Extend dead code analysis of impls and impl items to non-ADTs

compiler/rustc_passes/src/dead.rs

@@ -18,7 +18,7 @@ use rustc_hir::{self as hir, ImplItem, ImplItemKind, Node, PatKind, QPath, TyKin
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::middle::privacy::Level;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, AssocItemContainer, TyCtxt};
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::DEAD_CODE;
 use rustc_session::lint::{self, LintExpectationId};
@@ -58,6 +58,8 @@ fn local_adt_def_of_ty<'tcx>(ty: &hir::Ty<'tcx>) -> Option<LocalDefId> {
                 None
             }
         }
+        TyKind::Slice(ty) | TyKind::Array(ty, _) => local_adt_def_of_ty(ty),
+        TyKind::Ptr(ty) | TyKind::Ref(_, ty) => local_adt_def_of_ty(ty.ty),
         _ => None,
     }
 }
@@ -832,6 +834,19 @@ fn create_and_seed_worklist(
             effective_vis
                 .is_public_at_level(Level::Reachable)
                 .then_some(id)
+                .filter(|&id| match tcx.def_kind(id) {
+                    // Check impls of trait and impl items of trait later in `check_impl_or_impl_item_live`,
+                    // because `Self` may be non-ADTs and refer to local ADTs.
+                    DefKind::Impl { of_trait } => !of_trait,
+                    DefKind::AssocFn => {
+                        let assoc_item = tcx.associated_item(id);
+                        // Add reachable trait items and impl items not of traits
+                        // into the worklist directly.
+                        matches!(assoc_item.container, AssocItemContainer::Trait)
+                            || assoc_item.trait_item_def_id.is_none()
+                    }
+                    _ => true,
+                })
                 .map(|id| (id, ComesFromAllowExpect::No))
         })
         // Seed entry point

tests/ui/lint/dead-code/unused-impl-for-non-adts.rs

@@ -0,0 +1,45 @@
+#![deny(dead_code)]
+
+struct Foo; //~ ERROR struct `Foo` is never constructed
+
+trait Trait { //~ ERROR trait `Trait` is never used
+    fn foo(&self) {}
+}
+
+impl Trait for Foo {}
+
+impl Trait for [Foo] {}
+impl<const N: usize> Trait for [Foo; N] {}
+
+impl Trait for *const Foo {}
+impl Trait for *mut Foo {}
+
+impl Trait for &Foo {}
+impl Trait for &&Foo {}
+impl Trait for &mut Foo {}
+
+impl Trait for [&Foo] {}
+impl Trait for &[Foo] {}
+impl Trait for &*const Foo {}
+
+pub trait Trait2 {
+    fn foo(&self) {}
+}
+
+impl Trait2 for Foo {}
+
+impl Trait2 for [Foo] {}
+impl<const N: usize> Trait2 for [Foo; N] {}
+
+impl Trait2 for *const Foo {}
+impl Trait2 for *mut Foo {}
+
+impl Trait2 for &Foo {}
+impl Trait2 for &&Foo {}
+impl Trait2 for &mut Foo {}
+
+impl Trait2 for [&Foo] {}
+impl Trait2 for &[Foo] {}
+impl Trait2 for &*const Foo {}
+
+fn main() {}

tests/ui/lint/dead-code/unused-impl-for-non-adts.stderr

@@ -0,0 +1,20 @@
+error: struct `Foo` is never constructed
+  --> $DIR/unused-impl-for-non-adts.rs:3:8
+   |
+LL | struct Foo;
+   |        ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-impl-for-non-adts.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: trait `Trait` is never used
+  --> $DIR/unused-impl-for-non-adts.rs:5:7
+   |
+LL | trait Trait {
+   |       ^^^^^
+
+error: aborting due to 2 previous errors
+