Annotate dead code lint with notes about ignored derived impls

compiler/rustc_passes/src/dead.rs

@@ -3,6 +3,7 @@
 // from live codes are live, and everything else is dead.
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_errors::pluralize;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -47,6 +48,8 @@ struct MarkSymbolVisitor<'tcx> {
     ignore_variant_stack: Vec<DefId>,
     // maps from tuple struct constructors to tuple struct items
     struct_constructors: FxHashMap<LocalDefId, LocalDefId>,
+    // maps from ADTs to ignored derived traits (e.g. Debug and Clone)
+    ignored_derived_traits: FxHashMap<DefId, Vec<DefId>>,
 }
 
 impl<'tcx> MarkSymbolVisitor<'tcx> {
@@ -242,7 +245,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
     /// Automatically generated items marked with `rustc_trivial_field_reads`
     /// will be ignored for the purposes of dead code analysis (see PR #85200
     /// for discussion).
-    fn should_ignore_item(&self, def_id: DefId) -> bool {
+    fn should_ignore_item(&mut self, def_id: DefId) -> bool {
         if let Some(impl_of) = self.tcx.impl_of_method(def_id) {
             if !self.tcx.has_attr(impl_of, sym::automatically_derived) {
                 return false;
@@ -250,6 +253,14 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
 
             if let Some(trait_of) = self.tcx.trait_id_of_impl(impl_of) {
                 if self.tcx.has_attr(trait_of, sym::rustc_trivial_field_reads) {
+                    let trait_ref = self.tcx.impl_trait_ref(impl_of).unwrap();
+                    if let ty::Adt(adt_def, _) = trait_ref.self_ty().kind() {
+                        if let Some(v) = self.ignored_derived_traits.get_mut(&adt_def.did) {
+                            v.push(trait_of);
+                        } else {
+                            self.ignored_derived_traits.insert(adt_def.did, vec![trait_of]);
+                        }
+                    }
                     return true;
                 }
             }
@@ -577,7 +588,7 @@ fn create_and_seed_worklist<'tcx>(
 fn find_live<'tcx>(
     tcx: TyCtxt<'tcx>,
     access_levels: &privacy::AccessLevels,
-) -> FxHashSet<LocalDefId> {
+) -> (FxHashSet<LocalDefId>, FxHashMap<DefId, Vec<DefId>>) {
     let (worklist, struct_constructors) = create_and_seed_worklist(tcx, access_levels);
     let mut symbol_visitor = MarkSymbolVisitor {
         worklist,
@@ -590,14 +601,16 @@ fn find_live<'tcx>(
         pub_visibility: false,
         ignore_variant_stack: vec![],
         struct_constructors,
+        ignored_derived_traits: FxHashMap::default(),
     };
     symbol_visitor.mark_live_symbols();
-    symbol_visitor.live_symbols
+    (symbol_visitor.live_symbols, symbol_visitor.ignored_derived_traits)
 }
 
 struct DeadVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
     live_symbols: FxHashSet<LocalDefId>,
+    ignored_derived_traits: FxHashMap<DefId, Vec<DefId>>,
 }
 
