Rollup merge of #124827 - lcnr:generalize-incomplete, r=compiler-errors

generalize hr alias: avoid unconstrainable infer vars

fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/108

see inline comments for more details

r? `@compiler-errors` cc `@BoxyUwU`

compiler/rustc_trait_selection/src/solve/inspect/build.rs

@@ -376,7 +376,10 @@ impl<'tcx> ProofTreeBuilder<'tcx> {
                 (
                     DebugSolver::GoalEvaluation(goal_evaluation),
                     DebugSolver::CanonicalGoalEvaluation(canonical_goal_evaluation),
-                ) => goal_evaluation.evaluation = Some(canonical_goal_evaluation),
+                ) => {
+                    let prev = goal_evaluation.evaluation.replace(canonical_goal_evaluation);
+                    assert_eq!(prev, None);
+                }
                 _ => unreachable!(),
             }
         }