Normalize the result of Fields::ty_with_args

We were only instantiating before, which would leak an AliasTy.
I added a test case that reproduce the issue seen here:

https://github.com/model-checking/kani/issues/3113

compiler/rustc_smir/src/rustc_smir/context.rs

@@ -420,7 +420,10 @@ fn def_ty_with_args(&self, item: stable_mir::DefId, args: &GenericArgs) -> stabl
         let tcx = tables.tcx;
         let args = args.internal(&mut *tables, tcx);
         let def_ty = tables.tcx.type_of(item.internal(&mut *tables, tcx));
-        def_ty.instantiate(tables.tcx, args).stable(&mut *tables)
+        tables
+            .tcx
+            .instantiate_and_normalize_erasing_regions(args, ty::ParamEnv::reveal_all(), def_ty)
+            .stable(&mut *tables)
     }
 
     fn const_pretty(&self, cnst: &stable_mir::ty::Const) -> String {

compiler/stable_mir/src/ty.rs

@@ -654,7 +654,7 @@ pub fn ty(&self) -> Ty {
         with(|cx| cx.def_ty(self.0))
     }
 
-    /// Retrieve the type of this Adt instantiating the type with the given arguments.
+    /// Retrieve the type of this Adt by instantiating and normalizing it with the given arguments.
     ///
     /// This will assume the type can be instantiated with these arguments.
     pub fn ty_with_args(&self, args: &GenericArgs) -> Ty {
@@ -733,7 +733,7 @@ pub struct FieldDef {
 }
 
 impl FieldDef {
-    /// Retrieve the type of this field instantiating the type with the given arguments.
+    /// Retrieve the type of this field instantiating and normalizing it with the given arguments.
     ///
     /// This will assume the type can be instantiated with these arguments.
     pub fn ty_with_args(&self, args: &GenericArgs) -> Ty {

tests/ui-fulldeps/stable-mir/check_normalization.rs

@@ -0,0 +1,95 @@
+//@ run-pass
+//! Test that types are normalized in an instance body.
+
+//@ ignore-stage1
+//@ ignore-cross-compile
+//@ ignore-remote
+//@ ignore-windows-gnu mingw has troubles with linking https://github.com/rust-lang/rust/pull/116837
+//@ edition: 2021
+
+#![feature(rustc_private)]
+
+#[macro_use]
+extern crate rustc_smir;
+extern crate rustc_driver;
+extern crate rustc_interface;
+extern crate stable_mir;
+
+use mir::mono::Instance;
+use ty::{Ty, TyKind, RigidTy};
+use rustc_smir::rustc_internal;
+use stable_mir::*;
+use std::io::Write;
+use std::ops::ControlFlow;
+
+const CRATE_NAME: &str = "input";
+
+/// This function uses the Stable MIR APIs to get information about the test crate.
+fn test_stable_mir() -> ControlFlow<()> {
+    let items = stable_mir::all_local_items();
+
+    // Get all items and split generic vs monomorphic items.
+    let instances: Vec<_> =
+        items.into_iter().filter_map(|item| (!item.requires_monomorphization()).then(|| {
+            Instance::try_from(item).unwrap()
+        })).collect();
+    assert_eq!(instances.len(), 1, "Expected one constant");
+
+    for instance in instances {
+        check_ty(instance.ty());
+    }
+    ControlFlow::Continue(())
+}
+
+fn check_ty(ty: Ty) {
+    match ty.kind() {
+        TyKind::RigidTy(RigidTy::Adt(def, args)) if def.kind().is_struct() => {
+            // Ensure field type is also normalized
+            def.variants_iter().next().unwrap().fields().into_iter().for_each(|f| {
+                check_ty(f.ty_with_args(&args))
+            });
+        }
+        TyKind::RigidTy(RigidTy::Uint(..)) => {}
+        kind => unreachable!("Unexpected kind: {kind:?}")
+    }
+}
+
+
+/// This test will generate and analyze a dummy crate using the stable mir.
+/// For that, it will first write the dummy crate into a file.
+/// Then it will create a `StableMir` using custom arguments and then
+/// it will run the compiler.
+fn main() {
+    let path = "normalization_input.rs";
+    generate_input(&path).unwrap();
+    let args = vec![
+        "rustc".to_string(),
+        "-Cpanic=abort".to_string(),
+        "--crate-type=lib".to_string(),
+        "--crate-name".to_string(),
+        CRATE_NAME.to_string(),
+        path.to_string(),
+    ];
+    run!(args, test_stable_mir).unwrap();
+}
+
+fn generate_input(path: &str) -> std::io::Result<()> {
+    let mut file = std::fs::File::create(path)?;
+    write!(
+        file,
+        r#"
+        pub trait Primitive {{
+            type Base;
+        }}
+
+        impl Primitive for char {{
+            type Base = u32;
+        }}
+
+        pub struct Wrapper<T: Primitive>(T::Base);
+        pub type WrapperChar = Wrapper<char>;
+        pub const NULL_CHAR: WrapperChar = Wrapper::<char>(0);
+        "#
+    )?;
+    Ok(())
+}