Remove EvaluatedToErrStackDependent

compiler/rustc_middle/src/traits/select.rs

@@ -193,7 +193,6 @@ pub enum SelectionCandidate<'tcx> {
 /// The evaluation results are ordered:
 ///     - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions`
 ///       implies `EvaluatedToAmbig` implies `EvaluatedToAmbigStackDependent`
-///     - `EvaluatedToErr` implies `EvaluatedToErrStackDependent`
 ///     - the "union" of evaluation results is equal to their maximum -
 ///     all the "potential success" candidates can potentially succeed,
 ///     so they are noops when unioned with a definite error, and within
@@ -219,52 +218,9 @@ pub enum EvaluationResult {
     /// variables. We are somewhat imprecise there, so we don't actually
     /// know the real result.
     ///
-    /// This can't be trivially cached for the same reason as `EvaluatedToErrStackDependent`.
+    /// This can't be trivially cached because the result depends on the
+    /// stack results.
     EvaluatedToAmbigStackDependent,
-    /// Evaluation failed because we encountered an obligation we are already
-    /// trying to prove on this branch.
-    ///
-    /// We know this branch can't be a part of a minimal proof-tree for
-    /// the "root" of our cycle, because then we could cut out the recursion
-    /// and maintain a valid proof tree. However, this does not mean
-    /// that all the obligations on this branch do not hold -- it's possible
-    /// that we entered this branch "speculatively", and that there
-    /// might be some other way to prove this obligation that does not
-    /// go through this cycle -- so we can't cache this as a failure.
-    ///
-    /// For example, suppose we have this:
-    ///
-    /// ```rust,ignore (pseudo-Rust)
-    /// pub trait Trait { fn xyz(); }
-    /// // This impl is "useless", but we can still have
-    /// // an `impl Trait for SomeUnsizedType` somewhere.
-    /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
-    ///
-    /// pub fn foo<T: Trait + ?Sized>() {
-    ///     <T as Trait>::xyz();
-    /// }
-    /// ```
-    ///
-    /// When checking `foo`, we have to prove `T: Trait`. This basically
-    /// translates into this:
-    ///
-    /// ```plain,ignore
-    /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
-    /// ```
-    ///
-    /// When we try to prove it, we first go the first option, which
-    /// recurses. This shows us that the impl is "useless" -- it won't
-    /// tell us that `T: Trait` unless it already implemented `Trait`
-    /// by some other means. However, that does not prevent `T: Trait`
-    /// does not hold, because of the bound (which can indeed be satisfied
-    /// by `SomeUnsizedType` from another crate).
-    //
-    // FIXME: when an `EvaluatedToErrStackDependent` goes past its parent root, we
-    // ought to convert it to an `EvaluatedToErr`, because we know
-    // there definitely isn't a proof tree for that obligation. Not
-    // doing so is still sound -- there isn't any proof tree, so the
-    // branch still can't be a part of a minimal one -- but does not re-enable caching.
-    EvaluatedToErrStackDependent,
     /// Evaluation failed.
     EvaluatedToErr,
 }
@@ -290,13 +246,13 @@ impl EvaluationResult {
             | EvaluatedToAmbig
             | EvaluatedToAmbigStackDependent => true,
 
-            EvaluatedToErr | EvaluatedToErrStackDependent => false,
+            EvaluatedToErr => false,
         }
     }
 
     pub fn is_stack_dependent(self) -> bool {
         match self {
-            EvaluatedToAmbigStackDependent | EvaluatedToErrStackDependent => true,
+            EvaluatedToAmbigStackDependent => true,
 
             EvaluatedToOkModuloOpaqueTypes
             | EvaluatedToOk

compiler/rustc_trait_selection/src/infer.rs

@@ -49,8 +49,7 @@ impl<'tcx> InferCtxt<'tcx> {
     /// - the parameter environment
     ///
     /// Invokes `evaluate_obligation`, so in the event that evaluating
-    /// `Ty: Trait` causes overflow, EvaluatedToErrStackDependent
-    /// (or EvaluatedToAmbigStackDependent) will be returned.
+    /// `Ty: Trait` causes overflow, EvaluatedToAmbigStackDependent will be returned.
     #[instrument(level = "debug", skip(self, params), ret)]
     fn type_implements_trait(
         &self,

