From 9bc0e5b23c4ae397a6dbc32c3917605f17df2910 Mon Sep 17 00:00:00 2001
From: r0cky <mu001999@outlook.com>
Date: Sun, 26 May 2024 23:11:36 +0800
Subject: [PATCH] Detect unused associated constants in traits

---
 compiler/rustc_passes/src/dead.rs             | 23 ++++++-------------
 tests/ui/lint/dead-code/unused_assoc_const.rs | 17 ++++++++++++++
 .../lint/dead-code/unused_assoc_const.stderr  | 16 +++++++++++++
 3 files changed, 40 insertions(+), 16 deletions(-)
 create mode 100644 tests/ui/lint/dead-code/unused_assoc_const.rs
 create mode 100644 tests/ui/lint/dead-code/unused_assoc_const.stderr

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index f1e6528581ed..88c335435434 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -786,18 +786,9 @@ fn check_item<'tcx>(
                 // for trait impl blocks,
                 // mark the method live if the self_ty is public,
                 // or the method is public and may construct self
-                if of_trait
-                    && (!matches!(tcx.def_kind(local_def_id), DefKind::AssocFn)
-                        || tcx.visibility(local_def_id).is_public()
-                            && (ty_is_pure_pub || may_construct_self))
-                {
-                    // if the impl item is public,
-                    // and the ty may be constructed or can be constructed in foreign crates,
-                    // mark the impl item live
-                    worklist.push((local_def_id, ComesFromAllowExpect::No));
-                } else if !of_trait
-                    && tcx.visibility(local_def_id).is_public()
-                    && (ty_is_pure_pub || may_construct_self)
+                if of_trait && matches!(tcx.def_kind(local_def_id), DefKind::AssocTy)
+                    || tcx.visibility(local_def_id).is_public()
+                        && (ty_is_pure_pub || may_construct_self)
                 {
                     // if the impl item is public,
                     // and the ty may be constructed or can be constructed in foreign crates,
@@ -882,7 +873,7 @@ fn create_and_seed_worklist(
                     // checks impls, impl-items and pub structs with all public fields later
                     match tcx.def_kind(id) {
                         DefKind::Impl { .. } => false,
-                        DefKind::AssocFn => !matches!(tcx.associated_item(id).container, AssocItemContainer::ImplContainer),
+                        DefKind::AssocConst | DefKind::AssocFn => !matches!(tcx.associated_item(id).container, AssocItemContainer::ImplContainer),
                         DefKind::Struct => struct_all_fields_are_public(tcx, id.to_def_id()),
                         _ => true
                     })
@@ -1158,13 +1149,13 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) {
             || (def_kind == DefKind::Trait && live_symbols.contains(&item.owner_id.def_id))
         {
             for &def_id in tcx.associated_item_def_ids(item.owner_id.def_id) {
-                // We have diagnosed unused methods in traits
+                // We have diagnosed unused assoc consts and fns in traits
                 if matches!(def_kind, DefKind::Impl { of_trait: true })
-                    && tcx.def_kind(def_id) == DefKind::AssocFn
+                    && matches!(tcx.def_kind(def_id), DefKind::AssocConst | DefKind::AssocFn)
                     // skip unused public inherent methods,
                     // cause we have diagnosed unconstructed struct
                     || matches!(def_kind, DefKind::Impl { of_trait: false }) && tcx.visibility(def_id).is_public() && ty_ref_to_pub_struct(tcx, tcx.hir().item(item).expect_impl().self_ty)
-                    || def_kind == DefKind::Trait && tcx.def_kind(def_id) != DefKind::AssocFn
+                    || def_kind == DefKind::Trait && tcx.def_kind(def_id) == DefKind::AssocTy
                 {
                     continue;
                 }
diff --git a/tests/ui/lint/dead-code/unused_assoc_const.rs b/tests/ui/lint/dead-code/unused_assoc_const.rs
new file mode 100644
index 000000000000..ab911a0d0acf
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused_assoc_const.rs
@@ -0,0 +1,17 @@
+#![deny(dead_code)]
+
+trait Trait {
+    const I: i32; //~ ERROR associated constant `I` is never used
+
+    fn foo(&self) {}
+}
+
+pub struct T(());
+
+impl Trait for T {
+    const I: i32 = 0;
+}
+
+fn main() {
+    T(()).foo();
+}
diff --git a/tests/ui/lint/dead-code/unused_assoc_const.stderr b/tests/ui/lint/dead-code/unused_assoc_const.stderr
new file mode 100644
index 000000000000..3b15e5f8c31c
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused_assoc_const.stderr
@@ -0,0 +1,16 @@
+error: associated constant `I` is never used
+  --> $DIR/unused_assoc_const.rs:4:11
+   |
+LL | trait Trait {
+   |       ----- associated constant in this trait
+LL |     const I: i32;
+   |           ^
+   |
+note: the lint level is defined here
+  --> $DIR/unused_assoc_const.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to 1 previous error
+