From 450cb5eda6d1db8b5b45a3775cb5316bb623dc63 Mon Sep 17 00:00:00 2001
From: Lukas Markeffsky <@>
Date: Sun, 14 Jan 2024 23:23:30 +0100
Subject: [PATCH 1/2] Don't ICE if TAIT-defining fn contains a closure with `_`
 in return type

---
 .../src/collect/type_of/opaque.rs             | 31 +++++++++-------
 .../ui/type-alias-impl-trait/closure_infer.rs | 35 +++++++++++++++++++
 2 files changed, 54 insertions(+), 12 deletions(-)
 create mode 100644 tests/ui/type-alias-impl-trait/closure_infer.rs

diff --git a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
index 1f7ca48234a24..ade24259ed408 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
@@ -135,18 +135,25 @@ impl TaitConstraintLocator<'_> {
             return;
         }
 
-        if let Some(hir_sig) = self.tcx.hir_node_by_def_id(item_def_id).fn_decl() {
-            if hir_sig.output.get_infer_ret_ty().is_some() {
-                let guar = self.tcx.dcx().span_delayed_bug(
-                    hir_sig.output.span(),
-                    "inferring return types and opaque types do not mix well",
-                );
-                self.found = Some(ty::OpaqueHiddenType {
-                    span: DUMMY_SP,
-                    ty: Ty::new_error(self.tcx, guar),
-                });
-                return;
-            }
+        // Function items with `_` in their return type already emit an error, skip any
+        // "non-defining use" errors for them.
+        // Note that we use `Node::fn_sig` instead of `Node::fn_decl` here, because the former
+        // excludes closures, which are allowed to have `_` in their return type.
+        let hir_node = self.tcx.hir_node_by_def_id(item_def_id);
+        debug_assert!(
+            !matches!(hir_node, Node::ForeignItem(..)),
+            "foreign items cannot constrain opaque types",
+        );
+        if let Some(hir_sig) = hir_node.fn_sig()
+            && hir_sig.decl.output.get_infer_ret_ty().is_some()
+        {
+            let guar = self.tcx.dcx().span_delayed_bug(
+                hir_sig.decl.output.span(),
+                "inferring return types and opaque types do not mix well",
+            );
+            self.found =
+                Some(ty::OpaqueHiddenType { span: DUMMY_SP, ty: Ty::new_error(self.tcx, guar) });
+            return;
         }
 
         // Calling `mir_borrowck` can lead to cycle errors through
diff --git a/tests/ui/type-alias-impl-trait/closure_infer.rs b/tests/ui/type-alias-impl-trait/closure_infer.rs
new file mode 100644
index 0000000000000..04e2323ec4ae9
--- /dev/null
+++ b/tests/ui/type-alias-impl-trait/closure_infer.rs
@@ -0,0 +1,35 @@
+// check-pass
+
+// Regression test for an ICE: https://github.com/rust-lang/rust/issues/119916
+
+#![feature(impl_trait_in_assoc_type)]
+#![feature(type_alias_impl_trait)]
+
+// `impl_trait_in_assoc_type` example from the bug report.
+pub trait StreamConsumer {
+    type BarrierStream;
+    fn execute() -> Self::BarrierStream;
+}
+
+pub struct DispatchExecutor;
+
+impl StreamConsumer for DispatchExecutor {
+    type BarrierStream = impl Sized;
+    fn execute() -> Self::BarrierStream {
+        || -> _ {}
+    }
+}
+
+// Functions that constrain TAITs can contain closures with an `_` in the return type.
+type Foo = impl Sized;
+fn foo() -> Foo {
+    || -> _ {}
+}
+
+// The `_` in the closure return type can also be the TAIT itself.
+type Bar = impl Sized;
+fn bar() -> impl FnOnce() -> Bar {
+    || -> _ {}
+}
+
+fn main() {}

From 22833c177e23c1c80e7ad1cbea33d758dcd94c28 Mon Sep 17 00:00:00 2001
From: Lukas Markeffsky <@>
Date: Mon, 15 Jan 2024 01:21:03 +0100
Subject: [PATCH 2/2] add test for non-defining use of TAIT in foreign function
 item

---
 tests/ui/type-alias-impl-trait/issue-77179.rs     |  5 +++++
 tests/ui/type-alias-impl-trait/issue-77179.stderr | 11 ++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/tests/ui/type-alias-impl-trait/issue-77179.rs b/tests/ui/type-alias-impl-trait/issue-77179.rs
index 093aeb4b27911..3d6c826d6ac5a 100644
--- a/tests/ui/type-alias-impl-trait/issue-77179.rs
+++ b/tests/ui/type-alias-impl-trait/issue-77179.rs
@@ -12,3 +12,8 @@ fn test() -> Pointer<_> {
 fn main() {
     test();
 }
+
+extern "Rust" {
+    fn bar() -> Pointer<_>;
+    //~^ ERROR: the placeholder `_` is not allowed within types
+}
diff --git a/tests/ui/type-alias-impl-trait/issue-77179.stderr b/tests/ui/type-alias-impl-trait/issue-77179.stderr
index 68dd6570d00c0..c5cacfd3cd351 100644
--- a/tests/ui/type-alias-impl-trait/issue-77179.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-77179.stderr
@@ -7,6 +7,15 @@ LL | fn test() -> Pointer<_> {
    |              |       not allowed in type signatures
    |              help: replace with the correct return type: `Pointer<i32>`
 
-error: aborting due to 1 previous error
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
+  --> $DIR/issue-77179.rs:17:25
+   |
+LL |     fn bar() -> Pointer<_>;
+   |                         ^
+   |                         |
+   |                         not allowed in type signatures
+   |                         help: use type parameters instead: `T`
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0121`.