Auto merge of #709 - LeSeulArtichaut:fallible-folder, r=jackh726

Introduce `Folder::Error`

Equivalent to rust-lang/rust#85469, cc rust-lang/compiler-team#432 rust-lang/types-team#16.

This compiles, but there are implementations of `Folder` that still use `Fallible<T>` instead of `Result<T, Self::Error>`. I can make that change if it is deemed beneficial.

r? `@jackh726`

chalk-derive/src/lib.rs

@@ -299,11 +299,11 @@ fn derive_fold(mut s: synstructure::Structure) -> TokenStream {
         quote! {
             type Result = #result;
 
-            fn fold_with<'i>(
+            fn fold_with<'i, E>(
                 self,
-                folder: &mut dyn ::chalk_ir::fold::Folder < 'i, #interner >,
+                folder: &mut dyn ::chalk_ir::fold::Folder < 'i, #interner, Error = E >,
                 outer_binder: ::chalk_ir::DebruijnIndex,
-            ) -> ::chalk_ir::Fallible<Self::Result>
+            ) -> ::std::result::Result<Self::Result, E>
             where
                 #interner: 'i,
             {

chalk-engine/src/normalize_deep.rs

@@ -39,7 +39,9 @@ impl<'i, I: Interner> Folder<'i, I> for DeepNormalizer<'_, 'i, I>
 where
     I: 'i,
 {
-    fn as_dyn(&mut self) -> &mut dyn Folder<'i, I> {
+    type Error = NoSolution;
+
+    fn as_dyn(&mut self) -> &mut dyn Folder<'i, I, Error = Self::Error> {
         self
     }
 

chalk-engine/src/strand.rs

@@ -5,7 +5,7 @@ use std::fmt::Debug;
 use chalk_derive::HasInterner;
 use chalk_ir::fold::{Fold, Folder};
 use chalk_ir::interner::Interner;
-use chalk_ir::{Canonical, DebruijnIndex, Fallible, UniverseMap};
+use chalk_ir::{Canonical, DebruijnIndex, UniverseMap};
 
 #[derive(Clone, Debug, HasInterner)]
 pub(crate) struct Strand<I: Interner> {
@@ -37,11 +37,11 @@ pub(crate) struct SelectedSubgoal {
 
 impl<I: Interner> Fold<I> for Strand<I> {
     type Result = Strand<I>;
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {

chalk-ir/src/fold.rs

@@ -58,20 +58,24 @@ pub trait Folder<'i, I: Interner>
 where
     I: 'i,
 {
+    /// The type this folder returns when folding fails. This is
+    /// commonly [`NoSolution`].
+    type Error;
+
     /// Creates a `dyn` value from this folder. Unfortunately, this
     /// must be added manually to each impl of Folder; it permits the
     /// default implements below to create a `&mut dyn Folder` from
     /// `Self` without knowing what `Self` is (by invoking this
     /// method). Effectively, this limits impls of `Folder` to types
     /// for which we are able to create a dyn value (i.e., not `[T]`
     /// types).
-    fn as_dyn(&mut self) -> &mut dyn Folder<'i, I>;
+    fn as_dyn(&mut self) -> &mut dyn Folder<'i, I, Error = Self::Error>;
 
     /// Top-level callback: invoked for each `Ty<I>` that is
     /// encountered when folding. By default, invokes
     /// `super_fold_with`, which will in turn invoke the more
     /// specialized folding methods below, like `fold_free_var_ty`.
-    fn fold_ty(&mut self, ty: Ty<I>, outer_binder: DebruijnIndex) -> Fallible<Ty<I>> {
+    fn fold_ty(&mut self, ty: Ty<I>, outer_binder: DebruijnIndex) -> Result<Ty<I>, Self::Error> {
         ty.super_fold_with(self.as_dyn(), outer_binder)
     }
 
@@ -83,7 +87,7 @@ where
         &mut self,
         lifetime: Lifetime<I>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Lifetime<I>> {
+    ) -> Result<Lifetime<I>, Self::Error> {
         lifetime.super_fold_with(self.as_dyn(), outer_binder)
     }
 
@@ -95,7 +99,7 @@ where
         &mut self,
         constant: Const<I>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Const<I>> {
+    ) -> Result<Const<I>, Self::Error> {
         constant.super_fold_with(self.as_dyn(), outer_binder)
     }
 
@@ -104,12 +108,16 @@ where
         &mut self,
         clause: ProgramClause<I>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<ProgramClause<I>> {
+    ) -> Result<ProgramClause<I>, Self::Error> {
         clause.super_fold_with(self.as_dyn(), outer_binder)
     }
 
     /// Invoked for every goal. By default, recursively folds the goals contents.
-    fn fold_goal(&mut self, goal: Goal<I>, outer_binder: DebruijnIndex) -> Fallible<Goal<I>> {
+    fn fold_goal(
+        &mut self,
+        goal: Goal<I>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Goal<I>, Self::Error> {
         goal.super_fold_with(self.as_dyn(), outer_binder)
     }
 