 impl<'tcx> DeadVisitor<'tcx> {
@@ -666,7 +679,34 @@ impl<'tcx> DeadVisitor<'tcx> {
             self.tcx.struct_span_lint_hir(lint::builtin::DEAD_CODE, id, span, |lint| {
                 let def_id = self.tcx.hir().local_def_id(id);
                 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
-                lint.build(&format!("{} is never {}: `{}`", descr, participle, name)).emit()
+                let mut err = lint.build(&format!("{} is never {}: `{}`", descr, participle, name));
+                let hir = self.tcx.hir();
+                if let Some(encl_scope) = hir.get_enclosing_scope(id) {
+                    if let Some(encl_def_id) = hir.opt_local_def_id(encl_scope) {
+                        if let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id.to_def_id()) {
+                            let traits_str = ign_traits
+                                .iter()
+                                .map(|t| format!("`{}`", self.tcx.item_name(*t))).collect::<Vec<_>>()
+                                .join(" and ");
+                            let plural_s = pluralize!(ign_traits.len());
+                            let article = if ign_traits.len() > 1 { "" } else { "a " };
+                            let is_are = if ign_traits.len() > 1 { "these are" } else { "this is" };
+                            let msg = format!("`{}` has {}derived impl{} for the trait{} {}, but {} ignored during dead code analysis",
+                                              self.tcx.item_name(encl_def_id.to_def_id()),
+                                              article,
+                                              plural_s,
+                                              plural_s,
+                                              traits_str,
+                                              is_are);
+                            if let Some(span) = self.tcx.def_ident_span(encl_def_id) {
+                                err.span_note(span, &msg);
+                            } else {
+                                err.note(&msg);
+                            }
+                        }
+                    }
+                }
+                err.emit();
             });
         }
     }
@@ -796,7 +836,7 @@ impl<'tcx> Visitor<'tcx> for DeadVisitor<'tcx> {
 
 pub fn check_crate(tcx: TyCtxt<'_>) {
     let access_levels = &tcx.privacy_access_levels(());
-    let live_symbols = find_live(tcx, access_levels);
+    let (live_symbols, ignored_derived_traits) = find_live(tcx, access_levels);
-    let mut visitor = DeadVisitor { tcx, live_symbols };
+    let mut visitor = DeadVisitor { tcx, live_symbols, ignored_derived_traits };
     tcx.hir().walk_toplevel_module(&mut visitor);
 }

src/test/ui/derive-uninhabited-enum-38885.stderr

@@ -5,6 +5,11 @@ LL |     Void(Void),
    |     ^^^^^^^^^^
    |
    = note: `-W dead-code` implied by `-W unused`
+note: `Foo` has a derived impl for the trait `Debug`, but this is ignored during dead code analysis
+  --> $DIR/derive-uninhabited-enum-38885.rs:11:6
+   |
+LL | enum Foo {
+   |      ^^^
 
 warning: 1 warning emitted
 

src/test/ui/derives/clone-debug-dead-code.stderr

@@ -15,18 +15,36 @@ error: field is never read: `f`
    |
 LL | struct B { f: () }
    |            ^^^^^
+   |
+note: `B` has a derived impl for the trait `Clone`, but this is ignored during dead code analysis
+  --> $DIR/clone-debug-dead-code.rs:10:8
+   |
+LL | struct B { f: () }
+   |        ^
 
 error: field is never read: `f`
   --> $DIR/clone-debug-dead-code.rs:14:12
    |
 LL | struct C { f: () }
    |            ^^^^^
+   |
+note: `C` has a derived impl for the trait `Debug`, but this is ignored during dead code analysis
+  --> $DIR/clone-debug-dead-code.rs:14:8
+   |
+LL | struct C { f: () }
+   |        ^
 
 error: field is never read: `f`
   --> $DIR/clone-debug-dead-code.rs:18:12
    |
 LL | struct D { f: () }
    |            ^^^^^
+   |
+note: `D` has derived impls for the traits `Clone` and `Debug`, but these are ignored during dead code analysis
+  --> $DIR/clone-debug-dead-code.rs:18:8
+   |
+LL | struct D { f: () }
+   |        ^
 
 error: field is never read: `f`
   --> $DIR/clone-debug-dead-code.rs:21:12

src/test/ui/lint/dead-code/unused-variant.stderr

@@ -9,6 +9,11 @@ note: the lint level is defined here
    |
 LL | #![deny(dead_code)]
    |         ^^^^^^^^^
+note: `Enum` has a derived impl for the trait `Clone`, but this is ignored during dead code analysis
+  --> $DIR/unused-variant.rs:4:6
+   |
+LL | enum Enum {
+   |      ^^^^
 
 error: aborting due to previous error