From f580357863c1689ea1903b9570092a53f85a5c44 Mon Sep 17 00:00:00 2001
From: Jules Bertholet <julesbertholet@quoi.xyz>
Date: Fri, 21 Nov 2025 11:43:53 -0500
Subject: [PATCH] Handle cycles when checking impl candidates for `doc(hidden)`

Fixes https://github.com/rust-lang/rust/issues/149092
---
 .../traits/fulfillment_errors.rs              | 13 +++++++---
 compiler/rustc_trait_selection/src/lib.rs     |  1 +
 .../ui/suggestions/auxiliary/hidden-struct.rs |  3 +++
 .../dont-suggest-foreign-doc-hidden.rs        |  5 ++++
 .../dont-suggest-foreign-doc-hidden.stderr    | 26 +++++++++++++++++--
 5 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
index d2f2c92dda080..0f5607dd7fecb 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -1,12 +1,13 @@
 // ignore-tidy-filelength
 use core::ops::ControlFlow;
 use std::borrow::Cow;
+use std::collections::hash_set;
 use std::path::PathBuf;
 
 use rustc_abi::ExternAbi;
 use rustc_ast::ast::LitKind;
 use rustc_ast::{LitIntType, TraitObjectSyntax};
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::codes::*;
 use rustc_errors::{
@@ -1956,11 +1957,17 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         if self.tcx.visibility(did).is_accessible_from(body_def_id, self.tcx) {
                             // don't suggest foreign `#[doc(hidden)]` types
                             if !did.is_local() {
-                                while let Some(parent) = parent_map.get(&did) {
+                                let mut previously_seen_dids: FxHashSet<DefId> = Default::default();
+                                previously_seen_dids.insert(did);
+                                while let Some(&parent) = parent_map.get(&did)
+                                    && let hash_set::Entry::Vacant(v) =
+                                        previously_seen_dids.entry(parent)
+                                {
                                     if self.tcx.is_doc_hidden(did) {
                                         return false;
                                     }
-                                    did = *parent;
+                                    v.insert();
+                                    did = parent;
                                 }
                             }
                             true
diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs
index c5dfaa2a60d87..0184b2ccde784 100644
--- a/compiler/rustc_trait_selection/src/lib.rs
+++ b/compiler/rustc_trait_selection/src/lib.rs
@@ -17,6 +17,7 @@
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(default_field_values)]
+#![feature(hash_set_entry)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(iterator_try_reduce)]
diff --git a/tests/ui/suggestions/auxiliary/hidden-struct.rs b/tests/ui/suggestions/auxiliary/hidden-struct.rs
index 1f495a9f2224a..ee24389bebd7c 100644
--- a/tests/ui/suggestions/auxiliary/hidden-struct.rs
+++ b/tests/ui/suggestions/auxiliary/hidden-struct.rs
@@ -33,3 +33,6 @@ impl Marker for hidden::Foo {}
 impl Marker for hidden1::Bar {}
 impl Marker for Baz {}
 impl Marker for Quux {}
+
+
+pub use crate as library;
diff --git a/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.rs b/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.rs
index a83e496f2703d..0716e4e2e143b 100644
--- a/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.rs
+++ b/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.rs
@@ -21,7 +21,12 @@ pub fn test4(_: Quux) {}
 
 fn test5<T: hidden_struct::Marker>() {}
 
+fn test6<T: hidden_struct::library::Marker>() {}
+
 fn main() {
     test5::<i32>();
     //~^ ERROR [E0277]
+
+    test6::<i32>();
+    //~^ ERROR [E0277]
 }
diff --git a/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.stderr b/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.stderr
index e0444c93eeece..a111d0e5aa95a 100644
--- a/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.stderr
+++ b/tests/ui/suggestions/dont-suggest-foreign-doc-hidden.stderr
@@ -38,7 +38,7 @@ LL + use hidden_struct::Quux;
    |
 
 error[E0277]: the trait bound `i32: Marker` is not satisfied
-  --> $DIR/dont-suggest-foreign-doc-hidden.rs:25:13
+  --> $DIR/dont-suggest-foreign-doc-hidden.rs:27:13
    |
 LL |     test5::<i32>();
    |             ^^^ the trait `Marker` is not implemented for `i32`
@@ -59,7 +59,29 @@ note: required by a bound in `test5`
 LL | fn test5<T: hidden_struct::Marker>() {}
    |             ^^^^^^^^^^^^^^^^^^^^^ required by this bound in `test5`
 
-error: aborting due to 5 previous errors
+error[E0277]: the trait bound `i32: Marker` is not satisfied
+  --> $DIR/dont-suggest-foreign-doc-hidden.rs:30:13
+   |
+LL |     test6::<i32>();
+   |             ^^^ the trait `Marker` is not implemented for `i32`
+   |
+help: the following other types implement trait `Marker`
+  --> $DIR/auxiliary/hidden-struct.rs:31:1
+   |
+LL | impl Marker for Option<u32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Option<u32>`
+...
+LL | impl Marker for Baz {}
+   | ^^^^^^^^^^^^^^^^^^^ `Baz`
+LL | impl Marker for Quux {}
+   | ^^^^^^^^^^^^^^^^^^^^ `Quux`
+note: required by a bound in `test6`
+  --> $DIR/dont-suggest-foreign-doc-hidden.rs:24:13
+   |
+LL | fn test6<T: hidden_struct::library::Marker>() {}
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `test6`
+
+error: aborting due to 6 previous errors
 
 Some errors have detailed explanations: E0277, E0412.
 For more information about an error, try `rustc --explain E0277`.