An ugly, awful hack that works

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2101,8 +2101,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
                         let ty_fn_ptr_from = tcx.mk_fn_ptr(fn_sig);
 
+                        let ty = self.normalize(*ty, location);
                         if let Err(terr) = self.eq_types(
-                            *ty,
+                            ty,
                             ty_fn_ptr_from,
                             location.to_locations(),
                             ConstraintCategory::Cast,
@@ -2124,9 +2125,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             _ => bug!(),
                         };
                         let ty_fn_ptr_from = tcx.mk_fn_ptr(tcx.signature_unclosure(sig, *unsafety));
+                        let ty_fn_ptr_from = self.normalize(ty_fn_ptr_from, location);
 
+                        let ty = self.normalize(*ty, location);
                         if let Err(terr) = self.eq_types(
-                            *ty,
+                            ty,
                             ty_fn_ptr_from,
                             location.to_locations(),
                             ConstraintCategory::Cast,
@@ -2154,8 +2157,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
                         let ty_fn_ptr_from = tcx.safe_to_unsafe_fn_ty(fn_sig);
 
+                        let ty = self.normalize(*ty, location);
                         if let Err(terr) = self.eq_types(
-                            *ty,
+                            ty,
                             ty_fn_ptr_from,
                             location.to_locations(),
                             ConstraintCategory::Cast,

compiler/rustc_infer/src/infer/mod.rs

@@ -920,6 +920,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             .region_constraints_added_in_snapshot(&snapshot.undo_snapshot)
     }
 
+    pub fn any_instantiations(&self, snapshot: &CombinedSnapshot<'a, 'tcx>) -> bool {
+        self.inner.borrow_mut().any_instantiations(&snapshot.undo_snapshot)
+    }
+
     pub fn add_given(&self, sub: ty::Region<'tcx>, sup: ty::RegionVid) {
         self.inner.borrow_mut().unwrap_region_constraints().add_given(sub, sup);
     }

compiler/rustc_infer/src/infer/undo_log.rs

@@ -29,6 +29,23 @@ pub(crate) enum UndoLog<'tcx> {
     PushRegionObligation,
 }
 
+impl<'tcx> std::fmt::Debug for UndoLog<'tcx> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::TypeVariables(_) => f.debug_tuple("TypeVariables").finish(),
+            Self::ConstUnificationTable(_) => f.debug_tuple("ConstUnificationTable").finish(),
+            Self::IntUnificationTable(_) => f.debug_tuple("IntUnificationTable").finish(),
+            Self::FloatUnificationTable(_) => f.debug_tuple("FloatUnificationTable").finish(),
+            Self::RegionConstraintCollector(_) => {
+                f.debug_tuple("RegionConstraintCollector").finish()
+            }
+            Self::RegionUnificationTable(_) => f.debug_tuple("RegionUnificationTable").finish(),
+            Self::ProjectionCache(_) => f.debug_tuple("ProjectionCache").finish(),
+            Self::PushRegionObligation => write!(f, "PushRegionObligation"),
+        }
+    }
+}
+
 macro_rules! impl_from {
     ($($ctor: ident ($ty: ty),)*) => {
         $(
@@ -165,6 +182,10 @@ impl<'tcx> InferCtxtInner<'tcx> {
 
         self.undo_log.num_open_snapshots -= 1;
     }
+
+    pub fn any_instantiations(&mut self, snapshot: &Snapshot<'tcx>) -> bool {
+        self.undo_log.instantiations_in_snapshot(snapshot).next().is_some()
+    }
 }
 
 impl<'tcx> InferCtxtUndoLogs<'tcx> {
@@ -173,6 +194,16 @@ impl<'tcx> InferCtxtUndoLogs<'tcx> {
         Snapshot { undo_len: self.logs.len(), _marker: PhantomData }
     }
 
+    pub(crate) fn instantiations_in_snapshot(
+        &self,
+        s: &Snapshot<'tcx>,
+    ) -> impl Iterator<Item = &'_ type_variable::UndoLog<'tcx>> + Clone {
+        self.logs[s.undo_len..].iter().filter_map(|log| match log {
+            UndoLog::TypeVariables(log) => Some(log),
+            _ => None,
+        })
+    }
+
     pub(crate) fn region_constraints_in_snapshot(
         &self,
         s: &Snapshot<'tcx>,

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -55,7 +55,10 @@ pub use self::object_safety::is_vtable_safe_method;
 pub use self::object_safety::MethodViolationCode;
 pub use self::object_safety::ObjectSafetyViolation;
 pub use self::on_unimplemented::{OnUnimplementedDirective, OnUnimplementedNote};
-pub use self::project::{normalize, normalize_projection_type, normalize_to};
+pub use self::project::{
+    normalize, normalize_projection_type, normalize_to, normalize_with_depth_to,
+    project_and_unify_type,
+};
 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
 pub use self::specialize::specialization_graph::FutureCompatOverlapError;

compiler/rustc_trait_selection/src/traits/project.rs

@@ -42,7 +42,7 @@ pub type ProjectionObligation<'tcx> = Obligation<'tcx, ty::ProjectionPredicate<'
 
 pub type ProjectionTyObligation<'tcx> = Obligation<'tcx, ty::ProjectionTy<'tcx>>;
 
