Auto merge of #77274 - tmiasko:liveness-cnd, r=lcnr

Liveness refactoring continued

* Move body_owner field from IrMaps to Liveness (the only user of the field).
* Use upvars instead of FnKind to check for closures (avoids FnKind, will be useful when checking all bodies, not just fns).
* Use visit_param to add variables corresponding to params.
* Store upvars_mentioned inside Liveness struct.
* Inline visitor implementation for IrMaps, avoiding unnecessary indirection.
* Test interaction with automatically_derived attribute (not covered by any of existing tests).

No functional changes intended.

3 files changed, 244 insertions, 218 deletions

diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
index 9eac2f79e41..eb2f40a82d1 100644
--- a/compiler/rustc_passes/src/liveness.rs
+++ b/compiler/rustc_passes/src/liveness.rs
@@ -138,40 +138,8 @@ fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
     }
 }
 
-impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
-    type Map = Map<'tcx>;
-
-    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
-        NestedVisitorMap::OnlyBodies(self.tcx.hir())
-    }
-
-    fn visit_fn(
-        &mut self,
-        fk: FnKind<'tcx>,
-        fd: &'tcx hir::FnDecl<'tcx>,
-        b: hir::BodyId,
-        s: Span,
-        id: HirId,
-    ) {
-        visit_fn(self, fk, fd, b, s, id);
-    }
-
-    fn visit_local(&mut self, l: &'tcx hir::Local<'tcx>) {
-        visit_local(self, l);
-    }
-    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
-        visit_expr(self, ex);
-    }
-    fn visit_arm(&mut self, a: &'tcx hir::Arm<'tcx>) {
-        visit_arm(self, a);
-    }
-}
-
 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
-    tcx.hir().visit_item_likes_in_module(
-        module_def_id,
-        &mut IrMaps::new(tcx, module_def_id).as_deep_visitor(),
-    );
+    tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor());
 }
 
 pub fn provide(providers: &mut Providers) {
@@ -227,7 +195,6 @@ enum VarKind {
 
 struct IrMaps<'tcx> {
     tcx: TyCtxt<'tcx>,
-    body_owner: LocalDefId,
     live_node_map: HirIdMap<LiveNode>,
     variable_map: HirIdMap<Variable>,
     capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
@@ -236,10 +203,9 @@ struct IrMaps<'tcx> {
 }
 
 impl IrMaps<'tcx> {
-    fn new(tcx: TyCtxt<'tcx>, body_owner: LocalDefId) -> IrMaps<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>) -> IrMaps<'tcx> {
         IrMaps {
             tcx,
-            body_owner,
             live_node_map: HirIdMap::default(),
             variable_map: HirIdMap::default(),
             capture_info_map: Default::default(),
@@ -302,44 +268,117 @@ impl IrMaps<'tcx> {
     fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
         self.capture_info_map.insert(hir_id, Rc::new(cs));
     }
-}
 
-fn visit_fn<'tcx>(
-    ir: &mut IrMaps<'tcx>,
-    fk: FnKind<'tcx>,
-    decl: &'tcx hir::FnDecl<'tcx>,
-    body_id: hir::BodyId,
-    sp: Span,
-    id: hir::HirId,
-) {
-    debug!("visit_fn {:?}", id);
-
-    // swap in a new set of IR maps for this function body:
-    let def_id = ir.tcx.hir().local_def_id(id);
-    let mut fn_maps = IrMaps::new(ir.tcx, def_id);
-
-    // Don't run unused pass for #[derive()]
-    if let FnKind::Method(..) = fk {
-        let parent = ir.tcx.hir().get_parent_item(id);
-        if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
-            if i.attrs.iter().any(|a| ir.tcx.sess.check_name(a, sym::automatically_derived)) {
-                return;
+    fn add_from_pat(&mut self, pat: &hir::Pat<'tcx>) {
+        // For struct patterns, take note of which fields used shorthand
+        // (`x` rather than `x: x`).
+        let mut shorthand_field_ids = HirIdSet::default();
+        let mut pats = VecDeque::new();
+        pats.push_back(pat);
+        while let Some(pat) = pats.pop_front() {
+            use rustc_hir::PatKind::*;
+            match &pat.kind {
+                Binding(.., inner_pat) => {
+                    pats.extend(inner_pat.iter());
+                }
+                Struct(_, fields, _) => {
+                    let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
+                    shorthand_field_ids.extend(ids);
+                }
+                Ref(inner_pat, _) | Box(inner_pat) => {
+                    pats.push_back(inner_pat);
+                }
+                TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
+                    pats.extend(inner_pats.iter());
+                }
+                Slice(pre_pats, inner_pat, post_pats) => {
+                    pats.extend(pre_pats.iter());
+                    pats.extend(inner_pat.iter());
+                    pats.extend(post_pats.iter());
+                }
+                _ => {}
             }
         }
+
+        pat.each_binding(|_, hir_id, _, ident| {
+            self.add_live_node_for_node(hir_id, VarDefNode(ident.span));
+            self.add_variable(Local(LocalInfo {
+                id: hir_id,
+                name: ident.name,
+                is_shorthand: shorthand_field_ids.contains(&hir_id),
+            }));
+        });
     }
+}
 
