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_transmute/src/maybe_transmutable/mod.rs

@@ -31,7 +31,7 @@ where
 #[cfg(feature = "rustc")]
 mod rustc {
     use rustc_middle::ty::layout::LayoutCx;
-    use rustc_middle::ty::{ParamEnv, Ty, TyCtxt};
+    use rustc_middle::ty::{Ty, TyCtxt, TypingEnv};
 
     use super::*;
     use crate::layout::tree::rustc::Err;
@@ -43,7 +43,7 @@ mod rustc {
         pub(crate) fn answer(self) -> Answer<<TyCtxt<'tcx> as QueryContext>::Ref> {
             let Self { src, dst, assume, context } = self;
 
-            let layout_cx = LayoutCx::new(context, ParamEnv::reveal_all());
+            let layout_cx = LayoutCx::new(context, TypingEnv::fully_monomorphized());
 
             // Convert `src` and `dst` from their rustc representations, to `Tree`-based
             // representations.