matches!(index_kind, ty::BoundVarIndexKind::Canonical));
                        opt_values[b.var] = Some(*original_value);
                    }
                }
                GenericArgKind::Lifetime(result_value) => {
                    if let ty::ReBound(index_kind, b) = result_value.kind() {
                        // We only allow a `Canonical` index in generic parameters.
                        assert!(matches!(index_kind, ty::BoundVarIndexKind::Canonical));
                        opt_values[b.var] = Some(*original_value);
                    }
                }
                GenericArgKind::Const(result_value) => {
                    if let ty::ConstKind::Bound(index_kind, b) = result_value.kind() {
                        // We only allow a `Canonical` index in generic parameters.
                        assert!(matches!(index_kind, ty::BoundVarIndexKind::Canonical));
                        opt_values[b.var] = Some(*original_value);
                    }
                }
            }
        }

        // Create result arguments: if we found a value for a
        // given variable in the loop above, use that. Otherwise, use
        // a fresh inference variable.
        let tcx = self.tcx;
        let variables = query_response.variables;
        let var_values = CanonicalVarValues::instantiate(tcx, variables, |var_values, kind| {
            if kind.universe() != ty::UniverseIndex::ROOT {
                // A variable from inside a binder of the query. While ideally these shouldn't
                // exist at all, we have to deal with them for now.
                self.instantiate_canonical_var(cause.span, kind, &var_values, |u| {
                    universe_map[u.as_usize()]
                })
            } else if kind.is_existential() {
                match opt_values[BoundVar::new(var_values.len())] {
                    Some(k) => k,
                    None => self.instantiate_canonical_var(cause.span, kind, &var_values, |u| {
                        universe_map[u.as_usize()]
                    }),
                }
            } else {
                // For placeholders which were already part of the input, we simply map this
                // universal bound variable back the placeholder of the input.
                opt_values[BoundVar::new(var_values.len())]
                    .expect("expected placeholder to be unified with itself during response")
            }
        });

        let mut obligations = PredicateObligations::new();

        // Carry all newly resolved opaque types to the caller's scope
        for &(a, b) in &query_response.value.opaque_types {
            let a = instantiate_value(self.tcx, &var_values, a);
            let b = instantiate_value(self.tcx, &var_values, b);
            debug!(?a, ?b, "constrain opaque type");
            // We use equate here instead of, for example, just registering the
            // opaque type's hidden value directly, because the hidden type may have been an inference
            // variable that got constrained to the opaque type itself. In that case we want to equate
            // the generic args of the opaque with the generic params of its hidden type version.
            obligations.extend(
                self.at(cause, param_env)
                    .eq(
                        DefineOpaqueTypes::Yes,
                        Ty::new_opaque(self.tcx, a.def_id.to_def_id(), a.args),
                        b,
                    )?
                    .obligations,
            );
        }

        Ok(InferOk { value: var_values, obligations })
    }

    /// Given a "guess" at the values for the canonical variables in
    /// the input, try to unify with the *actual* values found in the
    /// query result. Often, but not always, this is a no-op, because
    /// we already found the mapping in the "guessing" step.
    ///
    /// See also: [`Self::query_response_instantiation_guess`]
    fn unify_query_response_instantiation_guess<R>(
        &self,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        original_values: &OriginalQueryValues<'tcx>,
        result_args: &CanonicalVarValues<'tcx>,
        query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
    ) -> InferResult<'tcx, ()>
    where
        R: Debug + TypeFoldable<TyCtxt<'tcx>>,
    {
        // A closure that yields the result value for the given
        // canonical variable; this is taken from
        // `query_response.var_values` after applying the instantiation
        // by `result_args`.
        let instantiated_query_response = |index: BoundVar| -> GenericArg<'tcx> {
            query_response.instantiate_projected(self.tcx, result_args, |v| v.var_values[index])
        };

        // Unify the original value for each variable with the value
        // taken from `query_response` (after applying `result_args`).
        self.unify_canonical_vars(cause, param_env, original_values, instantiated_query_response)
    }

    /// Given two sets of values for the same set of canonical variables, unify them.
    /// The second set is produced lazily by supplying indices from the first set.
    fn unify_canonical_vars(
        &self,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        variables1: &OriginalQueryValues<'tcx>,
        variables2: impl Fn(BoundVar) -> GenericArg<'tcx>,
    ) -> InferResult<'tcx, ()> {
        let mut obligations = PredicateObligations::new();
        for (index, value1) in variables1.var_values.iter().enumerate() {
            let value2 = variables2(BoundVar::new(index));

            match (value1.kind(), value2.kind()) {
                (GenericArgKind::Type(v1), GenericArgKind::Type(v2)) => {
                    obligations.extend(
                        self.at(cause, param_env)
                            .eq(DefineOpaqueTypes::Yes, v1, v2)?
                            .into_obligations(),
                    );
                }
                (GenericArgKind::Lifetime(re1), GenericArgKind::Lifetime(re2))
                    if re1.is_erased() && re2.is_erased() =>
                {
                    // no action needed
                }
                (GenericArgKind::Lifetime(v1), GenericArgKind::Lifetime(v2)) => {
                    self.inner.borrow_mut().unwrap_region_constraints().make_eqregion(
                        SubregionOrigin::RelateRegionParamBound(cause.span, None),
                        v1,
                        v2,
                    );
                }
                (GenericArgKind::Const(v1), GenericArgKind::Const(v2)) => {
                    let ok = self.at(cause, param_env).eq(DefineOpaqueTypes::Yes, v1, v2)?;
                    obligations.extend(ok.into_obligations());
                }
                _ => {
                    bug!("kind mismatch, cannot unify {:?} and {:?}", value1, value2,);
                }
            }
        }
        Ok(InferOk { value: (), obligations })
    }
}

/// Given the region obligations and constraints scraped from the infcx,
/// creates query region constraints.
pub fn make_query_region_constraints<'tcx>(
    outlives_obligations: Vec<TypeOutlivesConstraint<'tcx>>,
    region_constraints: &RegionConstraintData<'tcx>,
    assumptions: Vec<ty::ArgOutlivesPredicate<'tcx>>,
) -> QueryRegionConstraints<'tcx> {
    let RegionConstraintData { constraints, verifys } = region_constraints;

    assert!(verifys.is_empty());

    debug!(?constraints);

    let outlives: Vec<_> = constraints
        .iter()
        .map(|(c, origin)| {
            // Swap regions because we are going from sub (<=) to outlives (>=).
            let constraint = ty::OutlivesPredicate(c.sup.into(), c.sub);
            (constraint, origin.to_constraint_category())
        })
        .chain(outlives_obligations.into_iter().map(|obl| {
            (
                ty::OutlivesPredicate(obl.sup_type.into(), obl.sub_region),
                obl.origin.to_constraint_category(),
            )
        }))
        .collect();

    QueryRegionConstraints { outlives, assumptions }
}