-    debug!("creating fn_maps: {:p}", &fn_maps);
+impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
+    type Map = Map<'tcx>;
 
-    let body = ir.tcx.hir().body(body_id);
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::OnlyBodies(self.tcx.hir())
+    }
 
-    if let Some(upvars) = ir.tcx.upvars_mentioned(def_id) {
-        for (&var_hir_id, _upvar) in upvars {
-            let var_name = ir.tcx.hir().name(var_hir_id);
-            fn_maps.add_variable(Upvar(var_hir_id, var_name));
+    fn visit_fn(
+        &mut self,
+        fk: FnKind<'tcx>,
+        decl: &'tcx hir::FnDecl<'tcx>,
+        body_id: hir::BodyId,
+        sp: Span,
+        id: HirId,
+    ) {
+        debug!("visit_fn {:?}", id);
+
+        // swap in a new set of IR maps for this function body:
+        let def_id = self.tcx.hir().local_def_id(id);
+        let mut fn_maps = IrMaps::new(self.tcx);
+
+        // Don't run unused pass for #[derive()]
+        if let FnKind::Method(..) = fk {
+            let parent = self.tcx.hir().get_parent_item(id);
+            if let Some(Node::Item(i)) = self.tcx.hir().find(parent) {
+                if i.attrs.iter().any(|a| self.tcx.sess.check_name(a, sym::automatically_derived)) {
+                    return;
+                }
+            }
         }
+
+        debug!("creating fn_maps: {:p}", &fn_maps);
+
+        let body = self.tcx.hir().body(body_id);
+
+        if let Some(upvars) = self.tcx.upvars_mentioned(def_id) {
+            for (&var_hir_id, _upvar) in upvars {
+                let var_name = self.tcx.hir().name(var_hir_id);
+                fn_maps.add_variable(Upvar(var_hir_id, var_name));
+            }
+        }
+
+        // gather up the various local variables, significant expressions,
+        // and so forth:
+        intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
+
+        // compute liveness
+        let mut lsets = Liveness::new(&mut fn_maps, def_id);
+        let entry_ln = lsets.compute(&body, sp, id);
+        lsets.log_liveness(entry_ln, id);
+
+        // check for various error conditions
+        lsets.visit_body(body);
+        lsets.warn_about_unused_upvars(entry_ln);
+        lsets.warn_about_unused_args(body, entry_ln);
+    }
+
+    fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
+        self.add_from_pat(&local.pat);
+        intravisit::walk_local(self, local);
+    }
+
+    fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
+        self.add_from_pat(&arm.pat);
+        intravisit::walk_arm(self, arm);
     }
 
-    for param in body.params {
+    fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
         let is_shorthand = match param.pat.kind {
             rustc_hir::PatKind::Struct(..) => true,
             _ => false,
@@ -350,150 +389,83 @@ fn visit_fn<'tcx>(
             } else {
                 Param(hir_id, ident.name)
             };
-            fn_maps.add_variable(var);
-        })
+            self.add_variable(var);
+        });
+        intravisit::walk_param(self, param);
     }
 
