Auto merge of #91230 - eggyal:fallible-type-fold, r=jackh726

Make `TypeFolder::fold_*` return `Result`

Implements rust-lang/compiler-team#432.

Initially this is just a rebase of `@LeSeulArtichaut's` work in #85469 (abandoned; see https://github.com/rust-lang/rust/pull/85485#issuecomment-908781112).  At that time, it caused a regression in performance that required some further exploration... with this rebased PR bors can hopefully report some perf analysis from which we can investigate further (if the regression is indeed still present).

r? `@jackh726` cc `@nikomatsakis`

8 files changed, 220 insertions, 216 deletions

diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index 934ada9932e..27e73738b7f 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -278,7 +278,7 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
         self.tcx
     }
 
-    fn fold_binder<T>(&mut self, t: ty::Binder<'tcx, T>) -> ty::Binder<'tcx, T>
+    fn fold_binder<T>(&mut self, t: ty::Binder<'tcx, T>) -> Result<ty::Binder<'tcx, T>, Self::Error>
     where
         T: TypeFoldable<'tcx>,
     {
@@ -288,13 +288,13 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
         t
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
         match *r {
             ty::ReLateBound(index, ..) => {
                 if index >= self.binder_index {
                     bug!("escaping late-bound region during canonicalization");
                 } else {
-                    r
+                    Ok(r)
                 }
             }
 
@@ -311,7 +311,7 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                     vid, r
                 );
                 let r = self.tcx.reuse_or_mk_region(r, ty::ReVar(resolved_vid));
-                self.canonicalize_region_mode.canonicalize_free_region(self, r)
+                Ok(self.canonicalize_region_mode.canonicalize_free_region(self, r))
             }
 
             ty::ReStatic
@@ -319,11 +319,11 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
             | ty::ReFree(_)
             | ty::ReEmpty(_)
             | ty::RePlaceholder(..)
-            | ty::ReErased => self.canonicalize_region_mode.canonicalize_free_region(self, r),
+            | ty::ReErased => Ok(self.canonicalize_region_mode.canonicalize_free_region(self, r)),
         }
     }
 
-    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         match *t.kind() {
             ty::Infer(ty::TyVar(vid)) => {
                 debug!("canonical: type var found with vid {:?}", vid);
@@ -339,40 +339,40 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                     Err(mut ui) => {
                         // FIXME: perf problem described in #55921.
                         ui = ty::UniverseIndex::ROOT;
-                        self.canonicalize_ty_var(
+                        Ok(self.canonicalize_ty_var(
                             CanonicalVarInfo {
                                 kind: CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui)),
                             },
                             t,
-                        )
+                        ))
                     }
                 }
             }
 
-            ty::Infer(ty::IntVar(_)) => self.canonicalize_ty_var(
+            ty::Infer(ty::IntVar(_)) => Ok(self.canonicalize_ty_var(
                 CanonicalVarInfo { kind: CanonicalVarKind::Ty(CanonicalTyVarKind::Int) },
                 t,
-            ),
+            )),
 
-            ty::Infer(ty::FloatVar(_)) => self.canonicalize_ty_var(
+            ty::Infer(ty::FloatVar(_)) => Ok(self.canonicalize_ty_var(
                 CanonicalVarInfo { kind: CanonicalVarKind::Ty(CanonicalTyVarKind::Float) },
                 t,
-            ),
+            )),
 
             ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
                 bug!("encountered a fresh type during canonicalization")
             }
 
-            ty::Placeholder(placeholder) => self.canonicalize_ty_var(
+            ty::Placeholder(placeholder) => Ok(self.canonicalize_ty_var(
                 CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderTy(placeholder) },
                 t,
-            ),
+            )),
 
             ty::Bound(debruijn, _) => {
                 if debruijn >= self.binder_index {
                     bug!("escaping bound type during canonicalization")
                 } else {
-                    t
+                    Ok(t)
                 }
             }
 
