Auto merge of rust-lang#141128 - compiler-errors:fast-path, r=<try>

Fast path for processing some obligations in the new solver

r? lcnr

Author: bors

compiler/rustc_next_trait_solver/src/delegate.rs

@@ -17,6 +17,12 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
     where
         V: TypeFoldable<Self::Interner>;
 
+    fn compute_goal_fast_path(
+        &self,
+        goal: Goal<Self::Interner, <Self::Interner as Interner>::Predicate>,
+        span: <Self::Interner as Interner>::Span,
+    ) -> Option<()>;
+
     fn fresh_var_for_kind_with_span(
         &self,
         arg: <Self::Interner as Interner>::GenericArg,

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

@@ -12,6 +12,7 @@
 use std::iter;
 
 use rustc_index::IndexVec;
+use rustc_type_ir::data_structures::HashSet;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::relate::solver_relating::RelateExt;
 use rustc_type_ir::{
@@ -158,10 +159,12 @@ where
             self.compute_external_query_constraints(certainty, normalization_nested_goals);
         let (var_values, mut external_constraints) = (self.var_values, external_constraints)
             .fold_with(&mut EagerResolver::new(self.delegate));
-        // Remove any trivial region constraints once we've resolved regions
-        external_constraints
-            .region_constraints
-            .retain(|outlives| outlives.0.as_region().is_none_or(|re| re != outlives.1));
+
+        // Remove any trivial or duplicated region constraints once we've resolved regions
+        let mut unique = HashSet::default();
+        external_constraints.region_constraints.retain(|outlives| {
+            outlives.0.as_region().is_none_or(|re| re != outlives.1) && unique.insert(*outlives)
+        });
 
         let canonical = Canonicalizer::canonicalize_response(
             self.delegate,

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

@@ -603,6 +603,11 @@ where
         // If this loop did not result in any progress, what's our final certainty.
         let mut unchanged_certainty = Some(Certainty::Yes);
         for (source, goal) in mem::take(&mut self.nested_goals) {
+            if let Some(()) = self.delegate.compute_goal_fast_path(goal, self.origin_span) {
+                unchanged_certainty = None;
+                continue;
+            }
+
             // We treat normalizes-to goals specially here. In each iteration we take the
             // RHS of the projection, replace it with a fresh inference variable, and only
             // after evaluating that goal do we equate the fresh inference variable with the

compiler/rustc_trait_selection/src/solve/delegate.rs

@@ -6,14 +6,14 @@ use rustc_infer::infer::canonical::query_response::make_query_region_constraints
 use rustc_infer::infer::canonical::{
     Canonical, CanonicalExt as _, CanonicalQueryInput, CanonicalVarInfo, CanonicalVarValues,
 };
-use rustc_infer::infer::{InferCtxt, RegionVariableOrigin, TyCtxtInferExt};
+use rustc_infer::infer::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TyCtxtInferExt};
 use rustc_infer::traits::solve::Goal;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::Certainty;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitableExt as _, TypingMode};
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
 
-use crate::traits::{EvaluateConstErr, specialization_graph};
+use crate::traits::{EvaluateConstErr, ObligationCause, specialization_graph};
 
 #[repr(transparent)]
 pub struct SolverDelegate<'tcx>(InferCtxt<'tcx>);
@@ -55,6 +55,37 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
         (SolverDelegate(infcx), value, vars)
     }
 
+    fn compute_goal_fast_path(
+        &self,
+        goal: Goal<'tcx, ty::Predicate<'tcx>>,
+        span: Span,
+    ) -> Option<()> {
+        let pred = goal.predicate.kind();
+        match pred.no_bound_vars()? {
+            ty::PredicateKind::DynCompatible(def_id) if self.0.tcx.is_dyn_compatible(def_id) => {
+                Some(())
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(outlives)) => {
+                self.0.sub_regions(
+                    SubregionOrigin::RelateRegionParamBound(span, None),
+                    outlives.1,
+                    outlives.0,
+                );
+                Some(())
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(outlives)) => {
+                self.0.register_region_obligation_with_cause(
+                    outlives.0,
+                    outlives.1,
+                    &ObligationCause::dummy_with_span(span),
+                );
+
+                Some(())
+            }
+            _ => None,
+        }
+    }
+
     fn fresh_var_for_kind_with_span(
         &self,
         arg: ty::GenericArg<'tcx>,

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -12,6 +12,7 @@ use rustc_infer::traits::{
 use rustc_middle::ty::{
     self, DelayedSet, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor, TypingMode,
 };
+use rustc_next_trait_solver::delegate::SolverDelegate as _;
 use rustc_next_trait_solver::solve::{GenerateProofTree, HasChanged, SolverDelegateEvalExt as _};
 use rustc_span::Span;
 use tracing::instrument;
@@ -172,7 +173,13 @@ where
                 }
 
                 let goal = obligation.as_goal();
-                let result = <&SolverDelegate<'tcx>>::from(infcx)
+                let delegate = <&SolverDelegate<'tcx>>::from(infcx);
+                if let Some(()) = delegate.compute_goal_fast_path(goal, obligation.cause.span) {
+                    has_changed = true;
+                    continue;
+                }
+
+                let result = delegate
                     .evaluate_root_goal(goal, GenerateProofTree::No, obligation.cause.span)
                     .0;
                 self.inspect_evaluated_obligation(infcx, &obligation, &result);

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -701,9 +701,15 @@ pub fn impossible_predicates<'tcx>(tcx: TyCtxt<'tcx>, predicates: Vec<ty::Clause
         let obligation = Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate);
         ocx.register_obligation(obligation);
     }
-    let errors = ocx.select_all_or_error();
 
-    if !errors.is_empty() {
+    // Use `select_where_possible` to only return impossible for true errors,
+    // and not ambiguities or overflows. Since the new trait solver forces
+    // some currently undetected overlap between `dyn Trait: Trait` built-in
+    // vs user-written impls to AMBIGUOUS, this may return ambiguity even
+    // with no infer vars. There may also be ways to encounter ambiguity due
+    // to post-mono overflow.
+    let true_errors = ocx.select_where_possible();
+    if !true_errors.is_empty() {
         return true;
     }
 

tests/ui/traits/object/ambiguity-vtable-segfault.rs

@@ -0,0 +1,37 @@
+// In this example below, we have two overlapping candidates for `dyn Q: Q`.
+// Specifically, the user written impl for `<dyn Q as Mirror>::Assoc` and the
+// built-in impl for object types. Since they differ by their region responses,
+// the goal is ambiguous. This affects codegen since impossible obligations
+// for method dispatch will lead to a segfault, since we end up emitting dummy
+// call vtable offsets due to <https://github.com/rust-lang/rust/pull/136311>.
+
+// Test for <https://github.com/rust-lang/rust/issues/141119>.
+
+//@ run-pass
+
+trait Mirror {
+    type Assoc: ?Sized;
+}
+impl<T: ?Sized> Mirror for T {
+    type Assoc = T;
+}
+
+trait Q: 'static {
+    fn q(&self);
+}
+
+impl Q for i32 {
+    fn q(&self) { println!("i32"); }
+}
+
+impl Q for <dyn Q as Mirror>::Assoc where Self: 'static {
+    fn q(&self) { println!("dyn Q"); }
+}
+
+fn foo<T: Q + ?Sized>(t: &T) {
+    t.q();
+}
+
+fn main() {
+    foo(&1 as &dyn Q);
+}