-    // gather up the various local variables, significant expressions,
-    // and so forth:
-    intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
-
-    // compute liveness
-    let mut lsets = Liveness::new(&mut fn_maps, def_id);
-    let entry_ln = lsets.compute(fk, &body, sp, id);
-    lsets.log_liveness(entry_ln, id);
-
-    // check for various error conditions
-    lsets.visit_body(body);
-    lsets.warn_about_unused_upvars(entry_ln);
-    lsets.warn_about_unused_args(body, entry_ln);
-}
-
-fn add_from_pat(ir: &mut IrMaps<'_>, pat: &hir::Pat<'_>) {
-    // For struct patterns, take note of which fields used shorthand
-    // (`x` rather than `x: x`).
-    let mut shorthand_field_ids = HirIdSet::default();
-    let mut pats = VecDeque::new();
-    pats.push_back(pat);
-    while let Some(pat) = pats.pop_front() {
-        use rustc_hir::PatKind::*;
-        match &pat.kind {
-            Binding(.., inner_pat) => {
-                pats.extend(inner_pat.iter());
-            }
-            Struct(_, fields, _) => {
-                let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
-                shorthand_field_ids.extend(ids);
-            }
-            Ref(inner_pat, _) | Box(inner_pat) => {
-                pats.push_back(inner_pat);
-            }
-            TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
-                pats.extend(inner_pats.iter());
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        match expr.kind {
+            // live nodes required for uses or definitions of variables:
+            hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
+                debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
+                if let Res::Local(_var_hir_id) = path.res {
+                    self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
+                }
+                intravisit::walk_expr(self, expr);
             }
-            Slice(pre_pats, inner_pat, post_pats) => {
-                pats.extend(pre_pats.iter());
-                pats.extend(inner_pat.iter());
-                pats.extend(post_pats.iter());
+            hir::ExprKind::Closure(..) => {
+                // Interesting control flow (for loops can contain labeled
+                // breaks or continues)
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
+
+                // Make a live_node for each captured variable, with the span
+                // being the location that the variable is used.  This results
+                // in better error messages than just pointing at the closure
+                // construction site.
+                let mut call_caps = Vec::new();
+                let closure_def_id = self.tcx.hir().local_def_id(expr.hir_id);
+                if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
+                    call_caps.extend(upvars.iter().map(|(&var_id, upvar)| {
+                        let upvar_ln = self.add_live_node(UpvarNode(upvar.span));
+                        CaptureInfo { ln: upvar_ln, var_hid: var_id }
+                    }));
+                }
+                self.set_captures(expr.hir_id, call_caps);
+                intravisit::walk_expr(self, expr);
             }
-            _ => {}
-        }
-    }
-
-    pat.each_binding(|_, hir_id, _, ident| {
-        ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
-        ir.add_variable(Local(LocalInfo {
-            id: hir_id,
-            name: ident.name,
-            is_shorthand: shorthand_field_ids.contains(&hir_id),
-        }));
-    });
-}
-
-fn visit_local<'tcx>(ir: &mut IrMaps<'tcx>, local: &'tcx hir::Local<'tcx>) {
-    add_from_pat(ir, &local.pat);
-    intravisit::walk_local(ir, local);
-}
 
-fn visit_arm<'tcx>(ir: &mut IrMaps<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
-    add_from_pat(ir, &arm.pat);
-    intravisit::walk_arm(ir, arm);
-}
-
-fn visit_expr<'tcx>(ir: &mut IrMaps<'tcx>, expr: &'tcx Expr<'tcx>) {
-    match expr.kind {
-        // live nodes required for uses or definitions of variables:
-        hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
-            debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
-            if let Res::Local(_var_hir_id) = path.res {
-                ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
+            // live nodes required for interesting control flow:
+            hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
+                intravisit::walk_expr(self, expr);
             }
-            intravisit::walk_expr(ir, expr);
-        }
-        hir::ExprKind::Closure(..) => {
-            // Interesting control flow (for loops can contain labeled
-            // breaks or continues)
-            ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
-
-            // Make a live_node for each captured variable, with the span
-            // being the location that the variable is used.  This results
-            // in better error messages than just pointing at the closure
-            // construction site.
-            let mut call_caps = Vec::new();
-            let closure_def_id = ir.tcx.hir().local_def_id(expr.hir_id);
-            if let Some(upvars) = ir.tcx.upvars_mentioned(closure_def_id) {
-                call_caps.extend(upvars.iter().map(|(&var_id, upvar)| {
-                    let upvar_ln = ir.add_live_node(UpvarNode(upvar.span));
-                    CaptureInfo { ln: upvar_ln, var_hid: var_id }
-                }));
+            hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
+                intravisit::walk_expr(self, expr);
             }
-            ir.set_captures(expr.hir_id, call_caps);
-            let old_body_owner = ir.body_owner;
-            ir.body_owner = closure_def_id;
-            intravisit::walk_expr(ir, expr);
-            ir.body_owner = old_body_owner;
-        }
 
