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:

> 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.

1 files changed, 10 insertions, 0 deletions

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);
         }