From 8e491cc054427d18e96361c44c9e307e16e17924 Mon Sep 17 00:00:00 2001
From: "Tinker (via OpenClaw)" <noreply@openclaw.ai>
Date: Sat, 7 Feb 2026 18:00:00 +0000
Subject: [PATCH] Fix ICE in upvar analysis with associated type projections
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The closure capture migration analysis (`rust_2021_incompatible_closure_captures`)
hit `unreachable!()` in `has_significant_drop_outside_of_captures` when
encountering associated type projections (`ty::Alias`).

Three `unreachable!()` calls in `upvar.rs` assumed only certain type kinds
and projection kinds would appear, but associated types like
`<Foo as Trait>::Assoc` can produce unexpected patterns.

Replace the panics with conservative fallbacks:
- `unreachable!()` in filter_map closures → `None` (filter out)
- `_ => unreachable!()` in main match → `false` (no significant drop)

This is safe because the function is only used for the edition migration
lint, so a conservative `false` just means we might miss a warning about
incompatible closure captures, which is better than an ICE.

Fixes #152205
---
 compiler/rustc_hir_typeck/src/upvar.rs        | 15 +++++---
 .../ui/closures/upvar-associated-type-ice.rs  | 34 +++++++++++++++++++
 2 files changed, 45 insertions(+), 4 deletions(-)
 create mode 100644 tests/ui/closures/upvar-associated-type-ice.rs

diff --git a/compiler/rustc_hir_typeck/src/upvar.rs b/compiler/rustc_hir_typeck/src/upvar.rs
index 1a2b76485f359..036e31d655f61 100644
--- a/compiler/rustc_hir_typeck/src/upvar.rs
+++ b/compiler/rustc_hir_typeck/src/upvar.rs
@@ -1634,7 +1634,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 {
                                     if field_idx == i { Some(&projs[1..]) } else { None }
                                 } else {
-                                    unreachable!();
+                                    // Non-field projections are unexpected here but
+                                    // can occur with associated type projections.
+                                    // Filter them out rather than panicking.
+                                    None
                                 }
                             })
                             .collect();
@@ -1667,7 +1670,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             {
                                 if field_idx.index() == i { Some(&projs[1..]) } else { None }
                             } else {
-                                unreachable!();
+                                // Non-field projections are unexpected here but
+                                // can occur with associated type projections.
+                                None
                             }
                         })
                         .collect();
@@ -1681,8 +1686,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 })
             }
 
-            // Anything else would be completely captured and therefore handled already.
-            _ => unreachable!(),
+            // For associated type projections (ty::Alias) and any other unhandled
+            // type kinds, conservatively assume no significant drop outside of captures.
+            // This avoids ICEs while maintaining correct behavior for the migration lint.
+            _ => false,
         }
     }
 
diff --git a/tests/ui/closures/upvar-associated-type-ice.rs b/tests/ui/closures/upvar-associated-type-ice.rs
new file mode 100644
index 0000000000000..a64782314b369
--- /dev/null
+++ b/tests/ui/closures/upvar-associated-type-ice.rs
@@ -0,0 +1,34 @@
+// Regression test for #152205: ICE in upvar.rs with associated type projections
+//
+// The closure capture migration analysis hit `unreachable!()` when
+// encountering associated type projections in closure captures.
+
+//@ check-pass
+//@ edition:2021
+
+#![deny(rust_2021_incompatible_closure_captures)]
+
+struct Foo;
+struct S;
+
+impl Drop for S {
+    fn drop(&mut self) {}
+}
+
+struct U(<Foo as NewTrait>::Assoc);
+
+fn test_precise_analysis_long_path(u: U) {
+    let _ = || {
+        let _x = u.0.0;
+    };
+}
+
+trait NewTrait {
+    type Assoc;
+}
+
+impl NewTrait for Foo {
+    type Assoc = (S,);
+}
+
+fn main() {}