Revert rust-lang#43546

This effectively reverts rust-lang#43546 as it seems that it does affect performance more than the PR has anticipated. Follow-up changes from rust-lang#44269 are also reverted.

This also removes the deduplication code from rust-lang#48296 as duplications were primarily coming from rust-lang#43546 and after removing that code it probably doesn't worth paying the cost of using a hash map.

Author: ishitatsuyuki

compiler/rustc_infer/src/traits/project.rs

@@ -92,7 +92,7 @@ pub enum ProjectionCacheEntry<'tcx> {
     Ambiguous,
     Recur,
     Error,
-    NormalizedTy(NormalizedTy<'tcx>),
+    NormalizedTy(Ty<'tcx>),
 }
 
 impl<'tcx> ProjectionCacheStorage<'tcx> {
@@ -149,7 +149,7 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
             debug!("Not overwriting Recur");
             return;
         }
-        let fresh_key = map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
+        let fresh_key = map.insert(key, ProjectionCacheEntry::NormalizedTy(value.value));
         assert!(!fresh_key, "never started projecting `{:?}`", key);
     }
 
@@ -160,9 +160,9 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
     pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
         let mut map = self.map();
         let ty = match map.get(&key) {
-            Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
+            Some(&ProjectionCacheEntry::NormalizedTy(ty)) => {
                 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}", key, ty);
-                ty.value
+                ty
             }
             ref value => {
                 // Type inference could "strand behind" old cache entries. Leave
@@ -172,26 +172,7 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
             }
         };
 
-        map.insert(
-            key,
-            ProjectionCacheEntry::NormalizedTy(Normalized { value: ty, obligations: vec![] }),
-        );
-    }
-
-    /// A specialized version of `complete` for when the key's value is known
-    /// to be a NormalizedTy.
-    pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
-        // We want to insert `ty` with no obligations. If the existing value
-        // already has no obligations (as is common) we don't insert anything.
-        if !ty.obligations.is_empty() {
-            self.map().insert(
-                key,
-                ProjectionCacheEntry::NormalizedTy(Normalized {
-                    value: ty.value,
-                    obligations: vec![],
-                }),
-            );
-        }
+        map.insert(key, ProjectionCacheEntry::NormalizedTy(ty));
     }
 
     /// Indicates that trying to normalize `key` resulted in

compiler/rustc_trait_selection/src/traits/project.rs

@@ -519,18 +519,12 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
             // This is the hottest path in this function.
             //
-            // If we find the value in the cache, then return it along
-            // with the obligations that went along with it. Note
-            // that, when using a fulfillment context, these
-            // obligations could in principle be ignored: they have
-            // already been registered when the cache entry was
-            // created (and hence the new ones will quickly be
-            // discarded as duplicated). But when doing trait
-            // evaluation this is not the case, and dropping the trait
-            // evaluations can causes ICEs (e.g., #43132).
+            // If we find the value in the cache, then the obligations
+            // have already been returned from the previous entry (and
+            // should therefore have been honored).
             debug!(?ty, "found normalized ty");
-            obligations.extend(ty.obligations);
-            return Ok(Some(ty.value));
+
+            return Ok(Some(ty));
         }
         Err(ProjectionCacheEntry::Error) => {
             debug!("opt_normalize_projection_type: found error");

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

@@ -2083,28 +2083,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             });
         }
 
-        // We are performing deduplication here to avoid exponential blowups
-        // (#38528) from happening, but the real cause of the duplication is
-        // unknown. What we know is that the deduplication avoids exponential
-        // amount of predicates being propagated when processing deeply nested
-        // types.
-        //
-        // This code is hot enough that it's worth avoiding the allocation
-        // required for the FxHashSet when possible. Special-casing lengths 0,
-        // 1 and 2 covers roughly 75-80% of the cases.
-        if obligations.len() <= 1 {
-            // No possibility of duplicates.
-        } else if obligations.len() == 2 {
-            // Only two elements. Drop the second if they are equal.
-            if obligations[0] == obligations[1] {
-                obligations.truncate(1);
-            }
-        } else {
-            // Three or more elements. Use a general deduplication process.
-            let mut seen = FxHashSet::default();
-            obligations.retain(|i| seen.insert(i.clone()));
-        }
-
         obligations
     }
 }