remove obligation dedup

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

@@ -2328,28 +2328,6 @@ 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
     }
 }