Move fast paths around to gain a bit of new-solver perf

compiler/rustc_middle/src/traits/solve.rs

@@ -16,6 +16,10 @@ pub type CanonicalInput<'tcx, P = ty::Predicate<'tcx>> = ir::solve::CanonicalInp
 pub type CanonicalResponse<'tcx> = ir::solve::CanonicalResponse<TyCtxt<'tcx>>;
 pub type FetchEligibleAssocItemResponse<'tcx> =
     ir::solve::FetchEligibleAssocItemResponse<TyCtxt<'tcx>>;
+pub type ComputeGoalFastPathOutcome<'tcx> = ir::solve::ComputeGoalFastPathOutcome<TyCtxt<'tcx>>;
+pub type GoalStalledOn<'tcx> = ir::solve::GoalStalledOn<TyCtxt<'tcx>>;
+pub type GoalStalledOnOpaques<'tcx> = ir::solve::GoalStalledOnOpaques<TyCtxt<'tcx>>;
+pub type SucceededInErased<'tcx> = ir::solve::SucceededInErased<TyCtxt<'tcx>>;
 
 pub type PredefinedOpaques<'tcx> = &'tcx ty::List<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)>;
 

compiler/rustc_next_trait_solver/src/delegate.rs

@@ -1,7 +1,8 @@
 use std::ops::Deref;
 
 use rustc_type_ir::solve::{
-    Certainty, FetchEligibleAssocItemResponse, Goal, NoSolution, VisibleForLeakCheck,
+    Certainty, ComputeGoalFastPathOutcome, FetchEligibleAssocItemResponse, Goal, NoSolution,
+    VisibleForLeakCheck,
 };
 use rustc_type_ir::{self as ty, InferCtxtLike, Interner, TypeFoldable};
 
@@ -23,7 +24,7 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
         &self,
         goal: Goal<Self::Interner, <Self::Interner as Interner>::Predicate>,
         span: <Self::Interner as Interner>::Span,
-    ) -> Option<Certainty>;
+    ) -> ComputeGoalFastPathOutcome<Self::Interner>;
 
