Rollup merge of #102037 - jyn514:normalize-docs, r=lcnr

Make cycle errors recoverable

In particular, this allows rustdoc to recover from cycle errors when normalizing associated types for documentation.

In the past, ```@jackh726``` has said we need to be careful about overflow errors: https://github.com/rust-lang/rust/pull/91430#issuecomment-983997013

> Off the top of my head, we definitely should be careful about treating overflow errors the same as
"not implemented for some reason" errors. Otherwise, you could end up with behavior that is
different depending on recursion depth. But, that might be context-dependent.

But cycle errors should be safe to unconditionally report; they don't depend on the recursion depth, they will always be an error whenever they're encountered.

Helps with https://github.com/rust-lang/rust/issues/81091.

r? ```@lcnr``` cc ```@matthewjasper```

9 files changed, 58 insertions, 30 deletions

diff --git a/compiler/rustc_data_structures/src/obligation_forest/mod.rs b/compiler/rustc_data_structures/src/obligation_forest/mod.rs
index e351b650a16..10e673cd929 100644
--- a/compiler/rustc_data_structures/src/obligation_forest/mod.rs
+++ b/compiler/rustc_data_structures/src/obligation_forest/mod.rs
@@ -95,6 +95,10 @@ pub trait ForestObligation: Clone + Debug {
 pub trait ObligationProcessor {
     type Obligation: ForestObligation;
     type Error: Debug;
+    type OUT: OutcomeTrait<
+        Obligation = Self::Obligation,
+        Error = Error<Self::Obligation, Self::Error>,
+    >;
 
     fn needs_process_obligation(&self, obligation: &Self::Obligation) -> bool;
 
@@ -111,7 +115,11 @@ pub trait ObligationProcessor {
     /// In other words, if we had O1 which required O2 which required
     /// O3 which required O1, we would give an iterator yielding O1,
     /// O2, O3 (O1 is not yielded twice).
-    fn process_backedge<'c, I>(&mut self, cycle: I, _marker: PhantomData<&'c Self::Obligation>)
+    fn process_backedge<'c, I>(
+        &mut self,
+        cycle: I,
+        _marker: PhantomData<&'c Self::Obligation>,
+    ) -> Result<(), Self::Error>
     where
         I: Clone + Iterator<Item = &'c Self::Obligation>;
 }
@@ -402,12 +410,11 @@ impl<O: ForestObligation> ObligationForest<O> {
 
     /// Performs a fixpoint computation over the obligation list.
     #[inline(never)]
-    pub fn process_obligations<P, OUT>(&mut self, processor: &mut P) -> OUT
+    pub fn process_obligations<P>(&mut self, processor: &mut P) -> P::OUT
     where
         P: ObligationProcessor<Obligation = O>,
-        OUT: OutcomeTrait<Obligation = O, Error = Error<O, P::Error>>,
     {
-        let mut outcome = OUT::new();
+        let mut outcome = P::OUT::new();
 
         // Fixpoint computation: we repeat until the inner loop stalls.
         loop {
@@ -473,7 +480,7 @@ impl<O: ForestObligation> ObligationForest<O> {
             }
 
             self.mark_successes();
-            self.process_cycles(processor);
+            self.process_cycles(processor, &mut outcome);
             self.compress(|obl| outcome.record_completed(obl));
         }
 
@@ -558,7 +565,7 @@ impl<O: ForestObligation> ObligationForest<O> {
 
     /// Report cycles between all `Success` nodes, and convert all `Success`
     /// nodes to `Done`. This must be called after `mark_successes`.
-    fn process_cycles<P>(&mut self, processor: &mut P)
+    fn process_cycles<P>(&mut self, processor: &mut P, outcome: &mut P::OUT)
     where
         P: ObligationProcessor<Obligation = O>,
     {
@@ -568,7 +575,7 @@ impl<O: ForestObligation> ObligationForest<O> {
             // to handle the no-op cases immediately to avoid the cost of the
             // function call.
             if node.state.get() == NodeState::Success {
-                self.find_cycles_from_node(&mut stack, processor, index);
+                self.find_cycles_from_node(&mut stack, processor, index, outcome);
             }
         }
 
@@ -576,8 +583,13 @@ impl<O: ForestObligation> ObligationForest<O> {
         self.reused_node_vec = stack;
     }
 
