1 use std::collections::VecDeque;
3 use clippy_utils::diagnostics::span_lint_and_sugg;
4 use itertools::{izip, Itertools};
5 use rustc_ast::{walk_list, Label, Mutability};
6 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
7 use rustc_errors::Applicability;
8 use rustc_hir::def::{DefKind, Res};
9 use rustc_hir::def_id::DefId;
10 use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
11 use rustc_hir::intravisit::{walk_expr, FnKind, Visitor};
13 Arm, Block, Body, Expr, ExprKind, Guard, HirId, ImplicitSelfKind, Let, Local, Pat, PatKind, Path, PathSegment,
14 QPath, Stmt, StmtKind, TyKind, UnOp,
16 use rustc_lint::{LateContext, LateLintPass};
18 use rustc_middle::ty::{Ty, TyCtxt, TypeckResults};
19 use rustc_session::{declare_lint_pass, declare_tool_lint};
20 use rustc_span::symbol::kw;
21 use rustc_span::symbol::Ident;
24 declare_clippy_lint! {
26 /// Checks for arguments that are only used in recursion with no side-effects.
28 /// ### Why is this bad?
29 /// It could contain a useless calculation and can make function simpler.
31 /// The arguments can be involved in calculations and assignments but as long as
32 /// the calculations have no side-effects (function calls or mutating dereference)
33 /// and the assigned variables are also only in recursion, it is useless.
35 /// ### Known problems
36 /// In some cases, this would not catch all useless arguments.
39 /// fn foo(a: usize, b: usize) -> usize {
40 /// let f = |x| x + 1;
50 /// For example, the argument `b` is only used in recursion, but the lint would not catch it.
52 /// List of some examples that can not be caught:
53 /// - binary operation of non-primitive types
55 /// - some `break` relative operations
56 /// - struct pattern binding
58 /// Also, when you recurse the function name with path segments, it is not possible to detect.
62 /// fn f(a: usize, b: usize) -> usize {
70 /// # print!("{}", f(1, 1));
75 /// fn f(a: usize) -> usize {
83 /// # print!("{}", f(1));
86 #[clippy::version = "1.60.0"]
87 pub ONLY_USED_IN_RECURSION,
89 "arguments that is only used in recursion can be removed"
91 declare_lint_pass!(OnlyUsedInRecursion => [ONLY_USED_IN_RECURSION]);
93 impl<'tcx> LateLintPass<'tcx> for OnlyUsedInRecursion {
96 cx: &LateContext<'tcx>,
98 decl: &'tcx rustc_hir::FnDecl<'tcx>,
99 body: &'tcx Body<'tcx>,
103 if let FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) = kind {
104 let def_id = id.owner.to_def_id();
105 let data = cx.tcx.def_path(def_id).data;
108 match data.get(data.len() - 2) {
109 Some(DisambiguatedDefPathData {
110 data: DefPathData::Impl,
112 }) if *disambiguator != 0 => return,
117 let has_self = !matches!(decl.implicit_self, ImplicitSelfKind::None);
119 let ty_res = cx.typeck_results();
120 let param_span = body
124 let mut v = Vec::new();
125 param.pat.each_binding(|_, hir_id, span, ident| {
126 v.push((hir_id, span, ident));
130 .skip(if has_self { 1 } else { 0 })
131 .filter(|(_, _, ident)| !ident.name.as_str().starts_with('_'))
134 let params = body.params.iter().map(|param| param.pat).collect();
136 let mut visitor = SideEffectVisit {
137 graph: FxHashMap::default(),
138 has_side_effect: FxHashSet::default(),
139 ret_vars: Vec::new(),
140 contains_side_effect: false,
141 break_vars: FxHashMap::default(),
145 is_method: matches!(kind, FnKind::Method(..)),
151 visitor.visit_expr(&body.value);
152 let vars = std::mem::take(&mut visitor.ret_vars);
153 // this would set the return variables to side effect
154 visitor.add_side_effect(vars);
156 let mut queue = visitor.has_side_effect.iter().copied().collect::<VecDeque<_>>();
158 // a simple BFS to check all the variables that have side effect
159 while let Some(id) = queue.pop_front() {
160 if let Some(next) = visitor.graph.get(&id) {
162 if !visitor.has_side_effect.contains(i) {
163 visitor.has_side_effect.insert(*i);
170 for (id, span, ident) in param_span {
171 // if the variable is not used in recursion, it would be marked as unused
172 if !visitor.has_side_effect.contains(&id) {
173 let mut queue = VecDeque::new();
174 let mut visited = FxHashSet::default();
178 // a simple BFS to check the graph can reach to itself
179 // if it can't, it means the variable is never used in recursion
180 while let Some(id) = queue.pop_front() {
181 if let Some(next) = visitor.graph.get(&id) {
183 if !visited.contains(i) {
191 if visited.contains(&id) {
194 ONLY_USED_IN_RECURSION,
196 "parameter is only used in recursion",
197 "if this is intentional, prefix with an underscore",
198 format!("_{}", ident.name.as_str()),
199 Applicability::MaybeIncorrect,
208 pub fn is_primitive(ty: Ty<'_>) -> bool {
210 ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str => true,
211 ty::Ref(_, t, _) => is_primitive(*t),
216 pub fn is_array(ty: Ty<'_>) -> bool {
218 ty::Array(..) | ty::Slice(..) => true,
219 ty::Ref(_, t, _) => is_array(*t),
224 /// This builds the graph of side effect.
