Auto merge of #141763 - lcnr:fixme-gamer, r=BoxyUwU

`FIXME(-Znext-solver)` triage

r? `@BoxyUwU`

3 files changed, 10 insertions, 3 deletions

diff --git a/compiler/rustc_next_trait_solver/src/solve/mod.rs b/compiler/rustc_next_trait_solver/src/solve/mod.rs
index a7a984181d7..e4e0aba7b50 100644
--- a/compiler/rustc_next_trait_solver/src/solve/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/mod.rs
@@ -237,8 +237,8 @@ where
             return None;
         }
 
-        // FIXME(-Znext-solver): We should instead try to find a `Certainty::Yes` response with
-        // a subset of the constraints that all the other responses have.
+        // FIXME(-Znext-solver): Add support to merge region constraints in
+        // responses to deal with trait-system-refactor-initiative#27.
         let one = responses[0];
         if responses[1..].iter().all(|&resp| resp == one) {
             return Some(one);
diff --git a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs
index 2640238f5a9..26443c54e18 100644
--- a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs
@@ -41,6 +41,9 @@ where
         // and we tag the impl bounds with `GoalSource::ImplWhereBound`?
         // Right now this includes both the impl and the assoc item where bounds,
         // and I don't think the assoc item where-bounds are allowed to be coinductive.
+        //
+        // Projecting to the IAT also "steps out the impl contructor", so we would have
+        // to be very careful when changing the impl where-clauses to be productive.
         self.add_goals(
             GoalSource::Misc,
             cx.predicates_of(inherent.def_id)
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index 7c4e1dc2c12..110c67a8e21 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -235,7 +235,11 @@ where
                 .predicates_of(goal.predicate.def_id())
                 .iter_instantiated(cx, goal.predicate.trait_ref.args)
                 .map(|p| goal.with(cx, p));
-            // FIXME(-Znext-solver=coinductive): Should this be `GoalSource::ImplWhereBound`?
+            // While you could think of trait aliases to have a single builtin impl
+            // which uses its implied trait bounds as where-clauses, using
+            // `GoalSource::ImplWhereClause` here would be incorrect, as we also
+            // impl them, which means we're "stepping out of the impl constructor"
+            // again. To handle this, we treat these cycles as ambiguous for now.
             ecx.add_goals(GoalSource::Misc, nested_obligations);
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })