From 7b960b4d99b4235e4abcd2828158f79650ec8570 Mon Sep 17 00:00:00 2001
From: trevyn <230691+trevyn@users.noreply.github.com>
Date: Tue, 9 Jul 2024 10:30:09 +0400
Subject: [PATCH] Mark associated constants used in equality constraints as
 used

---
 compiler/rustc_passes/src/dead.rs             | 25 +++++++++++
 .../equality-unused-issue-126729.rs           | 44 +++++++++++++++++++
 2 files changed, 69 insertions(+)
 create mode 100644 tests/ui/associated-consts/equality-unused-issue-126729.rs

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index 55514883cb1f8..61f9f5231fd39 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -857,6 +857,31 @@ fn check_item<'tcx>(
             // global_asm! is always live.
             worklist.push((id.owner_id.def_id, ComesFromAllowExpect::No));
         }
+        DefKind::OpaqueTy => {
+            if let hir::ItemKind::OpaqueTy(ty) = tcx.hir().item(id).kind {
+                for bound in ty.bounds {
+                    if let hir::GenericBound::Trait(polytraitref, _) = bound
+                        && let Some(pathsegment) = polytraitref.trait_ref.path.segments.last()
+                        && let Some(args) = pathsegment.args
+                    {
+                        for constraint in args.constraints {
+                            if let Some(item) = tcx
+                                .associated_items(pathsegment.res.def_id())
+                                .filter_by_name_unhygienic(constraint.ident.name)
+                                .find(|i| {
+                                    i.kind == ty::AssocKind::Const
+                                        && i.ident(tcx).normalize_to_macros_2_0()
+                                            == constraint.ident
+                                })
+                                && let Some(local_def_id) = item.def_id.as_local()
+                            {
+                                worklist.push((local_def_id, ComesFromAllowExpect::No));
+                            }
+                        }
+                    }
+                }
+            }
+        }
         _ => {}
     }
 }
diff --git a/tests/ui/associated-consts/equality-unused-issue-126729.rs b/tests/ui/associated-consts/equality-unused-issue-126729.rs
new file mode 100644
index 0000000000000..1482b874b9d69
--- /dev/null
+++ b/tests/ui/associated-consts/equality-unused-issue-126729.rs
@@ -0,0 +1,44 @@
+//@ check-pass
+
+#![feature(associated_const_equality)]
+#![deny(dead_code)]
+
+trait Tr {
+    const I: i32;
+}
+
+impl Tr for () {
+    const I: i32 = 1;
+}
+
+fn foo() -> impl Tr<I = 1> {}
+
+trait Tr2 {
+    const J: i32;
+    const K: i32;
+}
+
+impl Tr2 for () {
+    const J: i32 = 1;
+    const K: i32 = 1;
+}
+
+fn foo2() -> impl Tr2<J = 1, K = 1> {}
+
+mod t {
+    pub trait Tr3 {
+        const L: i32;
+    }
+
+    impl Tr3 for () {
+        const L: i32 = 1;
+    }
+}
+
+fn foo3() -> impl t::Tr3<L = 1> {}
+
+fn main() {
+    foo();
+    foo2();
+    foo3();
+}