diff --git a/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs b/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
index ccd5c649d4c9e..da21ff66f8e15 100644
--- a/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
@@ -374,7 +374,7 @@ where
         _ecx: &mut EvalCtxt<'_, D>,
         _goal: Goal<I, Self>,
     ) -> Result<Candidate<I>, NoSolutionOrRerunNonErased> {
-        todo!("Iterator is not yet const")
+        Err(NoSolutionOrRerunNonErased::NoSolution(NoSolution))
     }
 
     fn consider_builtin_fused_iterator_candidate(
diff --git a/src/ci/docker/scripts/x86_64-gnu-llvm.sh b/src/ci/docker/scripts/x86_64-gnu-llvm.sh
index 5fa17d954c3bc..79177b96a4afc 100755
--- a/src/ci/docker/scripts/x86_64-gnu-llvm.sh
+++ b/src/ci/docker/scripts/x86_64-gnu-llvm.sh
@@ -13,8 +13,10 @@ set -ex
 # despite having different output on 32-bit vs 64-bit targets.
 ../x --stage 2 test tests/mir-opt --host='' --target=i686-unknown-linux-gnu
 
-# Run the UI test suite again, but in `--pass=check` mode
-#
-# This is intended to make sure that both `--pass=check` continues to
-# work.
+# Run the UI test suite in `--pass=check` mode, to ensure it continues to work.
 ../x.ps1 --stage 2 test tests/ui --pass=check --host='' --target=i686-unknown-linux-gnu
+
+# Rebuild the stdlib using the new trait solver, to ensure it doesn't regress
+# until stabilization.
+RUSTFLAGS_NOT_BOOTSTRAP="-Znext-solver=globally" ../x --stage 1 build library \
+    --host='' --target=i686-unknown-linux-gnu