@@ -403,13 +403,16 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                 if t.flags().intersects(self.needs_canonical_flags) {
                     t.super_fold_with(self)
                 } else {
-                    t
+                    Ok(t)
                 }
             }
         }
     }
 
-    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+    fn fold_const(
+        &mut self,
+        ct: &'tcx ty::Const<'tcx>,
+    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
         match ct.val {
             ty::ConstKind::Infer(InferConst::Var(vid)) => {
                 debug!("canonical: const var found with vid {:?}", vid);
@@ -424,10 +427,10 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                     Err(mut ui) => {
                         // FIXME: perf problem described in #55921.
                         ui = ty::UniverseIndex::ROOT;
-                        return self.canonicalize_const_var(
+                        return Ok(self.canonicalize_const_var(
                             CanonicalVarInfo { kind: CanonicalVarKind::Const(ui) },
                             ct,
-                        );
+                        ));
                     }
                 }
             }
@@ -438,20 +441,20 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                 if debruijn >= self.binder_index {
                     bug!("escaping bound type during canonicalization")
                 } else {
-                    return ct;
+                    return Ok(ct);
                 }
             }
             ty::ConstKind::Placeholder(placeholder) => {
-                return self.canonicalize_const_var(
+                return Ok(self.canonicalize_const_var(
                     CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderConst(placeholder) },
                     ct,
-                );
+                ));
             }
             _ => {}
         }
 
         let flags = FlagComputation::for_const(ct);
-        if flags.intersects(self.needs_canonical_flags) { ct.super_fold_with(self) } else { ct }
+        if flags.intersects(self.needs_canonical_flags) { ct.super_fold_with(self) } else { Ok(ct) }
     }
 }
 
@@ -500,7 +503,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
             indices: FxHashMap::default(),
             binder_index: ty::INNERMOST,
         };
-        let out_value = value.fold_with(&mut canonicalizer);
+        let out_value = value.fold_with(&mut canonicalizer).into_ok();
 
         // Once we have canonicalized `out_value`, it should not
         // contain anything that ties it to this inference context
@@ -618,7 +621,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         let infcx = self.infcx;
         let bound_to = infcx.shallow_resolve(ty_var);
         if bound_to != ty_var {
-            self.fold_ty(bound_to)
+            self.fold_ty(bound_to).into_ok()
         } else {
             let var = self.canonical_var(info, ty_var.into());
             self.tcx().mk_ty(ty::Bound(self.binder_index, var.into()))
@@ -637,12 +640,12 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         let infcx = self.infcx;
         let bound_to = infcx.shallow_resolve(const_var);
         if bound_to != const_var {
-            self.fold_const(bound_to)
+            self.fold_const(bound_to).into_ok()
         } else {
             let var = self.canonical_var(info, const_var.into());
             self.tcx().mk_const(ty::Const {
                 val: ty::ConstKind::Bound(self.binder_index, var),
-                ty: self.fold_ty(const_var.ty),
+                ty: self.fold_ty(const_var.ty).into_ok(),
             })
         }
     }
diff --git a/compiler/rustc_infer/src/infer/freshen.rs b/compiler/rustc_infer/src/infer/freshen.rs
index c40e409891b..7599e98167f 100644
--- a/compiler/rustc_infer/src/infer/freshen.rs
+++ b/compiler/rustc_infer/src/infer/freshen.rs
@@ -72,7 +72,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
         F: FnOnce(u32) -> ty::InferTy,
     {
         if let Some(ty) = opt_ty {
-            return ty.fold_with(self);
+            return ty.fold_with(self).into_ok();
         }
 
         match self.ty_freshen_map.entry(key) {
@@ -98,7 +98,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
         F: FnOnce(u32) -> ty::InferConst<'tcx>,
     {
         if let Some(ct) = opt_ct {
-            return ct.fold_with(self);
+            return ct.fold_with(self).into_ok();
         }
 
