rustc_trait_selection/solve/

fulfill.rs

use std::marker::PhantomData;
use std::mem;

use rustc_data_structures::thinvec::ExtractIf;
use rustc_infer::infer::InferCtxt;
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::{
    FromSolverError, PredicateObligation, PredicateObligations, TraitEngine,
};
use rustc_next_trait_solver::solve::{GenerateProofTree, HasChanged, SolverDelegateEvalExt as _};
use tracing::instrument;

use self::derive_errors::*;
use super::Certainty;
use super::delegate::SolverDelegate;
use crate::traits::{FulfillmentError, ScrubbedTraitError};

mod derive_errors;

/// A trait engine using the new trait solver.
///
/// This is mostly identical to how `evaluate_all` works inside of the
/// solver, except that the requirements are slightly different.
///
/// Unlike `evaluate_all` it is possible to add new obligations later on
/// and we also have to track diagnostics information by using `Obligation`
/// instead of `Goal`.
///
/// It is also likely that we want to use slightly different datastructures
/// here as this will have to deal with far more root goals than `evaluate_all`.
pub struct FulfillmentCtxt<'tcx, E: 'tcx> {
    obligations: ObligationStorage<'tcx>,

    /// The snapshot in which this context was created. Using the context
    /// outside of this snapshot leads to subtle bugs if the snapshot
    /// gets rolled back. Because of this we explicitly check that we only
    /// use the context in exactly this snapshot.
    usable_in_snapshot: usize,
    _errors: PhantomData<E>,
}

#[derive(Default)]
struct ObligationStorage<'tcx> {
    /// Obligations which resulted in an overflow in fulfillment itself.
    ///
    /// We cannot eagerly return these as error so we instead store them here
    /// to avoid recomputing them each time `select_where_possible` is called.
    /// This also allows us to return the correct `FulfillmentError` for them.
    overflowed: PredicateObligations<'tcx>,
    pending: PredicateObligations<'tcx>,
}

impl<'tcx> ObligationStorage<'tcx> {
    fn register(&mut self, obligation: PredicateObligation<'tcx>) {
        self.pending.push(obligation);
    }

    fn clone_pending(&self) -> PredicateObligations<'tcx> {
        let mut obligations = self.pending.clone();
        obligations.extend(self.overflowed.iter().cloned());
        obligations
    }

    fn take_pending(&mut self) -> PredicateObligations<'tcx> {
        let mut obligations = mem::take(&mut self.pending);
        obligations.append(&mut self.overflowed);
        obligations
    }

    fn unstalled_for_select(&mut self) -> impl Iterator<Item = PredicateObligation<'tcx>> {
        mem::take(&mut self.pending).into_iter()
    }

    fn on_fulfillment_overflow(&mut self, infcx: &InferCtxt<'tcx>) {
        infcx.probe(|_| {
            // IMPORTANT: we must not use solve any inference variables in the obligations
            // as this is all happening inside of a probe. We use a probe to make sure
            // we get all obligations involved in the overflow. We pretty much check: if
            // we were to do another step of `select_where_possible`, which goals would
            // change.
            // FIXME: <https://github.com/Gankra/thin-vec/pull/66> is merged, this can be removed.
            self.overflowed.extend(ExtractIf::new(&mut self.pending, |o| {
                let goal = o.clone().into();
                let result = <&SolverDelegate<'tcx>>::from(infcx)
                    .evaluate_root_goal(goal, GenerateProofTree::No, o.cause.span)
                    .0;
                matches!(result, Ok((HasChanged::Yes, _)))
            }));
        })
    }
}

impl<'tcx, E: 'tcx> FulfillmentCtxt<'tcx, E> {
    pub fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentCtxt<'tcx, E> {
        assert!(
            infcx.next_trait_solver(),
            "new trait solver fulfillment context created when \
            infcx is set up for old trait solver"
        );
        FulfillmentCtxt {
            obligations: Default::default(),
            usable_in_snapshot: infcx.num_open_snapshots(),
            _errors: PhantomData,
        }
    }

