use TypingEnv when no infcx is available

the behavior of the type system not only depends on the current
assumptions, but also the currentnphase of the compiler. This is
mostly necessary as we need to decide whether and how to reveal
opaque types. We track this via the `TypingMode`.

compiler/rustc_mir_transform/src/remove_zsts.rs

@@ -21,9 +21,9 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveZsts {
             return;
         }
 
-        let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
+        let typing_env = body.typing_env(tcx);
         let local_decls = &body.local_decls;
-        let mut replacer = Replacer { tcx, param_env, local_decls };
+        let mut replacer = Replacer { tcx, typing_env, local_decls };
         for var_debug_info in &mut body.var_debug_info {
             replacer.visit_var_debug_info(var_debug_info);
         }
@@ -35,7 +35,7 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveZsts {
 
 struct Replacer<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
+    typing_env: ty::TypingEnv<'tcx>,
     local_decls: &'a LocalDecls<'tcx>,
 }
 
@@ -61,7 +61,7 @@ impl<'tcx> Replacer<'_, 'tcx> {
         if !maybe_zst(ty) {
             return false;
         }
-        let Ok(layout) = self.tcx.layout_of(self.param_env.and(ty)) else {
+        let Ok(layout) = self.tcx.layout_of(self.typing_env.as_query_input(ty)) else {
             return false;
         };
         layout.is_zst()