Change ty_with_args to return Ty instead of Result

Although, we would like to avoid crashes whenever
possible, and that's why I wanted to make this API fallible. It's
looking pretty hard to do proper validation.

I think many of our APIs will unfortunately depend on the user doing
the correct thing since at the MIR level we are working on,
we expect types to have been checked already.

compiler/rustc_smir/src/rustc_smir/context.rs

@@ -255,16 +255,11 @@ impl<'tcx> Context for TablesWrapper<'tcx> {
         tables.tcx.type_of(item.internal(&mut *tables)).instantiate_identity().stable(&mut *tables)
     }
 
-    fn def_ty_with_args(
-        &self,
-        item: stable_mir::DefId,
-        args: &GenericArgs,
-    ) -> Result<stable_mir::ty::Ty, Error> {
+    fn def_ty_with_args(&self, item: stable_mir::DefId, args: &GenericArgs) -> stable_mir::ty::Ty {
         let mut tables = self.0.borrow_mut();
         let args = args.internal(&mut *tables);
         let def_ty = tables.tcx.type_of(item.internal(&mut *tables));
-        // FIXME(celinval): use try_fold instead to avoid crashing.
-        Ok(def_ty.instantiate(tables.tcx, args).stable(&mut *tables))
+        def_ty.instantiate(tables.tcx, args).stable(&mut *tables)
     }
 
     fn const_literal(&self, cnst: &stable_mir::ty::Const) -> String {

compiler/rustc_smir/src/rustc_smir/convert/mod.rs

@@ -75,24 +75,3 @@ impl<'tcx> Stable<'tcx> for rustc_span::Span {
         tables.create_span(*self)
     }
 }
-
-impl<'tcx, T> Stable<'tcx> for &[T]
-where
-    T: Stable<'tcx>,
-{
-    type T = Vec<T::T>;
-    fn stable(&self, tables: &mut Tables<'tcx>) -> Self::T {
-        self.iter().map(|e| e.stable(tables)).collect()
-    }
-}
-
-impl<'tcx, T, U> Stable<'tcx> for (T, U)
-where
-    T: Stable<'tcx>,
-    U: Stable<'tcx>,
-{
-    type T = (T::T, U::T);
-    fn stable(&self, tables: &mut Tables<'tcx>) -> Self::T {
-        (self.0.stable(tables), self.1.stable(tables))
-    }
-}

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -141,3 +141,24 @@ where
         }
     }
 }
+
+impl<'tcx, T> Stable<'tcx> for &[T]
+where
+    T: Stable<'tcx>,
+{
+    type T = Vec<T::T>;
+    fn stable(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        self.iter().map(|e| e.stable(tables)).collect()
+    }
+}
+
+impl<'tcx, T, U> Stable<'tcx> for (T, U)
+where
+    T: Stable<'tcx>,
+    U: Stable<'tcx>,
+{
+    type T = (T::T, U::T);
+    fn stable(&self, tables: &mut Tables<'tcx>) -> Self::T {
+        (self.0.stable(tables), self.1.stable(tables))
+    }
+}