@@ -133,7 +141,7 @@ where
         &mut self,
         bound_var: BoundVar,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Ty<I>> {
+    ) -> Result<Ty<I>, Self::Error> {
         if self.forbid_free_vars() {
             panic!(
                 "unexpected free variable with depth `{:?}` with outer binder {:?}",
@@ -150,7 +158,7 @@ where
         &mut self,
         bound_var: BoundVar,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Lifetime<I>> {
+    ) -> Result<Lifetime<I>, Self::Error> {
         if self.forbid_free_vars() {
             panic!(
                 "unexpected free variable with depth `{:?}` with outer binder {:?}",
@@ -168,7 +176,7 @@ where
         ty: Ty<I>,
         bound_var: BoundVar,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Const<I>> {
+    ) -> Result<Const<I>, Self::Error> {
         if self.forbid_free_vars() {
             panic!(
                 "unexpected free variable with depth `{:?}` with outer binder {:?}",
@@ -202,7 +210,7 @@ where
         &mut self,
         universe: PlaceholderIndex,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Ty<I>> {
+    ) -> Result<Ty<I>, Self::Error> {
         if self.forbid_free_placeholders() {
             panic!("unexpected placeholder type `{:?}`", universe)
         } else {
@@ -216,7 +224,7 @@ where
         &mut self,
         universe: PlaceholderIndex,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Lifetime<I>> {
+    ) -> Result<Lifetime<I>, Self::Error> {
         if self.forbid_free_placeholders() {
             panic!("unexpected placeholder lifetime `{:?}`", universe)
         } else {
@@ -231,7 +239,7 @@ where
         ty: Ty<I>,
         universe: PlaceholderIndex,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Const<I>> {
+    ) -> Result<Const<I>, Self::Error> {
         if self.forbid_free_placeholders() {
             panic!("unexpected placeholder const `{:?}`", universe)
         } else {
@@ -259,7 +267,7 @@ where
         var: InferenceVar,
         kind: TyVariableKind,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Ty<I>> {
+    ) -> Result<Ty<I>, Self::Error> {
         if self.forbid_inference_vars() {
             panic!("unexpected inference type `{:?}`", var)
         } else {
@@ -273,7 +281,7 @@ where
         &mut self,
         var: InferenceVar,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Lifetime<I>> {
+    ) -> Result<Lifetime<I>, Self::Error> {
         if self.forbid_inference_vars() {
             panic!("unexpected inference lifetime `'{:?}`", var)
         } else {
@@ -288,7 +296,7 @@ where
         ty: Ty<I>,
         var: InferenceVar,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Const<I>> {
+    ) -> Result<Const<I>, Self::Error> {
         if self.forbid_inference_vars() {
             panic!("unexpected inference const `{:?}`", var)
         } else {
@@ -318,11 +326,11 @@ pub trait Fold<I: Interner>: Debug {
     /// folder. Typically `binders` starts as 0, but is adjusted when
     /// we encounter `Binders<T>` in the IR or other similar
     /// constructs.
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i;
 }
@@ -332,11 +340,11 @@ pub trait Fold<I: Interner>: Debug {
 /// the contents of the type.
 pub trait SuperFold<I: Interner>: Fold<I> {
     /// Recursively folds the value.
-    fn super_fold_with<'i>(
+    fn super_fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i;
 }
@@ -347,11 +355,11 @@ pub trait SuperFold<I: Interner>: Fold<I> {
 impl<I: Interner> Fold<I> for Ty<I> {
     type Result = Ty<I>;
 
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {
@@ -364,11 +372,11 @@ impl<I> SuperFold<I> for Ty<I>
 where
     I: Interner,
 {
-    fn super_fold_with<'i>(
+    fn super_fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Ty<I>>
+    ) -> Result<Ty<I>, E>
     where
         I: 'i,
     {
@@ -477,11 +485,11 @@ where
 impl<I: Interner> Fold<I> for Lifetime<I> {
     type Result = Lifetime<I>;
 
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {
@@ -493,11 +501,11 @@ impl<I> SuperFold<I> for Lifetime<I>
 where
     I: Interner,
 {
-    fn super_fold_with<'i>(
+    fn super_fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Lifetime<I>>
+    ) -> Result<Lifetime<I>, E>
     where
         I: 'i,
     {
@@ -535,11 +543,11 @@ where
 impl<I: Interner> Fold<I> for Const<I> {
     type Result = Const<I>;
 
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {
@@ -551,11 +559,11 @@ impl<I> SuperFold<I> for Const<I>
 where
     I: Interner,
 {
-    fn super_fold_with<'i>(
+    fn super_fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Const<I>>
+    ) -> Result<Const<I>, E>
     where
         I: 'i,
     {
@@ -592,11 +600,11 @@ where
 impl<I: Interner> Fold<I> for Goal<I> {
     type Result = Goal<I>;
 
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {
@@ -606,11 +614,11 @@ impl<I: Interner> Fold<I> for Goal<I> {
 
 /// Superfold folds recursively.
 impl<I: Interner> SuperFold<I> for Goal<I> {
-    fn super_fold_with<'i>(
+    fn super_fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {
@@ -630,11 +638,11 @@ impl<I: Interner> SuperFold<I> for Goal<I> {
 impl<I: Interner> Fold<I> for ProgramClause<I> {
     type Result = ProgramClause<I>;
 
-    fn fold_with<'i>(
+    fn fold_with<'i, E>(
         self,
-        folder: &mut dyn Folder<'i, I>,
+        folder: &mut dyn Folder<'i, I, Error = E>,
         outer_binder: DebruijnIndex,
-    ) -> Fallible<Self::Result>
+    ) -> Result<Self::Result, E>
     where
         I: 'i,
     {