From 268c56e14455d8889b818c4ea55470b94649aebc Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 7 Apr 2025 20:35:41 +0000
Subject: [PATCH] Implement overflow for infinite implied lifetime bounds

---
 .../src/outlives/implicit_infer.rs            | 21 +++++++++++++++----
 tests/ui/implied-bounds/overflow.rs           | 11 ++++++++++
 tests/ui/implied-bounds/overflow.stderr       |  8 +++++++
 3 files changed, 36 insertions(+), 4 deletions(-)
 create mode 100644 tests/ui/implied-bounds/overflow.rs
 create mode 100644 tests/ui/implied-bounds/overflow.stderr

diff --git a/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs
index a0faa5e8429..780c27d4595 100644
--- a/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs
+++ b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs
@@ -24,8 +24,8 @@ pub(super) fn infer_predicates(
 
     // If new predicates were added then we need to re-calculate
     // all crates since there could be new implied predicates.
-    loop {
-        let mut predicates_added = false;
+    for i in 0.. {
+        let mut predicates_added = vec![];
 
         // Visit all the crates and infer predicates
         for id in tcx.hir_free_items() {
@@ -83,14 +83,27 @@ pub(super) fn infer_predicates(
                 .get(&item_did.to_def_id())
                 .map_or(0, |p| p.as_ref().skip_binder().len());
             if item_required_predicates.len() > item_predicates_len {
-                predicates_added = true;
+                predicates_added.push(item_did);
                 global_inferred_outlives
                     .insert(item_did.to_def_id(), ty::EarlyBinder::bind(item_required_predicates));
             }
         }
 
-        if !predicates_added {
+        if predicates_added.is_empty() {
+            // We've reached a fixed point.
             break;
+        } else if !tcx.recursion_limit().value_within_limit(i) {
+            let msg = if let &[id] = &predicates_added[..] {
+                format!("overflow computing implied lifetime bounds for `{}`", tcx.def_path_str(id),)
+            } else {
+                "overflow computing implied lifetime bounds".to_string()
+            };
+            tcx.dcx()
+                .struct_span_fatal(
+                    predicates_added.iter().map(|id| tcx.def_span(*id)).collect::<Vec<_>>(),
+                    msg,
+                )
+                .emit();
         }
     }
 
diff --git a/tests/ui/implied-bounds/overflow.rs b/tests/ui/implied-bounds/overflow.rs
new file mode 100644
index 00000000000..7c36998dd4d
--- /dev/null
+++ b/tests/ui/implied-bounds/overflow.rs
@@ -0,0 +1,11 @@
+trait Tailed<'a>: 'a {
+    type Tail: Tailed<'a>;
+}
+
+struct List<'a, T: Tailed<'a>> {
+    //~^ ERROR overflow computing implied lifetime bounds for `List`
+    next: Box<List<'a, T::Tail>>,
+    node: &'a T,
+}
+
+fn main() {}
diff --git a/tests/ui/implied-bounds/overflow.stderr b/tests/ui/implied-bounds/overflow.stderr
new file mode 100644
index 00000000000..2e5a9ab141c
--- /dev/null
+++ b/tests/ui/implied-bounds/overflow.stderr
@@ -0,0 +1,8 @@
+error: overflow computing implied lifetime bounds for `List`
+  --> $DIR/overflow.rs:5:1
+   |
+LL | struct List<'a, T: Tailed<'a>> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+