         match self.const_freshen_map.entry(key) {
@@ -119,11 +119,11 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
         match *r {
             ty::ReLateBound(..) => {
                 // leave bound regions alone
-                r
+                Ok(r)
             }
 
             ty::ReEarlyBound(..)
@@ -133,21 +133,21 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
             | ty::ReEmpty(_)
             | ty::ReErased => {
                 // replace all free regions with 'erased
-                self.tcx().lifetimes.re_erased
+                Ok(self.tcx().lifetimes.re_erased)
             }
             ty::ReStatic => {
                 if self.keep_static {
-                    r
+                    Ok(r)
                 } else {
-                    self.tcx().lifetimes.re_erased
+                    Ok(self.tcx().lifetimes.re_erased)
                 }
             }
         }
     }
 
-    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         if !t.needs_infer() && !t.has_erasable_regions(self.tcx()) {
-            return t;
+            return Ok(t);
         }
 
         let tcx = self.infcx.tcx;
@@ -155,10 +155,10 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
         match *t.kind() {
             ty::Infer(ty::TyVar(v)) => {
                 let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
-                self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy)
+                Ok(self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy))
             }
 
-            ty::Infer(ty::IntVar(v)) => self.freshen_ty(
+            ty::Infer(ty::IntVar(v)) => Ok(self.freshen_ty(
                 self.infcx
                     .inner
                     .borrow_mut()
@@ -167,9 +167,9 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                     .map(|v| v.to_type(tcx)),
                 ty::IntVar(v),
                 ty::FreshIntTy,
-            ),
+            )),
 
-            ty::Infer(ty::FloatVar(v)) => self.freshen_ty(
+            ty::Infer(ty::FloatVar(v)) => Ok(self.freshen_ty(
                 self.infcx
                     .inner
                     .borrow_mut()
@@ -178,7 +178,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                     .map(|v| v.to_type(tcx)),
                 ty::FloatVar(v),
                 ty::FreshFloatTy,
-            ),
+            )),
 
             ty::Infer(ty::FreshTy(ct) | ty::FreshIntTy(ct) | ty::FreshFloatTy(ct)) => {
                 if ct >= self.ty_freshen_count {
@@ -189,7 +189,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                         self.ty_freshen_count
                     );
                 }
-                t
+                Ok(t)
             }
 
             ty::Generator(..)
@@ -221,7 +221,10 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
         }
     }
 
-    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+    fn fold_const(
+        &mut self,
+        ct: &'tcx ty::Const<'tcx>,
+    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
         match ct.val {
             ty::ConstKind::Infer(ty::InferConst::Var(v)) => {
                 let opt_ct = self
@@ -232,12 +235,12 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                     .probe_value(v)
                     .val
                     .known();
-                return self.freshen_const(
+                return Ok(self.freshen_const(
                     opt_ct,
                     ty::InferConst::Var(v),
                     ty::InferConst::Fresh,
                     ct.ty,
-                );
+                ));
             }
             ty::ConstKind::Infer(ty::InferConst::Fresh(i)) => {
                 if i >= self.const_freshen_count {
@@ -248,7 +251,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                         self.const_freshen_count,
                     );
                 }
-                return ct;
+                return Ok(ct);
             }
 
             ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
diff --git a/compiler/rustc_infer/src/infer/fudge.rs b/compiler/rustc_infer/src/infer/fudge.rs
index 773753a0363..4e6f1315d10 100644
--- a/compiler/rustc_infer/src/infer/fudge.rs
+++ b/compiler/rustc_infer/src/infer/fudge.rs
@@ -161,7 +161,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         {
             Ok(value)
         } else {
-            Ok(value.fold_with(&mut fudger))
+            Ok(value.fold_with(&mut fudger).into_ok())
         }
     }
 }
@@ -180,7 +180,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for InferenceFudger<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         match *ty.kind() {
             ty::Infer(ty::InferTy::TyVar(vid)) => {
                 if self.type_vars.0.contains(&vid) {
@@ -188,7 +188,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for InferenceFudger<'a, 'tcx> {
                     // Recreate it with a fresh variable here.
                     let idx = (vid.as_usize() - self.type_vars.0.start.as_usize()) as usize;
                     let origin = self.type_vars.1[idx];
-                    self.infcx.next_ty_var(origin)
+                    Ok(self.infcx.next_ty_var(origin))
                 } else {
                     // This variable was created before the
                     // "fudging". Since we refresh all type
@@ -198,48 +198,43 @@ impl<'a, 'tcx> TypeFolder<'tcx> for InferenceFudger<'a, 'tcx> {
                     debug_assert!(
                         self.infcx.inner.borrow_mut().type_variables().probe(vid).is_unknown()
                     );
-                    ty
+                    Ok(ty)
                 }
             }
             ty::Infer(ty::InferTy::IntVar(vid)) => {
-                if self.int_vars.contains(&vid) {
-                    self.infcx.next_int_var()
-                } else {
-                    ty
-                }
+                Ok(if self.int_vars.contains(&vid) { self.infcx.next_int_var() } else { ty })
             }
             ty::Infer(ty::InferTy::FloatVar(vid)) => {
-                if self.float_vars.contains(&vid) {
-                    self.infcx.next_float_var()
-                } else {
-                    ty
-                }
+                Ok(if self.float_vars.contains(&vid) { self.infcx.next_float_var() } else { ty })
             }
             _ => ty.super_fold_with(self),
         }
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
         if let ty::ReVar(vid) = *r {
             if self.region_vars.0.contains(&vid) {
                 let idx = vid.index() - self.region_vars.0.start.index();
                 let origin = self.region_vars.1[idx];
-                return self.infcx.next_region_var(origin);
+                return Ok(self.infcx.next_region_var(origin));
             }
         }
-        r
+        Ok(r)
     }
 
-    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+    fn fold_const(
+        &mut self,
+        ct: &'tcx ty::Const<'tcx>,
+    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
         if let ty::Const { val: ty::ConstKind::Infer(ty::InferConst::Var(vid)), ty } = ct {
             if self.const_vars.0.contains(&vid) {
                 // This variable was created during the fudging.
                 // Recreate it with a fresh variable here.
                 let idx = (vid.index - self.const_vars.0.start.index) as usize;
                 let origin = self.const_vars.1[idx];
-                self.infcx.next_const_var(ty, origin)
+                Ok(self.infcx.next_const_var(ty, origin))
             } else {
-                ct
+                Ok(ct)
             }
         } else {
             ct.super_fold_with(self)
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 2fd01c2d595..4a9a63e1c76 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -681,7 +681,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     pub fn freshen<T: TypeFoldable<'tcx>>(&self, t: T) -> T {
-        t.fold_with(&mut self.freshener())
+        t.fold_with(&mut self.freshener()).into_ok()
     }
 
     /// Returns the origin of the type variable identified by `vid`, or `None`
@@ -1381,7 +1381,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        value.fold_with(&mut ShallowResolver { infcx: self })
+        value.fold_with(&mut ShallowResolver { infcx: self }).into_ok()
     }
 
     pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
@@ -1402,7 +1402,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             return value; // Avoid duplicated subst-folding.
         }
         let mut r = resolve::OpportunisticVarResolver::new(self);
-        value.fold_with(&mut r)
+        value.fold_with(&mut r).into_ok()
     }
 
     /// Returns the first unresolved variable contained in `T`. In the
@@ -1745,12 +1745,15 @@ impl<'a, 'tcx> TypeFolder<'tcx> for ShallowResolver<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        self.infcx.shallow_resolve_ty(ty)
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
+        Ok(self.infcx.shallow_resolve_ty(ty))
     }
 
