Add stable Instance::body() and RustcInternal trait

The `Instance::body()` returns a monomorphized body.

For that, we had to implement visitor that monomorphize types and
constants. We are also introducing the RustcInternal trait that will
allow us to convert back from Stable to Internal.

Note that this trait is not yet visible for our users as it depends on
Tables. We should probably add a new trait that can be exposed.

compiler/rustc_smir/src/rustc_internal/internal.rs

@@ -0,0 +1,61 @@
+//! Module containing the translation from stable mir constructs to the rustc counterpart.
+//!
+//! This module will only include a few constructs to allow users to invoke internal rustc APIs
+//! due to incomplete stable coverage.
+
+// Prefer importing stable_mir over internal rustc constructs to make this file more readable.
+use crate::rustc_smir::{MaybeStable, Tables};
+use rustc_middle::ty::{self as rustc_ty, Ty as InternalTy};
+use stable_mir::ty::{Const, GenericArgKind, GenericArgs, Region, Ty};
+use stable_mir::DefId;
+
+use super::RustcInternal;
+
+impl<'tcx> RustcInternal<'tcx> for DefId {
+    type T = rustc_span::def_id::DefId;
+    fn internal(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        tables.def_ids[*self]
+    }
+}
+
+impl<'tcx> RustcInternal<'tcx> for GenericArgs {
+    type T = rustc_ty::GenericArgsRef<'tcx>;
+    fn internal(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        tables.tcx.mk_args_from_iter(self.0.iter().map(|arg| arg.internal(tables)))
+    }
+}
+
+impl<'tcx> RustcInternal<'tcx> for GenericArgKind {
+    type T = rustc_ty::GenericArg<'tcx>;
+    fn internal(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        match self {
+            GenericArgKind::Lifetime(reg) => reg.internal(tables).into(),
+            GenericArgKind::Type(ty) => ty.internal(tables).into(),
+            GenericArgKind::Const(cnst) => cnst.internal(tables).into(),
+        }
+    }
+}
+
+impl<'tcx> RustcInternal<'tcx> for Region {
+    type T = rustc_ty::Region<'tcx>;
+    fn internal(&self, _tables: &mut Tables<'tcx>) -> Self::T {
+        todo!()
+    }
+}
+
+impl<'tcx> RustcInternal<'tcx> for Ty {
+    type T = InternalTy<'tcx>;
+    fn internal(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        match tables.types[self.0] {
+            MaybeStable::Stable(_) => todo!(),
+            MaybeStable::Rustc(ty) => ty,
+        }
+    }
+}
+
+impl<'tcx> RustcInternal<'tcx> for Const {
+    type T = rustc_ty::Const<'tcx>;
+    fn internal(&self, _tables: &mut Tables<'tcx>) -> Self::T {
+        todo!()
+    }
+}