From fb94e5883ed1b648ae00fd3bc85e3dc002068a78 Mon Sep 17 00:00:00 2001
From: Shoyu Vanilla <modulo641@gmail.com>
Date: Wed, 18 Feb 2026 19:12:34 +0900
Subject: [PATCH] Fix an ICE while checking param env shadowing on an erroneous
 trait impl

---
 .../src/error_reporting/infer/mod.rs          |  6 ++++
 ...uggestion-no-ice-on-missing-assoc-value.rs | 18 +++++++++++
 ...stion-no-ice-on-missing-assoc-value.stderr | 32 +++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs
 create mode 100644 tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.stderr

diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
index 6003461f35e8..2e1657a8dc2c 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
@@ -299,6 +299,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 let trait_def_id = alias.trait_def_id(tcx);
                 let rebased_args = alias.args.rebase_onto(tcx, trait_def_id, impl_substs);
 
+                // The impl is erroneous missing a definition for the associated type.
+                // Skipping it since calling `TyCtxt::type_of` on its assoc ty will trigger an ICE.
+                if !leaf_def.item.defaultness(tcx).has_value() {
+                    return false;
+                }
+
                 let impl_item_def_id = leaf_def.item.def_id;
                 let impl_assoc_ty = tcx.type_of(impl_item_def_id).instantiate(tcx, rebased_args);
 
diff --git a/tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs b/tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs
new file mode 100644
index 000000000000..6f0cd74d2ffe
--- /dev/null
+++ b/tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs
@@ -0,0 +1,18 @@
+// A regression test for https://github.com/rust-lang/rust/issues/152663
+// Previously triggered an ICE when checking whether the param-env
+// shadows a global impl. The crash occurred due to calling
+// `TyCtxt::type_of` on an erroneous associated type in a trait impl
+// that had no corresponding value.
+
+trait Iterable {
+    type Iter;
+}
+
+impl<T> Iterable for [T] {
+    //~^ ERROR: not all trait items implemented
+    fn iter() -> Self::Iter {}
+    //~^ ERROR: method `iter` is not a member of trait `Iterable`
+    //~| ERROR: mismatched types
+}
+
+fn main() {}
diff --git a/tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.stderr b/tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.stderr
new file mode 100644
index 000000000000..9d4f7c58bd7a
--- /dev/null
+++ b/tests/ui/associated-types/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.stderr
@@ -0,0 +1,32 @@
+error[E0407]: method `iter` is not a member of trait `Iterable`
+  --> $DIR/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs:13:5
+   |
+LL |     fn iter() -> Self::Iter {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ not a member of trait `Iterable`
+
+error[E0046]: not all trait items implemented, missing: `Iter`
+  --> $DIR/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs:11:1
+   |
+LL |     type Iter;
+   |     --------- `Iter` from trait
+...
+LL | impl<T> Iterable for [T] {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ missing `Iter` in implementation
+
+error[E0308]: mismatched types
+  --> $DIR/param-env-shadowing-suggestion-no-ice-on-missing-assoc-value.rs:13:18
+   |
+LL |     fn iter() -> Self::Iter {}
+   |        ----      ^^^^^^^^^^ expected associated type, found `()`
+   |        |
+   |        implicitly returns `()` as its body has no tail or `return` expression
+   |
+   = note: expected associated type `<[T] as Iterable>::Iter`
+                    found unit type `()`
+   = help: consider constraining the associated type `<[T] as Iterable>::Iter` to `()` or calling a method that returns `<[T] as Iterable>::Iter`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0046, E0308, E0407.
+For more information about an error, try `rustc --explain E0046`.