-    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
-        if let ty::Const { val: ty::ConstKind::Infer(InferConst::Var(vid)), .. } = ct {
+    fn fold_const(
+        &mut self,
+        ct: &'tcx ty::Const<'tcx>,
+    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
+        Ok(if let ty::Const { val: ty::ConstKind::Infer(InferConst::Var(vid)), .. } = ct {
             self.infcx
                 .inner
                 .borrow_mut()
@@ -1761,7 +1764,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for ShallowResolver<'a, 'tcx> {
                 .unwrap_or(ct)
         } else {
             ct
-        }
+        })
     }
 }
 
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index e2e07f2072e..932f26d5550 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -418,92 +418,94 @@ struct Instantiator<'a, 'tcx> {
 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
     fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
         let tcx = self.infcx.tcx;
-        value.fold_with(&mut BottomUpFolder {
-            tcx,
-            ty_op: |ty| {
-                if ty.references_error() {
-                    return tcx.ty_error();
-                } else if let ty::Opaque(def_id, substs) = ty.kind() {
-                    // Check that this is `impl Trait` type is
-                    // declared by `parent_def_id` -- i.e., one whose
-                    // value we are inferring.  At present, this is
-                    // always true during the first phase of
-                    // type-check, but not always true later on during
-                    // NLL. Once we support named opaque types more fully,
-                    // this same scenario will be able to arise during all phases.
-                    //
-                    // Here is an example using type alias `impl Trait`
-                    // that indicates the distinction we are checking for:
-                    //
-                    // ```rust
-                    // mod a {
-                    //   pub type Foo = impl Iterator;
-                    //   pub fn make_foo() -> Foo { .. }
-                    // }
-                    //
-                    // mod b {
-                    //   fn foo() -> a::Foo { a::make_foo() }
-                    // }
-                    // ```
-                    //
-                    // Here, the return type of `foo` references an
-                    // `Opaque` indeed, but not one whose value is
-                    // presently being inferred. You can get into a
-                    // similar situation with closure return types
-                    // today:
-                    //
-                    // ```rust
-                    // fn foo() -> impl Iterator { .. }
-                    // fn bar() {
-                    //     let x = || foo(); // returns the Opaque assoc with `foo`
-                    // }
-                    // ```
-                    if let Some(def_id) = def_id.as_local() {
-                        let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
-                        let parent_def_id = self.infcx.defining_use_anchor;
-                        let def_scope_default = || {
-                            let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
-                            parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
-                        };
-                        let (in_definition_scope, origin) =
-                            match tcx.hir().expect_item(opaque_hir_id).kind {
-                                // Anonymous `impl Trait`
-                                hir::ItemKind::OpaqueTy(hir::OpaqueTy {
-                                    impl_trait_fn: Some(parent),
-                                    origin,
-                                    ..
-                                }) => (parent == parent_def_id.to_def_id(), origin),
-                                // Named `type Foo = impl Bar;`
-                                hir::ItemKind::OpaqueTy(hir::OpaqueTy {
-                                    impl_trait_fn: None,
-                                    origin,
-                                    ..
-                                }) => (
-                                    may_define_opaque_type(tcx, parent_def_id, opaque_hir_id),
-                                    origin,
-                                ),
-                                _ => (def_scope_default(), hir::OpaqueTyOrigin::TyAlias),
+        value
+            .fold_with(&mut BottomUpFolder {
+                tcx,
+                ty_op: |ty| {
+                    if ty.references_error() {
+                        return tcx.ty_error();
+                    } else if let ty::Opaque(def_id, substs) = ty.kind() {
+                        // Check that this is `impl Trait` type is
+                        // declared by `parent_def_id` -- i.e., one whose
+                        // value we are inferring.  At present, this is
+                        // always true during the first phase of
+                        // type-check, but not always true later on during
+                        // NLL. Once we support named opaque types more fully,
+                        // this same scenario will be able to arise during all phases.
+                        //
+                        // Here is an example using type alias `impl Trait`
+                        // that indicates the distinction we are checking for:
+                        //
+                        // ```rust
+                        // mod a {
+                        //   pub type Foo = impl Iterator;
+                        //   pub fn make_foo() -> Foo { .. }
+                        // }
+                        //
+                        // mod b {
+                        //   fn foo() -> a::Foo { a::make_foo() }
+                        // }
+                        // ```
+                        //
+                        // Here, the return type of `foo` references an
+                        // `Opaque` indeed, but not one whose value is
+                        // presently being inferred. You can get into a
+                        // similar situation with closure return types
+                        // today:
+                        //
+                        // ```rust
+                        // fn foo() -> impl Iterator { .. }
+                        // fn bar() {
+                        //     let x = || foo(); // returns the Opaque assoc with `foo`
+                        // }
+                        // ```
+                        if let Some(def_id) = def_id.as_local() {
+                            let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+                            let parent_def_id = self.infcx.defining_use_anchor;
+                            let def_scope_default = || {
+                                let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
+                                parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
                             };
-                        if in_definition_scope {
-                            let opaque_type_key =
-                                OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
-                            return self.fold_opaque_ty(ty, opaque_type_key, origin);
-                        }
-
-                        debug!(
-                            "instantiate_opaque_types_in_map: \
+                            let (in_definition_scope, origin) =
+                                match tcx.hir().expect_item(opaque_hir_id).kind {
+                                    // Anonymous `impl Trait`
+                                    hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+                                        impl_trait_fn: Some(parent),
+                                        origin,
+                                        ..
+                                    }) => (parent == parent_def_id.to_def_id(), origin),
+                                    // Named `type Foo = impl Bar;`
+                                    hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+                                        impl_trait_fn: None,
+                                        origin,
+                                        ..
+                                    }) => (
+                                        may_define_opaque_type(tcx, parent_def_id, opaque_hir_id),
+                                        origin,
+                                    ),
+                                    _ => (def_scope_default(), hir::OpaqueTyOrigin::TyAlias),
+                                };
+                            if in_definition_scope {
+                                let opaque_type_key =
+                                    OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
+                                return self.fold_opaque_ty(ty, opaque_type_key, origin);
+                            }
+
+                            debug!(
+                                "instantiate_opaque_types_in_map: \
                              encountered opaque outside its definition scope \
                              def_id={:?}",
-                            def_id,
-                        );
+                                def_id,
+                            );
+                        }
                     }
-                }
 
