Auto merge of rust-lang#120463 - lcnr:eager-inference-replacement, r=<try>

never normalize without eager inference replacement

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,

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/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(),