Resolve any lifetime variables to 'static after inference

Chalk's unification can sometimes create lifetime variables, which we
currently don't really deal with, but at least we don't want to leak
them outside of inference.

Should fix #8919.

crates/hir_ty/src/infer/unify.rs

@@ -435,7 +435,7 @@ mod resolve {
     use super::InferenceTable;
     use crate::{
         ConcreteConst, Const, ConstData, ConstValue, DebruijnIndex, GenericArg, InferenceVar,
-        Interner, Ty, TyVariableKind, VariableKind,
+        Interner, Lifetime, Ty, TyVariableKind, VariableKind,
     };
     use chalk_ir::{
         cast::Cast,
@@ -524,5 +524,17 @@ fn fold_inference_const(
             };
             Ok(result)
         }
+
+        fn fold_inference_lifetime(
+            &mut self,
+            _var: InferenceVar,
+            _outer_binder: DebruijnIndex,
+        ) -> Fallible<Lifetime> {
+            // fall back all lifetimes to 'static -- currently we don't deal
+            // with any lifetimes, but we can sometimes get some lifetime
+            // variables through Chalk's unification, and this at least makes
+            // sure we don't leak them outside of inference
+            Ok(crate::static_lifetime())
+        }
     }
 }