-                ty
-            },
-            lt_op: |lt| lt,
-            ct_op: |ct| ct,
-        })
+                    ty
+                },
+                lt_op: |lt| lt,
+                ct_op: |ct| ct,
+            })
+            .into_ok()
     }
 
     #[instrument(skip(self), level = "debug")]
@@ -556,21 +558,23 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
             debug!(?predicate);
 
             // We can't normalize associated types from `rustc_infer`, but we can eagerly register inference variables for them.
-            let predicate = predicate.fold_with(&mut BottomUpFolder {
-                tcx,
-                ty_op: |ty| match ty.kind() {
-                    ty::Projection(projection_ty) => infcx.infer_projection(
-                        self.param_env,
-                        *projection_ty,
-                        traits::ObligationCause::misc(self.value_span, self.body_id),
-                        0,
-                        &mut self.obligations,
-                    ),
-                    _ => ty,
-                },
-                lt_op: |lt| lt,
-                ct_op: |ct| ct,
-            });
+            let predicate = predicate
+                .fold_with(&mut BottomUpFolder {
+                    tcx,
+                    ty_op: |ty| match ty.kind() {
+                        ty::Projection(projection_ty) => infcx.infer_projection(
+                            self.param_env,
+                            *projection_ty,
+                            traits::ObligationCause::misc(self.value_span, self.body_id),
+                            0,
+                            &mut self.obligations,
+                        ),
+                        _ => ty,
+                    },
+                    lt_op: |lt| lt,
+                    ct_op: |ct| ct,
+                })
+                .into_ok();
             debug!(?predicate);
 
