Rollup merge of #120128 - oli-obk:smir_internal_lift, r=celinval

Make stable_mir::with_tables sound

See the first commit for the actual soundness fix. The rest is just fallout from that and is entirely safe code. Includes most of #120120

The major difference to #120120 is that we don't need an unsafe trait, as we can now rely on the type system (the only unsafe part, and the actual source of the unsoundness was in `with_tables`)

r? `@celinval`

compiler/rustc_middle/src/mir/consts.rs

@@ -195,7 +195,7 @@ impl<'tcx> ConstValue<'tcx> {
 /// Constants
 
 #[derive(Clone, Copy, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable, Debug)]
-#[derive(TypeFoldable, TypeVisitable)]
+#[derive(TypeFoldable, TypeVisitable, Lift)]
 pub enum Const<'tcx> {
     /// This constant came from the type system.
     ///
@@ -456,7 +456,7 @@ impl<'tcx> Const<'tcx> {
 
 /// An unevaluated (potentially generic) constant used in MIR.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable)]
-#[derive(Hash, HashStable, TypeFoldable, TypeVisitable)]
+#[derive(Hash, HashStable, TypeFoldable, TypeVisitable, Lift)]
 pub struct UnevaluatedConst<'tcx> {
     pub def: DefId,
     pub args: GenericArgsRef<'tcx>,

compiler/rustc_middle/src/ty/context.rs

@@ -1416,6 +1416,7 @@ nop_lift! {const_; Const<'a> => Const<'tcx>}
 nop_lift! {const_allocation; ConstAllocation<'a> => ConstAllocation<'tcx>}
 nop_lift! {predicate; Predicate<'a> => Predicate<'tcx>}
 nop_lift! {predicate; Clause<'a> => Clause<'tcx>}
+nop_lift! {layout; Layout<'a> => Layout<'tcx>}
 
 nop_list_lift! {type_lists; Ty<'a> => Ty<'tcx>}
 nop_list_lift! {poly_existential_predicates; PolyExistentialPredicate<'a> => PolyExistentialPredicate<'tcx>}
@@ -1424,8 +1425,28 @@ nop_list_lift! {bound_variable_kinds; ty::BoundVariableKind => ty::BoundVariable
 // This is the impl for `&'a GenericArgs<'a>`.
 nop_list_lift! {args; GenericArg<'a> => GenericArg<'tcx>}
 
+macro_rules! nop_slice_lift {
+    ($ty:ty => $lifted:ty) => {
+        impl<'a, 'tcx> Lift<'tcx> for &'a [$ty] {
+            type Lifted = &'tcx [$lifted];
+            fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+                if self.is_empty() {
+                    return Some(&[]);
+                }
+                tcx.interners
+                    .arena
+                    .dropless
+                    .contains_slice(self)
+                    .then(|| unsafe { mem::transmute(self) })
+            }
+        }
+    };
+}
+
+nop_slice_lift! {ty::ValTree<'a> => ty::ValTree<'tcx>}
+
 TrivialLiftImpls! {
-    ImplPolarity,
+    ImplPolarity, Promoted
 }
 
 macro_rules! sty_debug_print {