Detect unused structs which implement private traits

compiler/rustc_middle/src/query/mod.rs

@@ -819,6 +819,32 @@ rustc_queries! {
         desc { |tcx| "comparing impl items against trait for `{}`", tcx.def_path_str(impl_id) }
     }
 
+    /// Maps from associated items on the impl specified by `impl_id`
+    /// to the corresponding associated item on the trait.
+    ///
+    /// For example, with the following code
+    ///
+    /// ```
+    /// struct Type {}
+    ///                         // DefId
+    /// trait Trait {           // trait_id
+    ///     fn f();             // trait_f
+    ///     fn g() {}           // trait_g
+    /// }
+    ///
+    /// impl Trait for Type {   // impl_id
+    ///     fn f() {}           // impl_f
+    ///     fn g() {}           // impl_g
+    /// }
+    /// ```
+    ///
+    /// The map returned for `tcx.impl_item_trait_item_ids(impl_id)` would be
+    ///`{ impl_f: trait_f, impl_g: trait_g }`
+    query impl_item_trait_item_ids(impl_id: DefId) -> &'tcx DefIdMap<DefId> {
+        arena_cache
+        desc { |tcx| "comparing impl items against trait for `{}`", tcx.def_path_str(impl_id) }
+    }
+
     /// Given `fn_def_id` of a trait or of an impl that implements a given trait:
     /// if `fn_def_id` is the def id of a function defined inside a trait, then it creates and returns
     /// the associated items that correspond to each impl trait in return position for that trait.

compiler/rustc_passes/src/dead.rs

@@ -427,10 +427,11 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                             && let ItemKind::Impl(impl_ref) =
                                 self.tcx.hir().expect_item(local_impl_id).kind
                         {
-                            if self.tcx.visibility(trait_id).is_public()
-                                && matches!(trait_item.kind, hir::TraitItemKind::Fn(..))
+                            if matches!(trait_item.kind, hir::TraitItemKind::Fn(..))
                                 && !ty_ref_to_pub_struct(self.tcx, impl_ref.self_ty)
                             {
+                                // skip methods of private ty,
+                                // they would be solved in `solve_rest_impl_items`
                                 continue;
                             }
 
@@ -487,32 +488,51 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
 
     fn solve_rest_impl_items(&mut self, mut unsolved_impl_items: Vec<(hir::ItemId, LocalDefId)>) {
         let mut ready;
-        (ready, unsolved_impl_items) = unsolved_impl_items
-            .into_iter()
-            .partition(|&(impl_id, _)| self.impl_item_with_used_self(impl_id));
+        (ready, unsolved_impl_items) =
+            unsolved_impl_items.into_iter().partition(|&(impl_id, impl_item_id)| {
+                self.impl_item_with_used_self_and_trait_term(impl_id, impl_item_id)
+            });
 
         while !ready.is_empty() {
             self.worklist =
                 ready.into_iter().map(|(_, id)| (id, ComesFromAllowExpect::No)).collect();
             self.mark_live_symbols();
 
-            (ready, unsolved_impl_items) = unsolved_impl_items
-                .into_iter()
-                .partition(|&(impl_id, _)| self.impl_item_with_used_self(impl_id));
+            (ready, unsolved_impl_items) =
+                unsolved_impl_items.into_iter().partition(|&(impl_id, impl_item_id)| {
+                    self.impl_item_with_used_self_and_trait_term(impl_id, impl_item_id)
+                });
         }
     }
 
-    fn impl_item_with_used_self(&mut self, impl_id: hir::ItemId) -> bool {
+    fn impl_item_with_used_self_and_trait_term(
+        &mut self,
+        impl_id: hir::ItemId,
+        impl_item_id: LocalDefId,
+    ) -> bool {
         if let TyKind::Path(hir::QPath::Resolved(_, path)) =
             self.tcx.hir().item(impl_id).expect_impl().self_ty.kind
             && let Res::Def(def_kind, def_id) = path.res
             && let Some(local_def_id) = def_id.as_local()
             && matches!(def_kind, DefKind::Struct | DefKind::Enum | DefKind::Union)
         {
-            self.live_symbols.contains(&local_def_id)
-        } else {
-            false
+            if self.tcx.visibility(impl_item_id).is_public() {
+                // for the public method, we don't know the trait item is used or not,
+                // so we mark the method live if the self is used
+                return self.live_symbols.contains(&local_def_id);
+            } else if let Some(trait_item_id) = self
+                .tcx
+                .impl_item_trait_item_ids(impl_id.owner_id.to_def_id())
+                .get(&impl_item_id.to_def_id())
+                && let Some(local_id) = trait_item_id.as_local()
+            {
+                // for the private method, we can know the trait item is ued or not,
+                // so we mark the method live if the self is used and the trait item is used
+                return self.live_symbols.contains(&local_id)
+                    && self.live_symbols.contains(&local_def_id);
+            }
         }
+        false
     }
 }
 
