Fix query cycle when encounter unevaluated const

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -635,9 +635,18 @@ pub fn try_evaluate_const<'tcx>(
                         return Err(EvaluateConstErr::HasGenericsOrInfers);
                     }
 
-                    let typing_env = infcx
+                    let mut typing_env;
+
+                    typing_env = infcx
                         .typing_env(tcx.erase_and_anonymize_regions(param_env))
                         .with_post_analysis_normalized(tcx);
+
+                    // Since there is no generic parameter, we can just drop the environment
+                    // to prevent query cycle.
+                    if !uv.args.has_non_region_param() {
+                        typing_env = infcx.typing_env(ty::ParamEnv::empty());
+                    }
+
                     (uv.args, typing_env)
                 }
             };

tests/ui/traits/next-solver/unevaluated_const_query_cycle.rs

@@ -0,0 +1,20 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Regression test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/249
+
+const CONST: &str = "hi";
+
+trait ToUnit {
+    type Assoc;
+}
+impl<T> ToUnit for T {
+    type Assoc = ();
+}
+
+fn foo()
+where
+    <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
+{}
+
+fn main(){}