-    fn find_cycles_from_node<P>(&self, stack: &mut Vec<usize>, processor: &mut P, index: usize)
-    where
+    fn find_cycles_from_node<P>(
+        &self,
+        stack: &mut Vec<usize>,
+        processor: &mut P,
+        index: usize,
+        outcome: &mut P::OUT,
+    ) where
         P: ObligationProcessor<Obligation = O>,
     {
         let node = &self.nodes[index];
@@ -586,17 +598,20 @@ impl<O: ForestObligation> ObligationForest<O> {
                 None => {
                     stack.push(index);
                     for &dep_index in node.dependents.iter() {
-                        self.find_cycles_from_node(stack, processor, dep_index);
+                        self.find_cycles_from_node(stack, processor, dep_index, outcome);
                     }
                     stack.pop();
                     node.state.set(NodeState::Done);
                 }
                 Some(rpos) => {
                     // Cycle detected.
-                    processor.process_backedge(
+                    let result = processor.process_backedge(
                         stack[rpos..].iter().map(|&i| &self.nodes[i].obligation),
                         PhantomData,
                     );
+                    if let Err(err) = result {
+                        outcome.record_error(Error { error: err, backtrace: self.error_at(index) });
+                    }
                 }
             }
         }
diff --git a/compiler/rustc_data_structures/src/obligation_forest/tests.rs b/compiler/rustc_data_structures/src/obligation_forest/tests.rs
index e2991aae174..bc252f772a1 100644
--- a/compiler/rustc_data_structures/src/obligation_forest/tests.rs
+++ b/compiler/rustc_data_structures/src/obligation_forest/tests.rs
@@ -64,6 +64,7 @@ where
 {
     type Obligation = O;
     type Error = E;
+    type OUT = TestOutcome<O, E>;
 
     fn needs_process_obligation(&self, _obligation: &Self::Obligation) -> bool {
         true
@@ -76,10 +77,15 @@ where
         (self.process_obligation)(obligation)
     }
 
-    fn process_backedge<'c, I>(&mut self, _cycle: I, _marker: PhantomData<&'c Self::Obligation>)
+    fn process_backedge<'c, I>(
+        &mut self,
+        _cycle: I,
+        _marker: PhantomData<&'c Self::Obligation>,
+    ) -> Result<(), Self::Error>
     where
         I: Clone + Iterator<Item = &'c Self::Obligation>,
     {
+        Ok(())
     }
 }
 
diff --git a/compiler/rustc_infer/src/traits/mod.rs b/compiler/rustc_infer/src/traits/mod.rs
index da3b45a4d6b..c8600ded987 100644
--- a/compiler/rustc_infer/src/traits/mod.rs
+++ b/compiler/rustc_infer/src/traits/mod.rs
@@ -113,6 +113,8 @@ pub struct FulfillmentError<'tcx> {
 
 #[derive(Clone)]
 pub enum FulfillmentErrorCode<'tcx> {
+    /// Inherently impossible to fulfill; this trait is implemented if and only if it is already implemented.
+    CodeCycle(Vec<Obligation<'tcx, ty::Predicate<'tcx>>>),
     CodeSelectionError(SelectionError<'tcx>),
     CodeProjectionError(MismatchedProjectionTypes<'tcx>),
     CodeSubtypeError(ExpectedFound<Ty<'tcx>>, TypeError<'tcx>), // always comes from a SubtypePredicate
diff --git a/compiler/rustc_infer/src/traits/structural_impls.rs b/compiler/rustc_infer/src/traits/structural_impls.rs
index 573d2d1e330..1c6ab6a082b 100644
--- a/compiler/rustc_infer/src/traits/structural_impls.rs
+++ b/compiler/rustc_infer/src/traits/structural_impls.rs
@@ -47,6 +47,7 @@ impl<'tcx> fmt::Debug for traits::FulfillmentErrorCode<'tcx> {
                 write!(f, "CodeConstEquateError({:?}, {:?})", a, b)
             }
             super::CodeAmbiguity => write!(f, "Ambiguity"),
+            super::CodeCycle(ref cycle) => write!(f, "Cycle({:?})", cycle),
         }
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/codegen.rs b/compiler/rustc_trait_selection/src/traits/codegen.rs
index 08adbcbd410..0155798c8b6 100644
--- a/compiler/rustc_trait_selection/src/traits/codegen.rs
+++ b/compiler/rustc_trait_selection/src/traits/codegen.rs
@@ -4,10 +4,12 @@
 // general routines.
 