             if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
diff --git a/compiler/rustc_infer/src/infer/resolve.rs b/compiler/rustc_infer/src/infer/resolve.rs
index 4b08c2eb9c1..cccdfb7452a 100644
--- a/compiler/rustc_infer/src/infer/resolve.rs
+++ b/compiler/rustc_infer/src/infer/resolve.rs
@@ -30,25 +30,28 @@ impl<'a, 'tcx> TypeFolder<'tcx> for OpportunisticVarResolver<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         if !t.has_infer_types_or_consts() {
-            t // micro-optimize -- if there is nothing in this type that this fold affects...
+            Ok(t) // micro-optimize -- if there is nothing in this type that this fold affects...
         } else {
             let t = self.infcx.shallow_resolve(t);
             t.super_fold_with(self)
         }
     }
 
-    fn fold_const(&mut self, ct: &'tcx Const<'tcx>) -> &'tcx Const<'tcx> {
+    fn fold_const(&mut self, ct: &'tcx Const<'tcx>) -> Result<&'tcx Const<'tcx>, Self::Error> {
         if !ct.has_infer_types_or_consts() {
-            ct // micro-optimize -- if there is nothing in this const that this fold affects...
+            Ok(ct) // micro-optimize -- if there is nothing in this const that this fold affects...
         } else {
             let ct = self.infcx.shallow_resolve(ct);
             ct.super_fold_with(self)
         }
     }
 
-    fn fold_mir_const(&mut self, constant: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+    fn fold_mir_const(
+        &mut self,
+        constant: mir::ConstantKind<'tcx>,
+    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
         constant.super_fold_with(self)
     }
 }
@@ -75,16 +78,16 @@ impl<'a, 'tcx> TypeFolder<'tcx> for OpportunisticRegionResolver<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         if !t.has_infer_regions() {
-            t // micro-optimize -- if there is nothing in this type that this fold affects...
+            Ok(t) // micro-optimize -- if there is nothing in this type that this fold affects...
         } else {
             t.super_fold_with(self)
         }
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        match *r {
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
+        Ok(match *r {
             ty::ReVar(rid) => {
                 let resolved = self
                     .infcx
@@ -95,12 +98,15 @@ impl<'a, 'tcx> TypeFolder<'tcx> for OpportunisticRegionResolver<'a, 'tcx> {
                 self.tcx().reuse_or_mk_region(r, ty::ReVar(resolved))
             }
             _ => r,
-        }
+        })
     }
 
-    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+    fn fold_const(
+        &mut self,
+        ct: &'tcx ty::Const<'tcx>,
+    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
         if !ct.has_infer_regions() {
-            ct // micro-optimize -- if there is nothing in this const that this fold affects...
+            Ok(ct) // micro-optimize -- if there is nothing in this const that this fold affects...
         } else {
             ct.super_fold_with(self)
         }
@@ -175,44 +181,31 @@ pub fn fully_resolve<'a, 'tcx, T>(infcx: &InferCtxt<'a, 'tcx>, value: T) -> Fixu
 where
     T: TypeFoldable<'tcx>,
 {
-    let mut full_resolver = FullTypeResolver { infcx, err: None };
-    let result = value.fold_with(&mut full_resolver);
-    match full_resolver.err {
-        None => Ok(result),
-        Some(e) => Err(e),
-    }
+    value.fold_with(&mut FullTypeResolver { infcx })
 }
 
 // N.B. This type is not public because the protocol around checking the
 // `err` field is not enforceable otherwise.
 struct FullTypeResolver<'a, 'tcx> {
     infcx: &'a InferCtxt<'a, 'tcx>,
-    err: Option<FixupError<'tcx>>,
 }
 
 impl<'a, 'tcx> TypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
+    type Error = FixupError<'tcx>;
+
     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
         self.infcx.tcx
     }
 
