Auto merge of rust-lang#120463 - lcnr:eager-inference-replacement, r=jackh726,compiler-errors

some trait system cleanups

Always eagerly replace projections with infer vars if normalization is ambig. Unsure why we previously didn't do so, wasn't able to find an explanation in rust-lang#90887. This adds some complexity to the trait system and is afaict unnecessary.

The second commit simplifies `pred_known_to_hold_modulo_regions`, afaict the optional `fulfill` isn't necessary anymore.

r? types cc `@jackh726`

Author: bors

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -312,7 +312,7 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 
         if obligation.predicate.has_projections() {
             let mut obligations = Vec::new();
-            let predicate = crate::traits::project::try_normalize_with_depth_to(
+            let predicate = crate::traits::project::normalize_with_depth_to(
                 &mut self.selcx,
                 obligation.param_env,
                 obligation.cause.clone(),

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -119,60 +119,16 @@ pub fn predicates_for_generics<'tcx>(
 
 /// Determines whether the type `ty` is known to meet `bound` and
 /// returns true if so. Returns false if `ty` either does not meet
-/// `bound` or is not known to meet bound (note that this is
-/// conservative towards *no impl*, which is the opposite of the
-/// `evaluate` methods).
+/// `bound` or is not known to meet bound.
 pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
     infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     ty: Ty<'tcx>,
     def_id: DefId,
 ) -> bool {
     let trait_ref = ty::TraitRef::new(infcx.tcx, def_id, [ty]);
-    pred_known_to_hold_modulo_regions(infcx, param_env, trait_ref)
-}
-
-/// FIXME(@lcnr): this function doesn't seem right and shouldn't exist?
-///
-/// Ping me on zulip if you want to use this method and need help with finding
-/// an appropriate replacement.
-#[instrument(level = "debug", skip(infcx, param_env, pred), ret)]
-fn pred_known_to_hold_modulo_regions<'tcx>(
-    infcx: &InferCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    pred: impl ToPredicate<'tcx>,
-) -> bool {
-    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, pred);
-
-    let result = infcx.evaluate_obligation_no_overflow(&obligation);
-    debug!(?result);
-
-    if result.must_apply_modulo_regions() {
-        true
-    } else if result.may_apply() {
-        // Sometimes obligations are ambiguous because the recursive evaluator
-        // is not smart enough, so we fall back to fulfillment when we're not certain
-        // that an obligation holds or not. Even still, we must make sure that
-        // the we do no inference in the process of checking this obligation.
-        let goal = infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
-        infcx.probe(|_| {
-            let ocx = ObligationCtxt::new(infcx);
-            ocx.register_obligation(obligation);
-
-            let errors = ocx.select_all_or_error();
-            match errors.as_slice() {
-                // Only known to hold if we did no inference.
-                [] => infcx.shallow_resolve(goal) == goal,
-
-                errors => {
-                    debug!(?errors);
-                    false
-                }
-            }
-        })
-    } else {
-        false
-    }
+    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, trait_ref);
+    infcx.predicate_must_hold_modulo_regions(&obligation)
 }
 
 #[instrument(level = "debug", skip(tcx, elaborated_env))]

compiler/rustc_trait_selection/src/traits/project.rs

@@ -383,32 +383,6 @@ where
     result
 }
 
-#[instrument(level = "info", skip(selcx, param_env, cause, obligations))]
-pub(crate) fn try_normalize_with_depth_to<'a, 'b, 'tcx, T>(
-    selcx: &'a mut SelectionContext<'b, 'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    cause: ObligationCause<'tcx>,
-    depth: usize,
-    value: T,
-    obligations: &mut Vec<PredicateObligation<'tcx>>,
-) -> T
-where
-    T: TypeFoldable<TyCtxt<'tcx>>,
-{
-    debug!(obligations.len = obligations.len());
-    let mut normalizer = AssocTypeNormalizer::new_without_eager_inference_replacement(
-        selcx,
-        param_env,
-        cause,
-        depth,
-        obligations,
-    );
-    let result = ensure_sufficient_stack(|| normalizer.fold(value));
-    debug!(?result, obligations.len = normalizer.obligations.len());
-    debug!(?normalizer.obligations,);
-    result
-}
-
 pub(crate) fn needs_normalization<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
     value: &T,
     reveal: Reveal,
@@ -435,10 +409,6 @@ struct AssocTypeNormalizer<'a, 'b, 'tcx> {
     obligations: &'a mut Vec<PredicateObligation<'tcx>>,
     depth: usize,
     universes: Vec<Option<ty::UniverseIndex>>,
-    /// If true, when a projection is unable to be completed, an inference
-    /// variable will be created and an obligation registered to project to that
-    /// inference variable. Also, constants will be eagerly evaluated.
-    eager_inference_replacement: bool,
 }
 
 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
@@ -450,33 +420,7 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
         obligations: &'a mut Vec<PredicateObligation<'tcx>>,
     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
         debug_assert!(!selcx.infcx.next_trait_solver());
-        AssocTypeNormalizer {
-            selcx,
-            param_env,
-            cause,
-            obligations,
-            depth,
-            universes: vec![],
-            eager_inference_replacement: true,
-        }
-    }
-
-    fn new_without_eager_inference_replacement(
-        selcx: &'a mut SelectionContext<'b, 'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-        cause: ObligationCause<'tcx>,
-        depth: usize,
-        obligations: &'a mut Vec<PredicateObligation<'tcx>>,
-    ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
-        AssocTypeNormalizer {
-            selcx,
-            param_env,
-            cause,
-            obligations,
-            depth,
-            universes: vec![],
-            eager_inference_replacement: false,
-        }
+        AssocTypeNormalizer { selcx, param_env, cause, obligations, depth, universes: vec![] }
     }
 
     fn fold<T: TypeFoldable<TyCtxt<'tcx>>>(&mut self, value: T) -> T {
@@ -579,28 +523,14 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                 // register an obligation to *later* project, since we know
                 // there won't be bound vars there.
                 let data = data.fold_with(self);
-                let normalized_ty = if self.eager_inference_replacement {
-                    normalize_projection_type(
-                        self.selcx,
-                        self.param_env,
-                        data,
-                        self.cause.clone(),
-                        self.depth,
-                        self.obligations,
-                    )
-                } else {
-                    opt_normalize_projection_type(
-                        self.selcx,
-                        self.param_env,
-                        data,
-                        self.cause.clone(),
-                        self.depth,
-                        self.obligations,
-                    )
-                    .ok()
-                    .flatten()
-                    .unwrap_or_else(|| ty.super_fold_with(self).into())
-                };
+                let normalized_ty = normalize_projection_type(
+                    self.selcx,
+                    self.param_env,
+                    data,
+                    self.cause.clone(),
+                    self.depth,
+                    self.obligations,
+                );
                 debug!(
                     ?self.depth,
                     ?ty,

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

@@ -22,7 +22,6 @@ use super::{
 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
 use crate::solve::InferCtxtSelectExt;
 use crate::traits::error_reporting::TypeErrCtxtExt;
-use crate::traits::project::try_normalize_with_depth_to;
 use crate::traits::project::ProjectAndUnifyResult;
 use crate::traits::project::ProjectionCacheKeyExt;
 use crate::traits::ProjectionCacheKey;
@@ -1069,7 +1068,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 && fresh_trait_pred.is_global()
             {
                 let mut nested_obligations = Vec::new();
-                let predicate = try_normalize_with_depth_to(
+                let predicate = normalize_with_depth_to(
                     this,
                     param_env,
                     obligation.cause.clone(),