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -210,7 +210,7 @@ fn overlap<'tcx>(
         .intercrate(true)
         .with_next_trait_solver(tcx.next_trait_solver_in_coherence())
         .build();
-    let selcx = &mut SelectionContext::with_treat_inductive_cycle_as_ambig(&infcx);
+    let selcx = &mut SelectionContext::new(&infcx);
     if track_ambiguity_causes.is_yes() {
         selcx.enable_tracking_intercrate_ambiguity_causes();
     }

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -126,8 +126,6 @@ pub struct SelectionContext<'cx, 'tcx> {
     /// policy. In essence, canonicalized queries need their errors propagated
     /// rather than immediately reported because we do not have accurate spans.
     query_mode: TraitQueryMode,
-
-    treat_inductive_cycle: TreatInductiveCycleAs,
 }
 
 // A stack that walks back up the stack frame.
@@ -208,47 +206,13 @@ enum BuiltinImplConditions<'tcx> {
     Ambiguous,
 }
 
-#[derive(Copy, Clone)]
-pub enum TreatInductiveCycleAs {
-    /// This is the previous behavior, where `Recur` represents an inductive
-    /// cycle that is known not to hold. This is not forwards-compatible with
-    /// coinduction, and will be deprecated. This is the default behavior
-    /// of the old trait solver due to back-compat reasons.
-    Recur,
-    /// This is the behavior of the new trait solver, where inductive cycles
-    /// are treated as ambiguous and possibly holding.
-    Ambig,
-}
-
-impl From<TreatInductiveCycleAs> for EvaluationResult {
-    fn from(treat: TreatInductiveCycleAs) -> EvaluationResult {
-        match treat {
-            TreatInductiveCycleAs::Ambig => EvaluatedToAmbigStackDependent,
-            TreatInductiveCycleAs::Recur => EvaluatedToErrStackDependent,
-        }
-    }
-}
-
 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
         SelectionContext {
             infcx,
             freshener: infcx.freshener(),
             intercrate_ambiguity_causes: None,
             query_mode: TraitQueryMode::Standard,
-            treat_inductive_cycle: TreatInductiveCycleAs::Ambig,
-        }
-    }
-
-    pub fn with_treat_inductive_cycle_as_ambig(
-        infcx: &'cx InferCtxt<'tcx>,
-    ) -> SelectionContext<'cx, 'tcx> {
-        // Should be executed in a context where caching is disabled,
-        // otherwise the cache is poisoned with the temporary result.
-        assert!(infcx.intercrate);
-        SelectionContext {
-            treat_inductive_cycle: TreatInductiveCycleAs::Ambig,
-            ..SelectionContext::new(infcx)
         }
     }
 
@@ -756,7 +720,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                 stack.update_reached_depth(stack_arg.1);
                                 return Ok(EvaluatedToOk);
                             } else {
-                                return Ok(self.treat_inductive_cycle.into());
+                                return Ok(EvaluatedToAmbigStackDependent);
                             }
                         }
                         return Ok(EvaluatedToOk);
@@ -875,7 +839,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             }
                         }
                         ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
-                        ProjectAndUnifyResult::Recursive => Ok(self.treat_inductive_cycle.into()),
+                        ProjectAndUnifyResult::Recursive => Ok(EvaluatedToAmbigStackDependent),
                         ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
                     }
                 }
@@ -1180,7 +1144,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Some(EvaluatedToOk)
             } else {
                 debug!("evaluate_stack --> recursive, inductive");
-                Some(self.treat_inductive_cycle.into())
+                Some(EvaluatedToAmbigStackDependent)
             }
         } else {
             None