-        // live nodes required for interesting control flow:
-        hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
-            ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
-            intravisit::walk_expr(ir, expr);
-        }
-        hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
-            ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
-            intravisit::walk_expr(ir, expr);
-        }
-
-        // otherwise, live nodes are not required:
-        hir::ExprKind::Index(..)
-        | hir::ExprKind::Field(..)
-        | hir::ExprKind::Array(..)
-        | hir::ExprKind::Call(..)
-        | hir::ExprKind::MethodCall(..)
-        | hir::ExprKind::Tup(..)
-        | hir::ExprKind::Binary(..)
-        | hir::ExprKind::AddrOf(..)
-        | hir::ExprKind::Cast(..)
-        | hir::ExprKind::DropTemps(..)
-        | hir::ExprKind::Unary(..)
-        | hir::ExprKind::Break(..)
-        | hir::ExprKind::Continue(_)
-        | hir::ExprKind::Lit(_)
-        | hir::ExprKind::Ret(..)
-        | hir::ExprKind::Block(..)
-        | hir::ExprKind::Assign(..)
-        | hir::ExprKind::AssignOp(..)
-        | hir::ExprKind::Struct(..)
-        | hir::ExprKind::Repeat(..)
-        | hir::ExprKind::InlineAsm(..)
-        | hir::ExprKind::LlvmInlineAsm(..)
-        | hir::ExprKind::Box(..)
-        | hir::ExprKind::Yield(..)
-        | hir::ExprKind::Type(..)
-        | hir::ExprKind::Err
-        | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
-        | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
-            intravisit::walk_expr(ir, expr);
+            // otherwise, live nodes are not required:
+            hir::ExprKind::Index(..)
+            | hir::ExprKind::Field(..)
+            | hir::ExprKind::Array(..)
+            | hir::ExprKind::Call(..)
+            | hir::ExprKind::MethodCall(..)
+            | hir::ExprKind::Tup(..)
+            | hir::ExprKind::Binary(..)
+            | hir::ExprKind::AddrOf(..)
+            | hir::ExprKind::Cast(..)
+            | hir::ExprKind::DropTemps(..)
+            | hir::ExprKind::Unary(..)
+            | hir::ExprKind::Break(..)
+            | hir::ExprKind::Continue(_)
+            | hir::ExprKind::Lit(_)
+            | hir::ExprKind::Ret(..)
+            | hir::ExprKind::Block(..)
+            | hir::ExprKind::Assign(..)
+            | hir::ExprKind::AssignOp(..)
+            | hir::ExprKind::Struct(..)
+            | hir::ExprKind::Repeat(..)
+            | hir::ExprKind::InlineAsm(..)
+            | hir::ExprKind::LlvmInlineAsm(..)
+            | hir::ExprKind::Box(..)
+            | hir::ExprKind::Yield(..)
+            | hir::ExprKind::Type(..)
+            | hir::ExprKind::Err
+            | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
+            | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
+                intravisit::walk_expr(self, expr);
+            }
         }
     }
 }
@@ -605,8 +577,10 @@ const ACC_USE: u32 = 4;
 
 struct Liveness<'a, 'tcx> {
     ir: &'a mut IrMaps<'tcx>,
+    body_owner: LocalDefId,
     typeck_results: &'a ty::TypeckResults<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
+    upvars: Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>>,
     successors: IndexVec<LiveNode, LiveNode>,
     rwu_table: RWUTable,
 
@@ -626,9 +600,10 @@ struct Liveness<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> Liveness<'a, 'tcx> {
-    fn new(ir: &'a mut IrMaps<'tcx>, def_id: LocalDefId) -> Liveness<'a, 'tcx> {
-        let typeck_results = ir.tcx.typeck(def_id);
-        let param_env = ir.tcx.param_env(def_id);
+    fn new(ir: &'a mut IrMaps<'tcx>, body_owner: LocalDefId) -> Liveness<'a, 'tcx> {
+        let typeck_results = ir.tcx.typeck(body_owner);
+        let param_env = ir.tcx.param_env(body_owner);
+        let upvars = ir.tcx.upvars_mentioned(body_owner);
 
         let closure_ln = ir.add_live_node(ClosureNode);
         let exit_ln = ir.add_live_node(ExitNode);
@@ -638,8 +613,10 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
         Liveness {
             ir,
+            body_owner,
             typeck_results,
             param_env,
+            upvars,
             successors: IndexVec::from_elem_n(INVALID_NODE, num_live_nodes),
             rwu_table: RWUTable::new(num_live_nodes * num_vars),
             closure_ln,
@@ -868,13 +845,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         self.rwu_table.assign_unpacked(idx, rwu);
     }
 
-    fn compute(
-        &mut self,
-        fk: FnKind<'_>,
-        body: &hir::Body<'_>,
-        span: Span,
-        id: hir::HirId,
-    ) -> LiveNode {
+    fn compute(&mut self, body: &hir::Body<'_>, span: Span, id: hir::HirId) -> LiveNode {
         debug!("compute: using id for body, {:?}", body.value);
 
         // # Liveness of captured variables
@@ -893,12 +864,12 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         // if they are live on the entry to the closure, since only the closure
         // itself can access them on subsequent calls.
 