-    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         if !t.needs_infer() {
-            t // micro-optimize -- if there is nothing in this type that this fold affects...
+            Ok(t) // micro-optimize -- if there is nothing in this type that this fold affects...
         } else {
             let t = self.infcx.shallow_resolve(t);
             match *t.kind() {
-                ty::Infer(ty::TyVar(vid)) => {
-                    self.err = Some(FixupError::UnresolvedTy(vid));
-                    self.tcx().ty_error()
-                }
-                ty::Infer(ty::IntVar(vid)) => {
-                    self.err = Some(FixupError::UnresolvedIntTy(vid));
-                    self.tcx().ty_error()
-                }
-                ty::Infer(ty::FloatVar(vid)) => {
-                    self.err = Some(FixupError::UnresolvedFloatTy(vid));
-                    self.tcx().ty_error()
-                }
+                ty::Infer(ty::TyVar(vid)) => Err(FixupError::UnresolvedTy(vid)),
+                ty::Infer(ty::IntVar(vid)) => Err(FixupError::UnresolvedIntTy(vid)),
+                ty::Infer(ty::FloatVar(vid)) => Err(FixupError::UnresolvedFloatTy(vid)),
                 ty::Infer(_) => {
                     bug!("Unexpected type in full type resolver: {:?}", t);
                 }
@@ -221,28 +214,30 @@ impl<'a, 'tcx> TypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
         }
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
         match *r {
-            ty::ReVar(rid) => self
+            ty::ReVar(rid) => Ok(self
                 .infcx
                 .lexical_region_resolutions
                 .borrow()
                 .as_ref()
                 .expect("region resolution not performed")
-                .resolve_var(rid),
-            _ => r,
+                .resolve_var(rid)),
+            _ => Ok(r),
         }
     }
 
-    fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+    fn fold_const(
+        &mut self,
+        c: &'tcx ty::Const<'tcx>,
+    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
         if !c.needs_infer() {
-            c // micro-optimize -- if there is nothing in this const that this fold affects...
+            Ok(c) // micro-optimize -- if there is nothing in this const that this fold affects...
         } else {
             let c = self.infcx.shallow_resolve(c);
             match c.val {
                 ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                    self.err = Some(FixupError::UnresolvedConst(vid));
-                    return self.tcx().const_error(c.ty);
+                    return Err(FixupError::UnresolvedConst(vid));
                 }
                 ty::ConstKind::Infer(InferConst::Fresh(_)) => {
                     bug!("Unexpected const in full const resolver: {:?}", c);
diff --git a/compiler/rustc_infer/src/lib.rs b/compiler/rustc_infer/src/lib.rs
index e4b407e7c11..5153427954c 100644
--- a/compiler/rustc_infer/src/lib.rs
+++ b/compiler/rustc_infer/src/lib.rs
@@ -24,6 +24,7 @@
 #![feature(control_flow_enum)]
 #![feature(min_specialization)]
 #![feature(label_break_value)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]
diff --git a/compiler/rustc_infer/src/traits/structural_impls.rs b/compiler/rustc_infer/src/traits/structural_impls.rs
index b48ca3bd0c0..544b8939779 100644
--- a/compiler/rustc_infer/src/traits/structural_impls.rs
+++ b/compiler/rustc_infer/src/traits/structural_impls.rs
@@ -60,13 +60,13 @@ impl<'tcx> fmt::Debug for traits::MismatchedProjectionTypes<'tcx> {
 // TypeFoldable implementations.
 
 impl<'tcx, O: TypeFoldable<'tcx>> TypeFoldable<'tcx> for traits::Obligation<'tcx, O> {
-    fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
-        traits::Obligation {
+    fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(traits::Obligation {
             cause: self.cause,
             recursion_depth: self.recursion_depth,
-            predicate: self.predicate.fold_with(folder),
-            param_env: self.param_env.fold_with(folder),
-        }
+            predicate: self.predicate.fold_with(folder)?,
+            param_env: self.param_env.fold_with(folder)?,
+        })
     }
 
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {