feat: move fulfillment into rustc_next_trait_solver

Signed-off-by: Amirhossein Akhlaghpour <m9.akhlaghpoor@gmail.com>

Author: amirHdev

compiler/rustc_infer/src/traits/mod.rs

@@ -18,6 +18,7 @@ use rustc_middle::traits::solve::Certainty;
 pub use rustc_middle::traits::*;
 use rustc_middle::ty::{self, Ty, TyCtxt, Upcast};
 use rustc_span::Span;
+use rustc_type_ir::solve::fulfill::FulfillmentObligation;
 use thin_vec::ThinVec;
 
 pub use self::engine::{FromSolverError, ScrubbedTraitError, TraitEngine, TraitErrors};
@@ -92,6 +93,28 @@ pub type PolyTraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tc
 
 pub type PredicateObligations<'tcx> = ThinVec<PredicateObligation<'tcx>>;
 
+impl<'tcx> FulfillmentObligation<TyCtxt<'tcx>> for PredicateObligation<'tcx> {
+    fn as_goal(&self) -> solve::Goal<'tcx, ty::Predicate<'tcx>> {
+        Obligation::as_goal(self)
+    }
+
+    fn span(&self) -> Span {
+        self.cause.span
+    }
+
+    fn recursion_depth(&self) -> usize {
+        self.recursion_depth
+    }
+
+    fn set_recursion_depth(&mut self, depth: usize) {
+        self.recursion_depth = depth;
+    }
+
+    fn set_predicate(&mut self, predicate: ty::Predicate<'tcx>) {
+        self.predicate = predicate;
+    }
+}
+
 impl<'tcx> PredicateObligation<'tcx> {
     /// Flips the polarity of the inner predicate.
     ///

