[DO NOT MERGE] -Znext-solver=globally experiments

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

@@ -338,18 +338,6 @@ fn maybe_evaluate_root_goal_with_higher_recursion_limit<D, I>(
         Ok(goal_evaluation) => goal_evaluation.goal.predicate,
     };
 
-    // Some goals no longer overflow after the stalled infers are resolved.
-    // Thus we don't have to rerun eagerly here.
-    let has_stalled_infers = match predicate.kind().skip_binder() {
-        ty::PredicateKind::Clause(ty::ClauseKind::Projection(projection)) => {
-            projection.projection_term.has_non_region_infer()
-        }
-        _ => predicate.has_non_region_infer(),
-    };
-    if has_stalled_infers {
-        return;
-    }
-
     let rerun_result = delegate.commit_if_ok(|| {
         let rerun_result =
             EvalCtxt::enter_root(delegate, delegate.cx().recursion_limit() * 2, span, |ecx| {
@@ -397,19 +385,6 @@ fn maybe_evaluate_root_goal_for_proof_tree_with_higher_recursion_limit<D, I>(
         Ok(_) => {}
     }
 
-    // Some goals no longer overflow after the stalled infers are resolved.
-    // Thus we don't have to rerun eagerly here.
-    let predicate: I::Predicate = goal_evaluation.uncanonicalized_goal.predicate;
-    let has_stalled_infers = match predicate.kind().skip_binder() {
-        ty::PredicateKind::Clause(ty::ClauseKind::Projection(projection)) => {
-            projection.projection_term.has_non_region_infer()
-        }
-        _ => predicate.has_non_region_infer(),
-    };
-    if has_stalled_infers {
-        return;
-    }
-
     let rerun_result = delegate.commit_if_ok(|| {
         let (new_result, new_goal_evaluation) = evaluate_root_goal_for_proof_tree(
             delegate,
@@ -426,6 +401,7 @@ fn maybe_evaluate_root_goal_for_proof_tree_with_higher_recursion_limit<D, I>(
         }
     });
     if let Ok(rerun_result) = rerun_result {
+        let predicate: I::Predicate = goal_evaluation.uncanonicalized_goal.predicate;
         delegate.cx().emit_next_solver_overflow_fcw(predicate, span);
         *initial_result = rerun_result;
     }

compiler/rustc_session/src/config.rs

@@ -1009,7 +1009,7 @@ pub struct NextSolverConfig {
     pub coherence: bool = true,
     /// Whether the new trait solver should be enabled everywhere.
     /// This is only `true` if `coherence` is also enabled.
-    pub globally: bool = false,
+    pub globally: bool = true,
 }
 
 #[derive(Clone)]

tests/ui/traits/next-solver/overflow-discards-constraints.rs

@@ -0,0 +1,75 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Previously we didn't rerun the goal with doubled recursion limit if the goal contained ty vars.
+// This is to avoid futile evaluation. However, sometimes the type inference progress relies on
+// successful trait solving response. If we don't evaluate with higher recursion limit, the type
+// inference would fail eventually.
+//
+// See the `recursion_depth_exceeding_limit` FCW for why we need the doubled recursion limit.
+
+// Setting it to 12 would make it compile.
+#![recursion_limit = "6"]
+
+trait Trait<T> {}
+
+struct W1<T>(T);
+struct W2<T>(T);
+struct W3<T>(T);
+struct W4<T>(T);
+struct W5<T>(T);
+struct W6<T>(T);
+struct W7<T>(T);
+
+
+impl<T> Trait<T> for ()
+    where
+    W1<T>: Trait<T>,
+{}
+
+impl<T> Trait<T> for W1<T>
+where
+    W2<T>: Trait<T>,
+{}
+
+impl<T> Trait<T> for W2<T>
+where
+    W3<T>: Trait<T>,
+{}
+
+impl<T> Trait<T> for W3<T>
+where
+    W4<T>: Trait<T>,
+{}
+
+impl<T> Trait<T> for W4<T>
+where
+    W5<T>: Trait<T>,
+{}
+
+impl<T> Trait<T> for W5<T>
+where
+    W6<T>: Trait<T>,
+{}
+
+impl<T> Trait<T> for W6<T>
+where
+    W7<T>: Trait<T>,
+{}
+
+impl Trait<i32> for W7<i32> {}
+
+fn foo<T>()
+    where
+        (): Trait<T>,
+{
+}
+
+fn main() {
+    foo(); // register a `(): Trait<?t>` obligation
+    //~^ WARN: overflow evaluating the requirement `(): Trait<_>` [recursion_depth_exceeding_limit]
+    //~| WARN: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~| WARN: overflow evaluating the requirement `(): Trait<i32>` [recursion_depth_exceeding_limit]
+    //~| WARN: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+}

tests/ui/traits/next-solver/overflow-discards-constraints.stderr

@@ -0,0 +1,27 @@
+warning: overflow evaluating the requirement `(): Trait<_>`
+  --> $DIR/overflow-discards-constraints.rs:69:5
+   |
+LL |     foo(); // register a `(): Trait<?t>` obligation
+   |     ^^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "12"]` attribute to your crate (`overflow_discards_constraints`)
+   = help: or consider adding a manual `impl` of auto traits like `Send` for intermediate types, if auto traits are involved
+   = note: this lint is attached to the whole crate and can't be disabled on a per-function basis
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #159228 <https://github.com/rust-lang/rust/issues/159228>
+   = note: `#[warn(recursion_depth_exceeding_limit)]` (part of `#[warn(future_incompatible)]`) on by default
+
+warning: overflow evaluating the requirement `(): Trait<i32>`
+  --> $DIR/overflow-discards-constraints.rs:69:5
+   |
+LL |     foo(); // register a `(): Trait<?t>` obligation
+   |     ^^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "12"]` attribute to your crate (`overflow_discards_constraints`)
+   = help: or consider adding a manual `impl` of auto traits like `Send` for intermediate types, if auto traits are involved
+   = note: this lint is attached to the whole crate and can't be disabled on a per-function basis
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #159228 <https://github.com/rust-lang/rust/issues/159228>
+
+warning: 2 warnings emitted
+