@@ -746,20 +766,22 @@ fn check_item<'tcx>(
                         matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None);
                 }
 
-                // for impl trait blocks, mark associate functions live if the trait is public
+                // 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_pub || may_construct_self))
                 {
                     worklist.push((local_def_id, ComesFromAllowExpect::No));
-                } else if of_trait && tcx.visibility(local_def_id).is_public() {
-                    // pub method && private ty & methods not construct self
-                    unsolved_impl_items.push((id, local_def_id));
                 } else if let Some(comes_from_allow) =
                     has_allow_dead_code_or_lang_attr(tcx, local_def_id)
                 {
                     worklist.push((local_def_id, comes_from_allow));
+                } else if of_trait {
+                    // private method || public method not constructs self
+                    unsolved_impl_items.push((id, local_def_id));
                 }
             }
         }

compiler/rustc_ty_utils/src/assoc.rs

@@ -15,6 +15,7 @@ pub(crate) fn provide(providers: &mut Providers) {
         associated_types_for_impl_traits_in_associated_fn,
         associated_type_for_impl_trait_in_trait,
         impl_item_implementor_ids,
+        impl_item_trait_item_ids,
         ..*providers
     };
 }
@@ -92,6 +93,13 @@ fn impl_item_implementor_ids(tcx: TyCtxt<'_>, impl_id: DefId) -> DefIdMap<DefId>
         .collect()
 }
 
+fn impl_item_trait_item_ids(tcx: TyCtxt<'_>, impl_id: DefId) -> DefIdMap<DefId> {
+    tcx.associated_items(impl_id)
+        .in_definition_order()
+        .filter_map(|item| item.trait_item_def_id.map(|trait_item| (item.def_id, trait_item)))
+        .collect()
+}
+
 fn associated_item(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::AssocItem {
     let id = tcx.local_def_id_to_hir_id(def_id);
     let parent_def_id = tcx.hir().get_parent_item(id);

tests/ui/lint/dead-code/unused-adt-impls-trait.rs

@@ -0,0 +1,34 @@
+#![deny(dead_code)]
+
+struct Used;
+struct Unused; //~ ERROR struct `Unused` is never constructed
+
+pub trait PubTrait {
+    fn foo(&self) -> Self;
+}
+
+impl PubTrait for Used {
+    fn foo(&self) -> Self { Used }
+}
+
+impl PubTrait for Unused {
+    fn foo(&self) -> Self { Unused }
+}
+
+trait PriTrait {
+    fn foo(&self) -> Self;
+}
+
+impl PriTrait for Used {
+    fn foo(&self) -> Self { Used }
+}
+
+impl PriTrait for Unused {
+    fn foo(&self) -> Self { Unused }
+}
+
+fn main() {
+    let t = Used;
+    let _t = <Used as PubTrait>::foo(&t);
+    let _t = <Used as PriTrait>::foo(&t);
+}

tests/ui/lint/dead-code/unused-adt-impls-trait.stderr

@@ -0,0 +1,14 @@
+error: struct `Unused` is never constructed
+  --> $DIR/unused-adt-impls-trait.rs:4:8
+   |
+LL | struct Unused;
+   |        ^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-adt-impls-trait.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/rust-2021/inherent-dyn-collision.fixed

@@ -25,6 +25,7 @@ mod inner {
     // having a struct of the same name as the trait in-scope, while *also*
     // implementing the trait for that struct but **without** importing the
     // trait itself into scope
+    #[allow(dead_code)]
     struct TryIntoU32;
 
     impl super::TryIntoU32 for TryIntoU32 {

tests/ui/rust-2021/inherent-dyn-collision.rs

@@ -25,6 +25,7 @@ mod inner {
     // having a struct of the same name as the trait in-scope, while *also*
     // implementing the trait for that struct but **without** importing the
     // trait itself into scope
+    #[allow(dead_code)]
     struct TryIntoU32;
 
     impl super::TryIntoU32 for TryIntoU32 {

tests/ui/rust-2021/inherent-dyn-collision.stderr

@@ -1,5 +1,5 @@
 warning: trait method `try_into` will become ambiguous in Rust 2021
-  --> $DIR/inherent-dyn-collision.rs:41:9
+  --> $DIR/inherent-dyn-collision.rs:42:9
    |
 LL |         get_dyn_trait().try_into().unwrap()
    |         ^^^^^^^^^^^^^^^ help: disambiguate the method call: `(&*get_dyn_trait())`