compiler/rustc_next_trait_solver/src/solve/fulfill.rs

@@ -0,0 +1,301 @@
+use rustc_type_ir::solve::fulfill::FulfillmentObligation;
+use rustc_type_ir::solve::{Certainty, Goal, NoSolution};
+use rustc_type_ir::{InferCtxtLike as _, Interner};
+use thin_vec::ThinVec;
+
+use super::fast_path::compute_goal_fast_path;
+use super::{
+    GoalEvaluation, GoalStalledOn, HasChanged, SolverDelegate, SolverDelegateEvalExt as _,
+};
+
+#[derive(Debug, Clone)]
+pub enum NextSolverError<O> {
+    TrueError(O),
+    Ambiguity(O),
+    Overflow(O),
+}
+
+// FIXME: Do we need to use a `ThinVec` here?
+type PendingObligations<I, O> = ThinVec<(O, Option<GoalStalledOn<I>>)>;
+
+#[derive(Debug)]
+struct ObligationStorage<I: Interner, O> {
+    /// Obligations which resulted in overflow in fulfillment itself.
+    ///
+    /// We cannot eagerly return these as errors, so we instead store them here
+    /// to avoid recomputing them each time `try_evaluate_obligations` is called.
+    /// This also allows the frontend to construct the correct error for them.
+    overflowed: Vec<O>,
+
+    pending: PendingObligations<I, O>,
+}
+
+impl<I: Interner, O> Default for ObligationStorage<I, O> {
+    fn default() -> Self {
+        Self { overflowed: Vec::new(), pending: ThinVec::new() }
+    }
+}
+
+impl<I: Interner, O> ObligationStorage<I, O> {
+    fn register(&mut self, obligation: O, stalled_on: Option<GoalStalledOn<I>>) {
+        self.pending.push((obligation, stalled_on));
+    }
+
+    fn has_pending_obligations(&self) -> bool {
+        !self.pending.is_empty() || !self.overflowed.is_empty()
+    }
+
+    fn clone_pending(&self) -> ThinVec<O>
+    where
+        O: Clone,
+    {
+        let mut obligations =
+            self.pending.iter().map(|(obligation, _)| obligation.clone()).collect::<ThinVec<_>>();
+
+        obligations.extend(self.overflowed.iter().cloned());
+        obligations
+    }
+
+    fn clone_pending_filtered<F>(&self, mut filter: F) -> ThinVec<O>
+    where
+        O: Clone,
+        F: FnMut(&O, &Option<GoalStalledOn<I>>) -> bool,
+    {
+        let mut obligations = self
+            .pending
+            .iter()
+            .filter_map(|(obligation, stalled_on)| {
+                filter(obligation, stalled_on).then(|| obligation.clone())
+            })
+            .collect::<ThinVec<_>>();
+
+        obligations.extend(self.overflowed.iter().cloned());
+        obligations
+    }
+
+    fn drain_pending<F>(&mut self, mut filter: F) -> ThinVec<O>
+    where
+        F: FnMut(&O, &Option<GoalStalledOn<I>>) -> bool,
+    {
+        let (drained, pending): (PendingObligations<I, O>, PendingObligations<I, O>) =
+            std::mem::take(&mut self.pending)
+                .into_iter()
+                .partition(|(obligation, stalled_on)| filter(obligation, stalled_on));
+
+        self.pending = pending;
+
+        drained.into_iter().map(|(obligation, _)| obligation).collect()
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn collect_remaining_errors<E>(
+        &mut self,
+        map: impl FnMut(NextSolverError<O>) -> E,
+    ) -> ThinVec<E> {
+        self.pending
+            .drain(..)
+            .map(|(obligation, _)| NextSolverError::Ambiguity(obligation))
+            .chain(self.overflowed.drain(..).map(NextSolverError::Overflow))
+            .map(map)
+            .collect()
+    }
+}
+
+/// A fulfillment engine using the new trait solver.
+///
+/// This is mostly identical to how `evaluate_all` works inside of the solver,
+/// except that it is possible to add new obligations later and the frontend
+/// needs to retain its obligation representation for diagnostics.
+///
+/// It is also likely that we want to use different data structures here, as
+/// fulfillment deals with far more root goals than `evaluate_all`.
+#[derive(Debug)]
+pub struct FulfillmentCtxt<I: Interner, O: FulfillmentObligation<I>> {
+    obligations: ObligationStorage<I, O>,
+}
+
+impl<I: Interner, O: FulfillmentObligation<I>> Default for FulfillmentCtxt<I, O> {
+    fn default() -> Self {
+        Self { obligations: ObligationStorage::default() }
+    }
+}
+
+impl<I: Interner, O: FulfillmentObligation<I>> FulfillmentCtxt<I, O> {
+    pub fn new() -> Self {
+        Self { obligations: Default::default() }
+    }
+
+    pub fn register<D>(&mut self, delegate: &D, obligation: O)
+    where
+        D: SolverDelegate<Interner = I>,
+    {
+        if let Some(GoalEvaluation { certainty, stalled_on, .. }) =
+            compute_goal_fast_path(delegate, obligation.as_goal(), obligation.span())
+        {
+            // If we can take the fast path, do not add a successful goal to
+            // the pending obligations. For `Certainty::Maybe`, retain the
+            // precise `stalled_on` information for later re-evaluation.
+            match certainty {
+                Certainty::Yes => {}
+                Certainty::Maybe(_) => {
+                    self.obligations.register(obligation, stalled_on);
+                }
+            }
+        } else {
+            self.obligations.register(obligation, None);
+        }
+    }
+
+    fn on_fulfillment_overflow<D>(&mut self, delegate: &D)
+    where
+        D: SolverDelegate<Interner = I>,
+    {
+        delegate.probe(|| {
+            // IMPORTANT: we must not resolve any inference variables in the
+            // obligations, as this is all happening inside of a probe. The
+            // probe makes sure we collect every obligation involved in the
+            // overflow. Conceptually, we check which goals would change if we
+            // performed one more fulfillment iteration.
+            let overflowed = self
+                .obligations
+                .pending