225 /// The edge `a -> b` means if `a` has side effect, `b` will have side effect.
227 /// There are some exmaple in following code:
230 /// let a = b; // a -> b
231 /// let (c, d) = (a, b); // c -> b, d -> b
233 /// let e = if a == 0 { // e -> a
239 pub struct SideEffectVisit<'tcx> {
240 graph: FxHashMap<HirId, FxHashSet<HirId>>,
241 has_side_effect: FxHashSet<HirId>,
242 // bool for if the variable was dereferenced from mutable reference
243 ret_vars: Vec<(HirId, bool)>,
244 contains_side_effect: bool,
246 break_vars: FxHashMap<Ident, Vec<(HirId, bool)>>,
247 params: Vec<&'tcx Pat<'tcx>>,
252 ty_res: &'tcx TypeckResults<'tcx>,
253 ty_ctx: TyCtxt<'tcx>,
256 impl<'tcx> Visitor<'tcx> for SideEffectVisit<'tcx> {
257 fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
258 b.stmts.iter().for_each(|stmt| {
259 self.visit_stmt(stmt);
260 self.ret_vars.clear();
262 walk_list!(self, visit_expr, b.expr);
265 fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
267 StmtKind::Local(Local {
268 pat, init: Some(init), ..
270 self.visit_pat_expr(pat, init, false);
272 StmtKind::Item(i) => {
273 let item = self.ty_ctx.hir().item(i);
274 self.visit_item(item);
276 StmtKind::Expr(e) | StmtKind::Semi(e) => self.visit_expr(e),
277 StmtKind::Local(_) => {},
281 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
282 debug_assert!(self.ret_vars.is_empty());
284 ExprKind::Array(exprs) | ExprKind::Tup(exprs) => {
285 self.ret_vars = exprs
288 self.visit_expr(expr);
289 std::mem::take(&mut self.ret_vars)
293 ExprKind::Call(callee, args) => self.visit_fn(callee, args),
294 ExprKind::MethodCall(path, args, _) => self.visit_method_call(path, args),
295 ExprKind::Binary(_, lhs, rhs) => {
296 self.visit_bin_op(lhs, rhs);
298 ExprKind::Unary(op, expr) => self.visit_un_op(op, expr),
299 ExprKind::Let(Let { pat, init, .. }) => self.visit_pat_expr(pat, init, false),
300 ExprKind::If(bind, then_expr, else_expr) => {
301 self.visit_if(bind, then_expr, else_expr);
303 ExprKind::Match(expr, arms, _) => self.visit_match(expr, arms),
304 // since analysing the closure is not easy, just set all variables in it to side-effect
305 ExprKind::Closure(_, _, body_id, _, _) => {
306 let body = self.ty_ctx.hir().body(body_id);
307 self.visit_body(body);
308 let vars = std::mem::take(&mut self.ret_vars);
309 self.add_side_effect(vars);
311 ExprKind::Loop(block, label, _, _) | ExprKind::Block(block, label) => {
312 self.visit_block_label(block, label);
314 ExprKind::Assign(bind, expr, _) => {
315 self.visit_assign(bind, expr);
317 ExprKind::AssignOp(_, bind, expr) => {
318 self.visit_assign(bind, expr);
319 self.visit_bin_op(bind, expr);
321 ExprKind::Field(expr, _) => {
322 self.visit_expr(expr);
323 if matches!(self.ty_res.expr_ty(expr).kind(), ty::Ref(_, _, Mutability::Mut)) {
324 self.ret_vars.iter_mut().for_each(|(_, b)| *b = true);
327 ExprKind::Index(expr, index) => {
328 self.visit_expr(expr);
329 let mut vars = std::mem::take(&mut self.ret_vars);
330 self.visit_expr(index);
331 self.ret_vars.append(&mut vars);
333 if !is_array(self.ty_res.expr_ty(expr)) {
334 self.add_side_effect(self.ret_vars.clone());
335 } else if matches!(self.ty_res.expr_ty(expr).kind(), ty::Ref(_, _, Mutability::Mut)) {
336 self.ret_vars.iter_mut().for_each(|(_, b)| *b = true);
339 ExprKind::Break(dest, Some(expr)) => {
340 self.visit_expr(expr);
341 if let Some(label) = dest.label {
344 .or_insert(Vec::new())
345 .append(&mut self.ret_vars);
347 self.contains_side_effect = true;
349 ExprKind::Ret(Some(expr)) => {
350 self.visit_expr(expr);
351 let vars = std::mem::take(&mut self.ret_vars);
352 self.add_side_effect(vars);
353 self.contains_side_effect = true;
355 ExprKind::Break(_, None) | ExprKind::Continue(_) | ExprKind::Ret(None) => {
356 self.contains_side_effect = true;
358 ExprKind::Struct(_, exprs, expr) => {
359 let mut ret_vars = exprs
362 self.visit_expr(field.expr);
363 std::mem::take(&mut self.ret_vars)
367 walk_list!(self, visit_expr, expr);
368 self.ret_vars.append(&mut ret_vars);
370 _ => walk_expr(self, ex),
374 fn visit_path(&mut self, path: &'tcx Path<'tcx>, _id: HirId) {
375 if let Res::Local(id) = path.res {
376 self.ret_vars.push((id, false));
381 impl<'tcx> SideEffectVisit<'tcx> {
382 fn visit_assign(&mut self, lhs: &'tcx Expr<'tcx>, rhs: &'tcx Expr<'tcx>) {
383 // Just support array and tuple unwrapping for now.
