Fast path for processing some obligations in the new solver

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs

@@ -12,6 +12,7 @@
 use std::iter;
 
 use rustc_index::IndexVec;
+use rustc_type_ir::data_structures::HashSet;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::relate::solver_relating::RelateExt;
 use rustc_type_ir::{
@@ -158,10 +159,12 @@ where
             self.compute_external_query_constraints(certainty, normalization_nested_goals);
         let (var_values, mut external_constraints) = (self.var_values, external_constraints)
             .fold_with(&mut EagerResolver::new(self.delegate));
-        // Remove any trivial region constraints once we've resolved regions
-        external_constraints
-            .region_constraints
-            .retain(|outlives| outlives.0.as_region().is_none_or(|re| re != outlives.1));
+
+        // Remove any trivial or duplicated region constraints once we've resolved regions
+        let mut unique = HashSet::default();
+        external_constraints.region_constraints.retain(|outlives| {
+            outlives.0.as_region().is_none_or(|re| re != outlives.1) && unique.insert(*outlives)
+        });
 
         let canonical = Canonicalizer::canonicalize_response(
             self.delegate,

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -603,6 +603,14 @@ where
         // If this loop did not result in any progress, what's our final certainty.
         let mut unchanged_certainty = Some(Certainty::Yes);
         for (source, goal) in mem::take(&mut self.nested_goals) {
+            if let Some(has_changed) = self.delegate.compute_goal_fast_path(goal, self.origin_span)
+            {
+                if matches!(has_changed, HasChanged::Yes) {
+                    unchanged_certainty = None;
+                }
+                continue;
+            }
+
             // We treat normalizes-to goals specially here. In each iteration we take the
             // RHS of the projection, replace it with a fresh inference variable, and only
             // after evaluating that goal do we equate the fresh inference variable with the