+                .extract_if(.., |(obligation, stalled_on)| {
+                    let result = delegate.evaluate_root_goal(
+                        obligation.as_goal(),
+                        obligation.span(),
+                        stalled_on.take(),
+                    );
+
+                    matches!(result, Ok(GoalEvaluation { has_changed: HasChanged::Yes, .. }))
+                })
+                .map(|(obligation, _)| obligation)
+                .collect::<Vec<_>>();
+
+            self.obligations.overflowed.extend(overflowed);
+        });
+    }
+
+    pub fn try_evaluate_obligations<D, Inspect, OnSuccess>(
+        &mut self,
+        delegate: &D,
+        mut inspect: Inspect,
+        mut on_success: OnSuccess,
+    ) -> ThinVec<NextSolverError<O>>
+    where
+        D: SolverDelegate<Interner = I>,
+        Inspect: FnMut(&O, Goal<I, I::Predicate>, &Result<GoalEvaluation<I>, NoSolution>),
+        OnSuccess: FnMut(&O),
+    {
+        let mut errors = ThinVec::new();
+
+        loop {
+            let mut any_changed = false;
+            let mut overflowed = false;
+
+            self.obligations.pending.retain_mut(|(obligation, opt_stalled_on)| {
+                if overflowed {
+                    return false;
+                }
+
+                // Common case: still stalled; keep the obligation. This path is extremely hot in
+                // some cases; there can be thousands of pending obligations.
+                if let Some(stalled_on) = opt_stalled_on
+                    && let Some(certainty) = delegate.goal_remains_stalled(stalled_on)
+                    && matches!(certainty, Certainty::Maybe(_))
+                {
+                    return true;
+                }
+
+                let goal = obligation.as_goal();
+                let result =
+                    delegate.evaluate_root_goal(goal, obligation.span(), opt_stalled_on.take());
+
+                inspect(obligation, goal, &result);
+
+                let GoalEvaluation { goal, certainty, has_changed, stalled_on } = match result {
+                    Ok(result) => result,
+                    Err(NoSolution) => {
+                        errors.push(NextSolverError::TrueError(obligation.clone()));
+                        return false;
+                    }
+                };
+
+                // We resolved the goal in `evaluate_root_goal`; retain the eagerly resolved
+                // predicate to avoid repeating this work in the next iteration.
+                obligation.set_predicate(goal.predicate);
+
+                if has_changed == HasChanged::Yes {
+                    // Track the number of times this root goal resulted in inference progress.
+                    let depth = obligation.recursion_depth() + 1;
+                    obligation.set_recursion_depth(depth);
+
+                    if depth > delegate.cx().recursion_limit() {
+                        // We cannot break out of `retain_mut`, so use a flag and handle
+                        // fulfillment overflow after the iteration.
+                        overflowed = true;
+                        return false;
+                    }
+
+                    any_changed = true;
+                }
+
+                match certainty {
+                    Certainty::Yes => {
+                        on_success(obligation);
+                        false
+                    }
+                    Certainty::Maybe(_) => {
+                        *opt_stalled_on = stalled_on;
+                        true
+                    }
+                }
+            });
+
+            if overflowed {
+                self.on_fulfillment_overflow(delegate);
+                // Only return true errors accumulated while processing.
+                return errors;
+            }
+
+            if !any_changed {
+                break;
+            }
+        }
+
+        errors
+    }
+
+    pub fn has_pending_obligations(&self) -> bool {
+        self.obligations.has_pending_obligations()
+    }
+
+    pub fn pending_obligations(&self) -> ThinVec<O>
+    where
+        O: Clone,
+    {
+        self.obligations.clone_pending()
+    }
+
+    pub fn pending_obligations_filtered<F>(&self, filter: F) -> ThinVec<O>
+    where
+        O: Clone,
+        F: FnMut(&O, &Option<GoalStalledOn<I>>) -> bool,
+    {
+        self.obligations.clone_pending_filtered(filter)
+    }
+
+    pub fn drain_pending_obligations<F>(&mut self, filter: F) -> ThinVec<O>
+    where
+        F: FnMut(&O, &Option<GoalStalledOn<I>>) -> bool,
+    {
+        self.obligations.drain_pending(filter)
+    }
+
+    pub fn collect_remaining_errors<E>(
+        &mut self,
+        map: impl FnMut(NextSolverError<O>) -> E,
+    ) -> ThinVec<E> {
+        self.obligations.collect_remaining_errors(map)
+    }
+}

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -14,6 +14,7 @@
 mod assembly;
 mod effect_goals;
 mod eval_ctxt;
+pub mod fulfill;
 pub mod inspect;
 mod normalizes_to;
 mod project_goals;

compiler/rustc_trait_selection/src/solve.rs

@@ -1,18 +1,18 @@
 pub use rustc_next_trait_solver::solve::*;
 
 mod delegate;
-mod fulfill;
 pub mod inspect;
 mod normalize;
+mod rustc_fulfill;
 mod select;
 
 pub(crate) use delegate::SolverDelegate;
-pub use fulfill::{FulfillmentCtxt, NextSolverError};
 pub(crate) use normalize::deeply_normalize_for_diagnostics;
 pub use normalize::{
     deeply_normalize, deeply_normalize_with_skipped_universes,
     deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals, normalize,
 };
+pub use rustc_fulfill::{FulfillmentCtxt, NextSolverError};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 pub use select::InferCtxtSelectExt;