remove stability assert in evaluate_goal

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

@@ -333,7 +333,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
  let (orig_values, canonical_goal) = self.canonicalize_goal(goal);
  let mut goal_evaluation =
  self.inspect.new_goal_evaluation(goal, &orig_values, goal_evaluation_kind);
- let encountered_overflow = self.search_graph.encountered_overflow();
  let canonical_response = EvalCtxt::evaluate_canonical_goal(
  self.tcx(),
  self.search_graph,
@@ -368,75 +367,19 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
  bug!("an unchanged goal shouldn't have any side-effects on instantiation");
  }
 
- // Check that rerunning this query with its inference constraints applied
- // doesn't result in new inference constraints and has the same result.
+ // FIXME: We previously had an assert here that checked that recomputing
+ // a goal after applying its constraints did not change its response.
  //
- // If we have projection goals like `<T as Trait>::Assoc == u32` we recursively
- // call `exists<U> <T as Trait>::Assoc == U` to enable better caching. This goal
- // could constrain `U` to `u32` which would cause this check to result in a
- // solver cycle.
- if cfg!(debug_assertions)
- && has_changed
- && !matches!(
- goal_evaluation_kind,
- GoalEvaluationKind::Nested { is_normalizes_to_hack: IsNormalizesToHack::Yes }
- )
- && !self.search_graph.in_cycle()
- {
- // The nested evaluation has to happen with the original state
- // of `encountered_overflow`.
- let from_original_evaluation =
- self.search_graph.reset_encountered_overflow(encountered_overflow);
- self.check_evaluate_goal_stable_result(goal, canonical_goal, canonical_response);
- // In case the evaluation was unstable, we manually make sure that this
- // debug check does not influence the result of the parent goal.
- self.search_graph.reset_encountered_overflow(from_original_evaluation);
- }
+ // This assert was removed as it did not hold for goals constraining
+ // an inference variable to a recursive alias, e.g. in
+ // tests/ui/traits/new-solver/overflow/recursive-self-normalization.rs.
+ //
+ // Once we have decided on how to handle trait-system-refactor-initiative#75,
+ // we should re-add an assert here.
 
  Ok((has_changed, certainty, nested_goals))
  }
 
- fn check_evaluate_goal_stable_result(
- &mut self,
- goal: Goal<'tcx, ty::Predicate<'tcx>>,
- original_input: CanonicalInput<'tcx>,
- original_result: CanonicalResponse<'tcx>,
- ) {
- let (_orig_values, canonical_goal) = self.canonicalize_goal(goal);
- let result = EvalCtxt::evaluate_canonical_goal(
- self.tcx(),
- self.search_graph,
- canonical_goal,
- // FIXME(-Ztrait-solver=next): we do not track what happens in `evaluate_canonical_goal`
- &mut ProofTreeBuilder::new_noop(),
- );
-
- macro_rules! fail {
- ($msg:expr) => {{
- let msg = $msg;
- warn!(
- "unstable result: {msg}\n\
- original goal: {original_input:?},\n\
- original result: {original_result:?}\n\
- re-canonicalized goal: {canonical_goal:?}\n\
- second response: {result:?}"
- );
- return;
- }};
- }
-
- let Ok(new_canonical_response) = result else { fail!("second response was error") };
- // We only check for modulo regions as we convert all regions in
- // the input to new existentials, even if they're expected to be
- // `'static` or a placeholder region.
- if !new_canonical_response.value.var_values.is_identity_modulo_regions() {
- fail!("additional constraints from second response")
- }
- if original_result.value.certainty != new_canonical_response.value.certainty {
- fail!("unstable certainty")
- }
- }
-
  fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
  let Goal { param_env, predicate } = goal;
  let kind = predicate.kind();

compiler/rustc_trait_selection/src/solve/search_graph.rs

@@ -110,39 +110,6 @@ impl<'tcx> SearchGraph<'tcx> {
  self.stack.is_empty()
  }
 
- /// Whether we're currently in a cycle. This should only be used
- /// for debug assertions.
- pub(super) fn in_cycle(&self) -> bool {
- if let Some(stack_depth) = self.stack.last_index() {
- // Either the current goal on the stack is the root of a cycle
- // or it depends on a goal with a lower depth.
- self.stack[stack_depth].has_been_used
- || self.stack[stack_depth].cycle_root_depth != stack_depth
- } else {
- false
- }
- }
-
- /// Fetches whether the current goal encountered overflow.
- ///
- /// This should only be used for the check in `evaluate_goal`.
- pub(super) fn encountered_overflow(&self) -> bool {
- if let Some(last) = self.stack.raw.last() { last.encountered_overflow } else { false }
- }
-
- /// Resets `encountered_overflow` of the current goal.
- ///
- /// This should only be used for the check in `evaluate_goal`.
- pub(super) fn reset_encountered_overflow(&mut self, encountered_overflow: bool) -> bool {
- if let Some(last) = self.stack.raw.last_mut() {
- let prev = last.encountered_overflow;
- last.encountered_overflow = encountered_overflow;
- prev
- } else {
- false
- }
- }
-
  /// Returns the remaining depth allowed for nested goals.
  ///
  /// This is generally simply one less than the current depth.