Auto merge of #120463 - lcnr:eager-inference-replacement, r=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 #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_hir_analysis/src/astconv/mod.rs

@@ -1425,17 +1425,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             vec![]
         };
 
-        let (impl_, (assoc_item, def_scope)) =
-            crate::traits::project::with_replaced_escaping_bound_vars(
-                infcx,
-                &mut universes,
-                self_ty,
-                |self_ty| {
-                    self.select_inherent_assoc_type_candidates(
-                        infcx, name, span, self_ty, param_env, candidates,
-                    )
-                },
-            )?;
+        let (impl_, (assoc_item, def_scope)) = crate::traits::with_replaced_escaping_bound_vars(
+            infcx,
+            &mut universes,
+            self_ty,
+            |self_ty| {
+                self.select_inherent_assoc_type_candidates(
+                    infcx, name, span, self_ty, param_env, candidates,
+                )
+            },
+        )?;
 
         self.check_assoc_ty(assoc_item, name, def_scope, block, span);
 

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -1,6 +1,6 @@
 use crate::traits::error_reporting::TypeErrCtxtExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
-use crate::traits::{needs_normalization, BoundVarReplacer, PlaceholderReplacer};
+use crate::traits::{BoundVarReplacer, PlaceholderReplacer};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::infer::at::At;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
@@ -205,10 +205,9 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
     }
 
     fn try_fold_const(&mut self, ct: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
-        let reveal = self.at.param_env.reveal();
         let infcx = self.at.infcx;
         debug_assert_eq!(ct, infcx.shallow_resolve(ct));
-        if !needs_normalization(&ct, reveal) {
+        if !ct.has_projections() {
             return Ok(ct);
         }
 

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -1,5 +1,6 @@
 use crate::infer::{InferCtxt, TyOrConstInferVar};
 use crate::traits::error_reporting::TypeErrCtxtExt;
+use crate::traits::normalize::normalize_with_depth_to;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::obligation_forest::ProcessResult;
 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
@@ -312,7 +313,7 @@ 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 = normalize_with_depth_to(
                 &mut self.selcx,
                 obligation.param_env,
                 obligation.cause.clone(),

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -9,6 +9,7 @@ mod engine;
 pub mod error_reporting;
 mod fulfill;
 pub mod misc;
+pub mod normalize;
 mod object_safety;
 pub mod outlives_bounds;
 pub mod project;
@@ -40,17 +41,15 @@ use rustc_span::Span;
 use std::fmt::Debug;
 use std::ops::ControlFlow;
 
-pub(crate) use self::project::{needs_normalization, BoundVarReplacer, PlaceholderReplacer};
-
 pub use self::coherence::{add_placeholder_note, orphan_check, overlapping_impls};
 pub use self::coherence::{OrphanCheckErr, OverlapResult};
 pub use self::engine::{ObligationCtxt, TraitEngineExt};
 pub use self::fulfill::{FulfillmentContext, PendingPredicateObligation};
+pub use self::normalize::NormalizeExt;
 pub use self::object_safety::astconv_object_safety_violations;
 pub use self::object_safety::is_vtable_safe_method;
 pub use self::object_safety::object_safety_violations_for_assoc_item;
 pub use self::object_safety::ObjectSafetyViolation;
-pub use self::project::NormalizeExt;
 pub use self::project::{normalize_inherent_projection, normalize_projection_type};
 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
@@ -68,6 +67,7 @@ pub use self::util::{
 };
 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
 pub use self::util::{get_vtable_index_of_object_method, impl_item_is_final, upcast_choices};
+pub use self::util::{with_replaced_escaping_bound_vars, BoundVarReplacer, PlaceholderReplacer};
 
 pub use rustc_infer::traits::*;
 
@@ -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))]