From 026eb3dd6457f223ff1c3a3b40bf59898a25061a Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 13 Mar 2024 13:44:00 -0400
Subject: [PATCH] Delay a bug for stranded opaques

---
 compiler/rustc_hir_analysis/src/check/check.rs |  8 +++++++-
 tests/ui/impl-trait/stranded-opaque.rs         | 13 +++++++++++++
 tests/ui/impl-trait/stranded-opaque.stderr     |  9 +++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/impl-trait/stranded-opaque.rs
 create mode 100644 tests/ui/impl-trait/stranded-opaque.stderr

diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index 1b8174d3d18ff..d1fed13ee9feb 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -381,11 +381,17 @@ fn check_opaque_meets_bounds<'tcx>(
     match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
         Ok(()) => {}
         Err(ty_err) => {
+            // Some types may be left "stranded" if they can't be reached
+            // from an astconv'd bound but they're mentioned in the HIR. This
+            // will happen, e.g., when a nested opaque is inside of a non-
+            // existent associated type, like `impl Trait<Missing = impl Trait>`.
+            // See <tests/ui/impl-trait/stranded-opaque.rs>.
             let ty_err = ty_err.to_string(tcx);
-            tcx.dcx().span_bug(
+            let guar = tcx.dcx().span_delayed_bug(
                 span,
                 format!("could not unify `{hidden_ty}` with revealed type:\n{ty_err}"),
             );
+            return Err(guar);
         }
     }
 
diff --git a/tests/ui/impl-trait/stranded-opaque.rs b/tests/ui/impl-trait/stranded-opaque.rs
new file mode 100644
index 0000000000000..c7ab390e1fd0c
--- /dev/null
+++ b/tests/ui/impl-trait/stranded-opaque.rs
@@ -0,0 +1,13 @@
+trait Trait {}
+
+impl Trait for i32 {}
+
+// Since `Assoc` doesn't actually exist, it's "stranded", and won't show up in
+// the list of opaques that may be defined by the function. Make sure we don't
+// ICE in this case.
+fn produce<T>() -> impl Trait<Assoc = impl Trait> {
+    //~^ ERROR associated type `Assoc` not found for `Trait`
+    16
+}
+
+fn main () {}
diff --git a/tests/ui/impl-trait/stranded-opaque.stderr b/tests/ui/impl-trait/stranded-opaque.stderr
new file mode 100644
index 0000000000000..75f5480bc8b81
--- /dev/null
+++ b/tests/ui/impl-trait/stranded-opaque.stderr
@@ -0,0 +1,9 @@
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded-opaque.rs:8:31
+   |
+LL | fn produce<T>() -> impl Trait<Assoc = impl Trait> {
+   |                               ^^^^^ associated type `Assoc` not found
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0220`.