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(
+    fn def_ty_with_args(&self, item: stable_mir::DefId, args: &GenericArgs) -> stable_mir::ty::Ty {
-        &self,
-        item: stable_mir::DefId,
-        args: &GenericArgs,
-    ) -> Result<stable_mir::ty::Ty, Error> {
         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.
+        def_ty.instantiate(tables.tcx, args).stable(&mut *tables)
-        Ok(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))
+    }
+}

compiler/stable_mir/src/compiler_interface.rs

@@ -91,7 +91,7 @@ pub trait Context {
     fn def_ty(&self, item: DefId) -> Ty;
 
     /// Returns the type of given definition instantiated with the given arguments.
-    fn def_ty_with_args(&self, item: DefId, args: &GenericArgs) -> Result<Ty, Error>;
+    fn def_ty_with_args(&self, item: DefId, args: &GenericArgs) -> Ty;
 
     /// Returns literal value of a const as a string.
     fn const_literal(&self, cnst: &Const) -> String;

compiler/stable_mir/src/error.rs

@@ -28,7 +28,7 @@ pub enum CompilerError<T> {
 }
 
 /// A generic error to represent an API request that cannot be fulfilled.
-#[derive(Debug)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Error(pub(crate) String);
 
 impl Error {

compiler/stable_mir/src/mir/visit.rs

@@ -452,7 +452,7 @@ impl Location {
 }
 
 /// Information about a place's usage.
-#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct PlaceContext {
     /// Whether the access is mutable or not. Keep this private so we can increment the type in a
     /// backward compatible manner.

compiler/stable_mir/src/ty.rs

@@ -382,7 +382,9 @@ impl AdtDef {
     }
 
     /// Retrieve the type of this Adt instantiating the type with the given arguments.
-    pub fn ty_with_args(&self, args: &GenericArgs) -> Result<Ty, Error> {
+    ///
+    /// This will assume the type can be instantiated with these arguments.
+    pub fn ty_with_args(&self, args: &GenericArgs) -> Ty {
         with(|cx| cx.def_ty_with_args(self.0, args))
     }
 
@@ -441,6 +443,7 @@ impl VariantDef {
     }
 }
 
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub struct FieldDef {
     /// The field definition.
     ///
@@ -454,7 +457,9 @@ pub struct FieldDef {
 
 impl FieldDef {
     /// Retrieve the type of this field instantiating the type with the given arguments.
-    pub fn ty_with_args(&self, args: &GenericArgs) -> Result<Ty, Error> {
+    ///
+    /// This will assume the type can be instantiated with these arguments.
+    pub fn ty_with_args(&self, args: &GenericArgs) -> Ty {
         with(|cx| cx.def_ty_with_args(self.def, args))
     }
 

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

@@ -0,0 +1,115 @@
+// run-pass
+//! Test that users are able to use stable mir APIs to retrieve monomorphized types, and that
+//! we have an error handling for trying to instantiate types with incorrect arguments.
+
+// 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)]
+#![feature(assert_matches)]
+#![feature(control_flow_enum)]
+
+extern crate rustc_middle;
+#[macro_use]
+extern crate rustc_smir;
+extern crate rustc_driver;
+extern crate rustc_interface;
+extern crate stable_mir;
+
+use rustc_middle::ty::TyCtxt;
+use rustc_smir::rustc_internal;
+use stable_mir::ty::{RigidTy, TyKind, Ty, };
+use stable_mir::mir::{Body, MirVisitor, FieldIdx, Place, ProjectionElem, visit::{Location,
+                                                                            PlaceContext}};
+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(_tcx: TyCtxt<'_>) -> ControlFlow<()> {
+    let main_fn = stable_mir::entry_fn();
+    let body = main_fn.unwrap().body();
+    let mut visitor = PlaceVisitor{ body: &body, tested: false};
+    visitor.visit_body(&body);
+    assert!(visitor.tested);
+    ControlFlow::Continue(())
+}
+
+struct PlaceVisitor<'a> {
+    body: &'a Body,
+    /// Used to ensure that the test was reachable. Otherwise this test would vacuously succeed.
+    tested: bool,
+}
+
+/// Check that `wrapper.inner` place projection can be correctly interpreted.
+/// Ensure that instantiation is correct.
+fn check_tys(local_ty: Ty, idx: FieldIdx, expected_ty: Ty) {
+    let TyKind::RigidTy(RigidTy::Adt(def, args)) = local_ty.kind() else { unreachable!() };
+    assert_eq!(def.ty_with_args(&args), local_ty);
+
+    let field_def = &def.variants_iter().next().unwrap().fields()[idx];
+    let field_ty = field_def.ty_with_args(&args);
+    assert_eq!(field_ty, expected_ty);
+
+    // Check that the generic version is different than the instantiated one.
+    let field_ty_gen = field_def.ty();
+    assert_ne!(field_ty_gen, field_ty);
+}
+
+impl<'a> MirVisitor for PlaceVisitor<'a> {
+    fn visit_place(&mut self, place: &Place, _ptx: PlaceContext, _loc: Location) {
+        let start_ty = self.body.locals()[place.local].ty;
+        match place.projection.as_slice() {
+            [ProjectionElem::Field(idx, ty)] => {
+                check_tys(start_ty, *idx, *ty);
+                self.tested = true;
+            }
+            _ => {}
+        }
+    }
+}
+
+/// 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 = "ty_fold_input.rs";
+    generate_input(&path).unwrap();
+    let args = vec![
+        "rustc".to_string(),
+        "-Cpanic=abort".to_string(),
+        "--crate-name".to_string(),
+        CRATE_NAME.to_string(),
+        path.to_string(),
+    ];
+    run!(args, tcx, test_stable_mir(tcx)).unwrap();
+}
+
+fn generate_input(path: &str) -> std::io::Result<()> {
+    let mut file = std::fs::File::create(path)?;
+    write!(
+        file,
+        r#"
+        struct Wrapper<T: Default> {{
+            pub inner: T
+        }}
+
+        impl<T: Default> Wrapper<T> {{
+            pub fn new() -> Wrapper<T> {{
+                Wrapper {{ inner: T::default() }}
+            }}
+        }}
+
+        fn main() {{
+            let wrapper = Wrapper::<u8>::new();
+            let _inner = wrapper.inner;
+        }}
+    "#
+    )?;
+    Ok(())
+}