-pub(super) struct InProgress;
+pub struct InProgress;
 
 /// When attempting to resolve `<T as TraitRef>::Name` ...
 #[derive(Debug)]
@@ -188,7 +188,7 @@ pub(super) fn poly_project_and_unify_type<'cx, 'tcx>(
 /// If successful, this may result in additional obligations.
 ///
 /// See [poly_project_and_unify_type] for an explanation of the return value.
-fn project_and_unify_type<'cx, 'tcx>(
+pub fn project_and_unify_type<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionObligation<'tcx>,
 ) -> Result<
@@ -484,7 +484,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                 // there won't be bound vars there.
 
                 let data = data.super_fold_with(self);
-                let normalized_ty = if self.eager_inference_replacement {
+                let normalized_ty = if self.selcx.normalization_mode.eager_inference_replacement {
                     normalize_projection_type(
                         self.selcx,
                         self.param_env,
@@ -914,7 +914,8 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
     // Don't use the projection cache in intercrate mode -
     // the `infcx` may be re-used between intercrate in non-intercrate
     // mode, which could lead to using incorrect cache results.
-    let use_cache = !selcx.is_intercrate();
+    let use_cache =
+        !selcx.is_intercrate() && selcx.normalization_mode.allow_infer_constraint_during_projection;
 
     let projection_ty = infcx.resolve_vars_if_possible(projection_ty);
     let cache_key = ProjectionCacheKey::new(projection_ty);
@@ -1168,7 +1169,7 @@ fn project<'cx, 'tcx>(
 
     match candidates {
         ProjectionCandidateSet::Single(candidate) => {
-            Ok(Projected::Progress(confirm_candidate(selcx, obligation, candidate)))
+            Ok(confirm_candidate(selcx, obligation, candidate))
         }
         ProjectionCandidateSet::None => Ok(Projected::NoProgress(
             // FIXME(associated_const_generics): this may need to change in the future?
@@ -1583,7 +1584,7 @@ fn confirm_candidate<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
     candidate: ProjectionCandidate<'tcx>,
-) -> Progress<'tcx> {
+) -> Projected<'tcx> {
     debug!(?obligation, ?candidate, "confirm_candidate");
     let mut progress = match candidate {
         ProjectionCandidate::ParamEnv(poly_projection)
@@ -1596,7 +1597,7 @@ fn confirm_candidate<'cx, 'tcx>(
         }
 
         ProjectionCandidate::Select(impl_source) => {
-            confirm_select_candidate(selcx, obligation, impl_source)
+            Projected::Progress(confirm_select_candidate(selcx, obligation, impl_source))
         }
     };
 
@@ -1605,9 +1606,11 @@ fn confirm_candidate<'cx, 'tcx>(
     // with new region variables, we need to resolve them to existing variables
     // when possible for this to work. See `auto-trait-projection-recursion.rs`
     // for a case where this matters.
-    if progress.term.has_infer_regions() {
-        progress.term =
-            progress.term.fold_with(&mut OpportunisticRegionResolver::new(selcx.infcx()));
+    if let Projected::Progress(progress) = &mut progress {
+        if progress.term.has_infer_regions() {
+            progress.term =
+                progress.term.fold_with(&mut OpportunisticRegionResolver::new(selcx.infcx()));
+        }
     }
     progress
 }
@@ -1688,9 +1691,12 @@ fn confirm_generator_candidate<'cx, 'tcx>(
         }
     });
 
-    confirm_param_env_candidate(selcx, obligation, predicate, false)
-        .with_addl_obligations(impl_source.nested)
-        .with_addl_obligations(obligations)
+    let progress = confirm_param_env_candidate(selcx, obligation, predicate, false);
+    let progress = match progress {
+        Projected::Progress(progress) => progress,
+        Projected::NoProgress(_) => bug!(),
+    };
+    progress.with_addl_obligations(impl_source.nested).with_addl_obligations(obligations)
 }
 
 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
@@ -1715,7 +1721,12 @@ fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
 
     // We get here from `poly_project_and_unify_type` which replaces bound vars
     // with placeholders, so dummy is okay here.
-    confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
+    let progress =
+        confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false);
+    match progress {
+        Projected::Progress(progress) => progress,
+        Projected::NoProgress(_) => bug!(),
+    }
 }
 
 fn confirm_pointee_candidate<'cx, 'tcx>(
@@ -1746,8 +1757,12 @@ fn confirm_pointee_candidate<'cx, 'tcx>(
         term: metadata_ty.into(),
     };
 
-    confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
-        .with_addl_obligations(obligations)
+    let progress =
+        confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false);
+    match progress {
+        Projected::Progress(progress) => progress.with_addl_obligations(obligations),
+        Projected::NoProgress(_) => bug!(),
+    }
 }
 