-        if let Some(upvars) = self.ir.tcx.upvars_mentioned(self.ir.body_owner) {
+        if let Some(upvars) = self.upvars {
             // Mark upvars captured by reference as used after closure exits.
             for (&var_hir_id, upvar) in upvars.iter().rev() {
                 let upvar_id = ty::UpvarId {
                     var_path: ty::UpvarPath { hir_id: var_hir_id },
-                    closure_expr_id: self.ir.body_owner,
+                    closure_expr_id: self.body_owner,
                 };
                 match self.typeck_results.upvar_capture(upvar_id) {
                     ty::UpvarCapture::ByRef(_) => {
@@ -912,9 +883,11 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
         let succ = self.propagate_through_expr(&body.value, self.exit_ln);
 
-        match fk {
-            FnKind::Method(..) | FnKind::ItemFn(..) => return succ,
-            FnKind::Closure(..) => {}
+        if self.upvars.is_none() {
+            // Either not a closure, or closure without any captured variables.
+            // No need to determine liveness of captured variables, since there
+            // are none.
+            return succ;
         }
 
         let ty = self.typeck_results.node_type(id);
@@ -926,7 +899,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             },
             ty::Generator(..) => return succ,
             _ => {
-                span_bug!(span, "type of closure expr {:?} is not a closure {:?}", id, ty,);
+                span_bug!(span, "{} has upvars so it should have a closure type: {:?}", id, ty);
             }
         };
 
@@ -1565,7 +1538,7 @@ impl<'tcx> Liveness<'_, 'tcx> {
     }
 
     fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
-        let upvars = match self.ir.tcx.upvars_mentioned(self.ir.body_owner) {
+        let upvars = match self.upvars {
             None => return,
             Some(upvars) => upvars,
         };
@@ -1573,7 +1546,7 @@ impl<'tcx> Liveness<'_, 'tcx> {
             let var = self.variable(var_hir_id, upvar.span);
             let upvar_id = ty::UpvarId {
                 var_path: ty::UpvarPath { hir_id: var_hir_id },
-                closure_expr_id: self.ir.body_owner,
+                closure_expr_id: self.body_owner,
             };
             match self.typeck_results.upvar_capture(upvar_id) {
                 ty::UpvarCapture::ByValue(_) => {}
diff --git a/src/test/ui/liveness/liveness-derive.rs b/src/test/ui/liveness/liveness-derive.rs
new file mode 100644
index 00000000000..66d0b7090ff
--- /dev/null
+++ b/src/test/ui/liveness/liveness-derive.rs
@@ -0,0 +1,38 @@
+// Test for interaction between #[automatically_derived] attribute used by
+// built-in derives and lints generated by liveness pass.
+//
+// edition:2018
+// check-pass
+#![warn(unused)]
+
+pub trait T: Sized {
+    const N: usize;
+    fn t(&self) -> Self;
+}
+
+impl T for u32 {
+    const N: usize = {
+        let a = 0; // FIXME should warn about unused variable
+        4
+    };
+
+    fn t(&self) -> Self {
+        let b = 16; //~ WARN unused variable: `b`
+        0
+    }
+}
+
+#[automatically_derived]
+impl T for i32 {
+    const N: usize = {
+        let c = 0;
+        4
+    };
+
+    fn t(&self) -> Self {
+        let d = 17;
+        0
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/liveness/liveness-derive.stderr b/src/test/ui/liveness/liveness-derive.stderr
new file mode 100644
index 00000000000..d4f45a0a313
--- /dev/null
+++ b/src/test/ui/liveness/liveness-derive.stderr
@@ -0,0 +1,15 @@
+warning: unused variable: `b`
+  --> $DIR/liveness-derive.rs:20:13
+   |
+LL |         let b = 16;
+   |             ^ help: if this is intentional, prefix it with an underscore: `_b`
+   |
+note: the lint level is defined here
+  --> $DIR/liveness-derive.rs:6:9
+   |
+LL | #![warn(unused)]
+   |         ^^^^^^
+   = note: `#[warn(unused_variables)]` implied by `#[warn(unused)]`
+
+warning: 1 warning emitted
+