    fn inspect_evaluated_obligation(
        &self,
        infcx: &InferCtxt<'tcx>,
        obligation: &PredicateObligation<'tcx>,
        result: &Result<(HasChanged, Certainty), NoSolution>,
    ) {
        if let Some(inspector) = infcx.obligation_inspector.get() {
            let result = match result {
                Ok((_, c)) => Ok(*c),
                Err(NoSolution) => Err(NoSolution),
            };
            (inspector)(infcx, &obligation, result);
        }
    }
}

impl<'tcx, E> TraitEngine<'tcx, E> for FulfillmentCtxt<'tcx, E>
where
    E: FromSolverError<'tcx, NextSolverError<'tcx>>,
{
    #[instrument(level = "trace", skip(self, infcx))]
    fn register_predicate_obligation(
        &mut self,
        infcx: &InferCtxt<'tcx>,
        obligation: PredicateObligation<'tcx>,
    ) {
        assert_eq!(self.usable_in_snapshot, infcx.num_open_snapshots());
        self.obligations.register(obligation);
    }

    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
        self.obligations
            .pending
            .drain(..)
            .map(|obligation| NextSolverError::Ambiguity(obligation))
            .chain(
                self.obligations
                    .overflowed
                    .drain(..)
                    .map(|obligation| NextSolverError::Overflow(obligation)),
            )
            .map(|e| E::from_solver_error(infcx, e))
            .collect()
    }

    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
        assert_eq!(self.usable_in_snapshot, infcx.num_open_snapshots());
        let mut errors = Vec::new();
        for i in 0.. {
            if !infcx.tcx.recursion_limit().value_within_limit(i) {
                self.obligations.on_fulfillment_overflow(infcx);
                // Only return true errors that we have accumulated while processing.
                return errors;
            }

            let mut has_changed = false;
            for obligation in self.obligations.unstalled_for_select() {
                let goal = obligation.clone().into();
                let result = <&SolverDelegate<'tcx>>::from(infcx)
                    .evaluate_root_goal(goal, GenerateProofTree::No, obligation.cause.span)
                    .0;
                self.inspect_evaluated_obligation(infcx, &obligation, &result);
                let (changed, certainty) = match result {
                    Ok(result) => result,
                    Err(NoSolution) => {
                        errors.push(E::from_solver_error(
                            infcx,
                            NextSolverError::TrueError(obligation),
                        ));
                        continue;
                    }
                };

                if changed == HasChanged::Yes {
                    has_changed = true;
                }

                match certainty {
                    Certainty::Yes => {}
                    Certainty::Maybe(_) => self.obligations.register(obligation),
                }
            }

            if !has_changed {
                break;
            }
        }

        errors
    }

    fn has_pending_obligations(&self) -> bool {
        !self.obligations.pending.is_empty() || !self.obligations.overflowed.is_empty()
    }

    fn pending_obligations(&self) -> PredicateObligations<'tcx> {
        self.obligations.clone_pending()
    }

    fn drain_unstalled_obligations(&mut self, _: &InferCtxt<'tcx>) -> PredicateObligations<'tcx> {
        self.obligations.take_pending()
    }
}

pub enum NextSolverError<'tcx> {
    TrueError(PredicateObligation<'tcx>),
    Ambiguity(PredicateObligation<'tcx>),
    Overflow(PredicateObligation<'tcx>),
}

impl<'tcx> FromSolverError<'tcx, NextSolverError<'tcx>> for FulfillmentError<'tcx> {
    fn from_solver_error(infcx: &InferCtxt<'tcx>, error: NextSolverError<'tcx>) -> Self {
        match error {
            NextSolverError::TrueError(obligation) => {
                fulfillment_error_for_no_solution(infcx, obligation)
            }
            NextSolverError::Ambiguity(obligation) => {
                fulfillment_error_for_stalled(infcx, obligation)
            }
            NextSolverError::Overflow(obligation) => {
                fulfillment_error_for_overflow(infcx, obligation)
            }
        }
    }
}

impl<'tcx> FromSolverError<'tcx, NextSolverError<'tcx>> for ScrubbedTraitError<'tcx> {
    fn from_solver_error(_infcx: &InferCtxt<'tcx>, error: NextSolverError<'tcx>) -> Self {
        match error {
            NextSolverError::TrueError(_) => ScrubbedTraitError::TrueError,
            NextSolverError::Ambiguity(_) | NextSolverError::Overflow(_) => {
                ScrubbedTraitError::Ambiguity
            }
        }
    }
}