 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
@@ -1819,15 +1834,19 @@ fn confirm_callable_candidate<'cx, 'tcx>(
         term: ret_type.into(),
     });
 
-    confirm_param_env_candidate(selcx, obligation, predicate, true)
+    let progress = confirm_param_env_candidate(selcx, obligation, predicate, true);
+    match progress {
+        Projected::Progress(progress) => progress,
+        Projected::NoProgress(_) => bug!(),
+    }
 }
 
 fn confirm_param_env_candidate<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
     poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
     potentially_unnormalized_candidate: bool,
-) -> Progress<'tcx> {
+) -> Projected<'tcx> {
     let infcx = selcx.infcx();
     let cause = &obligation.cause;
     let param_env = obligation.param_env;
@@ -1868,23 +1887,42 @@ fn confirm_param_env_candidate<'cx, 'tcx>(
 
     debug!(?cache_projection, ?obligation_projection);
 
-    match infcx.at(cause, param_env).eq(cache_projection, obligation_projection) {
-        Ok(InferOk { value: _, obligations }) => {
-            nested_obligations.extend(obligations);
-            assoc_ty_own_obligations(selcx, obligation, &mut nested_obligations);
-            // FIXME(associated_const_equality): Handle consts here as well? Maybe this progress type should just take
-            // a term instead.
-            Progress { term: cache_entry.term, obligations: nested_obligations }
-        }
-        Err(e) => {
-            let msg = format!(
-                "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
-                obligation, poly_cache_entry, e,
-            );
-            debug!("confirm_param_env_candidate: {}", msg);
-            let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
-            Progress { term: err.into(), obligations: vec![] }
+    match infcx.commit_if_ok(|snapshot| {
+        let progress = match infcx.at(cause, param_env).eq(cache_projection, obligation_projection)
+        {
+            Ok(InferOk { value: _, obligations }) => {
+                nested_obligations.extend(obligations);
+                assoc_ty_own_obligations(selcx, obligation, &mut nested_obligations);
+                // FIXME(associated_const_equality): Handle consts here as well? Maybe this progress type should just take
+                // a term instead.
+                Progress { term: cache_entry.term, obligations: nested_obligations }
+            }
+            Err(e) => {
+                let msg = format!(
+                    "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
+                    obligation, poly_cache_entry, e,
+                );
+                debug!("confirm_param_env_candidate: {}", msg);
+                let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
+                Progress { term: err.into(), obligations: vec![] }
+            }
+        };
+
+        let any_instantiations = infcx.any_instantiations(&snapshot);
+
+        if any_instantiations && !selcx.normalization_mode.allow_infer_constraint_during_projection
+        {
+            Err(ty::Term::Ty(
+                infcx
+                    .tcx
+                    .mk_projection(obligation_projection.item_def_id, obligation_projection.substs),
+            ))
+        } else {
+            Ok(progress)
         }
+    }) {
+        Ok(p) => Projected::Progress(p),
+        Err(p) => Projected::NoProgress(p),
     }
 }
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -132,6 +132,17 @@ pub struct SelectionContext<'cx, 'tcx> {
     /// policy. In essence, canonicalized queries need their errors propagated
     /// rather than immediately reported because we do not have accurate spans.
     query_mode: TraitQueryMode,
+
+    pub normalization_mode: NormalizationMode,
+}
+
+#[derive(Copy, Clone)]
+pub struct NormalizationMode {
+    pub allow_infer_constraint_during_projection: bool,
+    /// If true, when a projection is unable to be completed, an inference
+    /// variable will be created and an obligation registered to project to that
+    /// inference variable. Also, constants will be eagerly evaluated.
+    pub eager_inference_replacement: bool,
 }
 
 // A stack that walks back up the stack frame.
@@ -221,6 +232,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
             query_mode: TraitQueryMode::Standard,
+            normalization_mode: NormalizationMode {
+                allow_infer_constraint_during_projection: true,
+                eager_inference_replacement: true,
+            },
         }
     }
 
@@ -232,6 +247,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
             query_mode: TraitQueryMode::Standard,
+            normalization_mode: NormalizationMode {
+                allow_infer_constraint_during_projection: true,
+                eager_inference_replacement: true,
+            },
         }
     }
 
@@ -247,6 +266,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate_ambiguity_causes: None,
             allow_negative_impls,
             query_mode: TraitQueryMode::Standard,
+            normalization_mode: NormalizationMode {
+                allow_infer_constraint_during_projection: true,
+                eager_inference_replacement: true,
+            },
         }
     }
 
@@ -262,6 +285,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
             query_mode,
+            normalization_mode: NormalizationMode {
+                allow_infer_constraint_during_projection: true,
+                eager_inference_replacement: true,
+            },
         }
     }
 
@@ -297,6 +324,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.intercrate
     }
 
+    pub fn normalization_mode(&self) -> NormalizationMode {
+        self.normalization_mode
+    }
+
     ///////////////////////////////////////////////////////////////////////////
     // Selection
     //