 use crate::infer::{DefiningAnchor, TyCtxtInferExt};
+use crate::traits::error_reporting::InferCtxtExt;
 use crate::traits::{
     ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine, TraitEngineExt,
     Unimplemented,
 };
+use rustc_infer::traits::FulfillmentErrorCode;
 use rustc_middle::traits::CodegenObligationError;
 use rustc_middle::ty::{self, TyCtxt};
 
@@ -62,6 +64,14 @@ pub fn codegen_select_candidate<'tcx>(
         // optimization to stop iterating early.
         let errors = fulfill_cx.select_all_or_error(&infcx);
         if !errors.is_empty() {
+            // `rustc_monomorphize::collector` assumes there are no type errors.
+            // Cycle errors are the only post-monomorphization errors possible; emit them now so
+            // `rustc_ty_utils::resolve_associated_item` doesn't return `None` post-monomorphization.
+            for err in errors {
+                if let FulfillmentErrorCode::CodeCycle(cycle) = err.code {
+                    infcx.report_overflow_error_cycle(&cycle);
+                }
+            }
             return Err(CodegenObligationError::FulfillmentError);
         }
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index efdb1ace139..d62b399c1b5 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1540,6 +1540,9 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 }
                 diag.emit();
             }
+            FulfillmentErrorCode::CodeCycle(ref cycle) => {
+                self.report_overflow_error_cycle(cycle);
+            }
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index a81fef60aed..6f3a9412dde 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -25,10 +25,9 @@ use super::Unimplemented;
 use super::{FulfillmentError, FulfillmentErrorCode};
 use super::{ObligationCause, PredicateObligation};
 
-use crate::traits::error_reporting::InferCtxtExt as _;
 use crate::traits::project::PolyProjectionObligation;
 use crate::traits::project::ProjectionCacheKeyExt as _;
-use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 
 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
     /// Note that we include both the `ParamEnv` and the `Predicate`,
@@ -224,6 +223,7 @@ fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligatio
 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
     type Obligation = PendingPredicateObligation<'tcx>;
     type Error = FulfillmentErrorCode<'tcx>;
+    type OUT = Outcome<Self::Obligation, Self::Error>;
 
     /// Identifies whether a predicate obligation needs processing.
     ///
@@ -594,14 +594,16 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
         &mut self,
         cycle: I,
         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
-    ) where
+    ) -> Result<(), FulfillmentErrorCode<'tcx>>
+    where
         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
     {
         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
             debug!("process_child_obligations: coinductive match");
+            Ok(())
         } else {
             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
-            self.selcx.infcx().report_overflow_error_cycle(&cycle);
+            Err(FulfillmentErrorCode::CodeCycle(cycle))
         }
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index e4e02584561..5e32a27cdb1 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -226,13 +226,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     }
 
     pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: true,
-            intercrate_ambiguity_causes: None,
-            query_mode: TraitQueryMode::Standard,
-        }
+        SelectionContext { intercrate: true, ..SelectionContext::new(infcx) }
     }
 
     pub fn with_query_mode(
@@ -240,13 +234,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         query_mode: TraitQueryMode,
     ) -> SelectionContext<'cx, 'tcx> {
         debug!(?query_mode, "with_query_mode");
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: false,
-            intercrate_ambiguity_causes: None,
-            query_mode,
-        }
+        SelectionContext { query_mode, ..SelectionContext::new(infcx) }
     }
 
     /// Enables tracking of intercrate ambiguity causes. See
diff --git a/src/test/rustdoc-ui/normalize-cycle.rs b/src/test/rustdoc-ui/normalize-cycle.rs
index 14ffac1e1dc..1ed9ac6bc34 100644
--- a/src/test/rustdoc-ui/normalize-cycle.rs
+++ b/src/test/rustdoc-ui/normalize-cycle.rs
@@ -1,4 +1,5 @@
 // check-pass
+// compile-flags: -Znormalize-docs
 // Regression test for <https://github.com/rust-lang/rust/issues/79459>.
 pub trait Query {}