     fn fresh_var_for_kind_with_span(
         &self,

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/fast_path.rs

@@ -0,0 +1,155 @@
+//! This file contains a number of standalone functions useful for taking _fast paths_ in the trait
+//! solver. The exact place where we check for these fast paths changes, and matters a lot for
+//! performance. Ideally we'd only check them in `evaluate_goal`, but when evaluating root goals
+//! we can check them earlier and save some time creating an `EvalCtxt` in the first place.
+//!
+//! For debugging, fast paths can be disabled using `-Zdisable-fast-paths`.
+
+use rustc_type_ir::inherent::*;
+use rustc_type_ir::solve::{
+    Certainty, ComputeGoalFastPathOutcome, Goal, GoalStalledOn, GoalStalledOnOpaques,
+    SucceededInErased,
+};
+use rustc_type_ir::{InferCtxtLike, Interner};
+
+use crate::delegate::SolverDelegate;
+use crate::solve::eval_ctxt::{RerunDecision, should_rerun_after_erased_canonicalization};
+use crate::solve::{GoalEvaluation, HasChanged};
+
+#[derive(Debug, Clone, Copy)]
+pub(super) enum RerunStalled {
+    WontMakeProgress(Certainty),
+    MayMakeProgress,
+}
+
+/// If we have run a goal before, and it was stalled, check that any of the goal's
+/// args have changed. This is a cheap way to determine that if we were to rerun this goal now,
+/// it will remain stalled since it'll canonicalize the same way and evaluation is pure.
+/// Therefore, we can skip this rerun
+#[inline]
+pub(super) fn rerunning_stalled_goal_may_make_progress<D, I>(
+    delegate: &D,
+    stalled_on: Option<&GoalStalledOn<I>>,
+) -> RerunStalled
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
+{
+    use RerunStalled::*;
+
+    // If fast paths are turned off, then we assume all goals can always make progress
+    if delegate.disable_trait_solver_fast_paths() {
+        return MayMakeProgress;
+    }
+
+    // If the goal isn't stalled, we should definitely run it.
+    let Some(&GoalStalledOn { ref opaques, ref stalled_vars, ref sub_roots, stalled_certainty }) =
+        stalled_on
+    else {
+        return MayMakeProgress;
+    };
+
+    // If any of the stalled goal's generic arguments changed,
+    // rerunning might make progress so we should rerun.
+    if stalled_vars.iter().any(|value| delegate.is_changed_arg(*value)) {
+        return MayMakeProgress;
+    }
+
+    // If some inference took place in any of the sub roots,
+    // rerunning might make progress so we should rerun.
+    if sub_roots.iter().any(|&vid| delegate.sub_unification_table_root_var(vid) != vid) {
+        return MayMakeProgress;
+    }
+
+    match opaques {
+        GoalStalledOnOpaques::No => {}
+        &GoalStalledOnOpaques::Yes {
+            num_opaques_in_storage,
+            ref previously_succeeded_in_erased,
+        } => {
+            // If any opaques changed in the opaque type storage,
+            // rerunning might make progress so we should rerun.
+            if delegate
+                .opaque_types_storage_num_entries()
+                .needs_reevaluation(num_opaques_in_storage)
+            {
+                // Unless this goal previously succeeded in erased mode.
+                // If the stalled goal successfully evaluated while erasing opaque types,
+                // and the current state of the opaque type storage is not different in a way that is
+                // relevant, this stalled goal cannot make any progress and we set this variable to true.
+                let mut previous_erased_run_is_still_valid = false;
+
+                if let &SucceededInErased::Yes { accessed_opaques } = previously_succeeded_in_erased
+                {
+                    match should_rerun_after_erased_canonicalization(
+                        accessed_opaques,
+                        delegate.typing_mode_raw(),
+                        &delegate.clone_opaque_types_lookup_table(),
+                    ) {
+                        RerunDecision::Yes => {}
+                        RerunDecision::EagerlyPropagateToParent => {
+                            unreachable!("we never retry stalled queries if the parent was erased")
+                        }
+                        RerunDecision::No => {
+                            previous_erased_run_is_still_valid = true;
+                        }
+                    }
+                }
+
+                if !previous_erased_run_is_still_valid {
+                    return MayMakeProgress;
+                }
+            }
+        }
+    }
+
+    // Otherwise, we can be sure that this stalled goal cannot make any progress
+    // and we can exit early.
+    WontMakeProgress(stalled_certainty)
+}
+
+#[cold]
+#[inline(never)]
+pub(super) fn compute_goal_fast_path_cold<D, I>(
+    delegate: &D,
+    goal: Goal<I, I::Predicate>,
+    origin_span: I::Span,
+) -> Option<GoalEvaluation<I>>
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
+{
+    compute_goal_fast_path(delegate, goal, origin_span)
+}
+
+/// This is a fast path optimization:
+/// See the docs on [`ComputeGoalFastPathOutcome`]
+pub fn compute_goal_fast_path<D, I>(
+    delegate: &D,
+    goal: Goal<I, I::Predicate>,
+    origin_span: I::Span,
+) -> Option<GoalEvaluation<I>>
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
+{
+    if delegate.disable_trait_solver_fast_paths() {
+        return None;
+    }
+
+    match delegate.compute_goal_fast_path(goal, origin_span) {
+        ComputeGoalFastPathOutcome::NoFastPath => None,
+        ComputeGoalFastPathOutcome::TriviallyHolds => Some(GoalEvaluation {
+            goal,
+            certainty: Certainty::Yes,
+            has_changed: HasChanged::No,
+            stalled_on: None,
+        }),
+        ComputeGoalFastPathOutcome::TriviallyStalled { stalled_on } => Some(GoalEvaluation {
+            goal,
+            certainty: Certainty::AMBIGUOUS,
+            has_changed: HasChanged::No,
+            stalled_on: Some(stalled_on),
+        }),
+    }
+}