385 // ex) `(a, b) = (c, d);`
386 // The graph would look like this:
390 // This would minimize the connection of the side-effect graph.
391 match (&lhs.kind, &rhs.kind) {
392 (ExprKind::Array(lhs), ExprKind::Array(rhs)) | (ExprKind::Tup(lhs), ExprKind::Tup(rhs)) => {
393 // if not, it is a compile error
394 debug_assert!(lhs.len() == rhs.len());
395 izip!(*lhs, *rhs).for_each(|(lhs, rhs)| self.visit_assign(lhs, rhs));
397 // in other assigns, we have to connect all each other
398 // because they can be connected somehow
400 self.visit_expr(lhs);
401 let lhs_vars = std::mem::take(&mut self.ret_vars);
402 self.visit_expr(rhs);
403 let rhs_vars = std::mem::take(&mut self.ret_vars);
404 self.connect_assign(&lhs_vars, &rhs_vars, false);
409 fn visit_block_label(&mut self, block: &'tcx Block<'tcx>, label: Option<Label>) {
410 self.visit_block(block);
411 let _ = label.and_then(|label| {
413 .remove(&label.ident)
414 .map(|mut break_vars| self.ret_vars.append(&mut break_vars))
418 fn visit_bin_op(&mut self, lhs: &'tcx Expr<'tcx>, rhs: &'tcx Expr<'tcx>) {
419 self.visit_expr(lhs);
420 let mut ret_vars = std::mem::take(&mut self.ret_vars);
421 self.visit_expr(rhs);
422 self.ret_vars.append(&mut ret_vars);
424 // the binary operation between non primitive values are overloaded operators
425 // so they can have side-effects
426 if !is_primitive(self.ty_res.expr_ty(lhs)) || !is_primitive(self.ty_res.expr_ty(rhs)) {
427 self.ret_vars.iter().for_each(|id| {
428 self.has_side_effect.insert(id.0);
430 self.contains_side_effect = true;
434 fn visit_un_op(&mut self, op: UnOp, expr: &'tcx Expr<'tcx>) {
435 self.visit_expr(expr);
436 let ty = self.ty_res.expr_ty(expr);
437 // dereferencing a reference has no side-effect
438 if !is_primitive(ty) && !matches!((op, ty.kind()), (UnOp::Deref, ty::Ref(..))) {
439 self.add_side_effect(self.ret_vars.clone());
442 if matches!((op, ty.kind()), (UnOp::Deref, ty::Ref(_, _, Mutability::Mut))) {
443 self.ret_vars.iter_mut().for_each(|(_, b)| *b = true);
447 fn visit_pat_expr(&mut self, pat: &'tcx Pat<'tcx>, expr: &'tcx Expr<'tcx>, connect_self: bool) {
448 match (&pat.kind, &expr.kind) {
449 (PatKind::Tuple(pats, _), ExprKind::Tup(exprs)) => {
450 self.ret_vars = izip!(*pats, *exprs)
451 .flat_map(|(pat, expr)| {
452 self.visit_pat_expr(pat, expr, connect_self);
453 std::mem::take(&mut self.ret_vars)
457 (PatKind::Slice(front_exprs, _, back_exprs), ExprKind::Array(exprs)) => {
458 let mut vars = izip!(*front_exprs, *exprs)
459 .flat_map(|(pat, expr)| {
460 self.visit_pat_expr(pat, expr, connect_self);
461 std::mem::take(&mut self.ret_vars)
464 self.ret_vars = izip!(back_exprs.iter().rev(), exprs.iter().rev())
465 .flat_map(|(pat, expr)| {
466 self.visit_pat_expr(pat, expr, connect_self);
467 std::mem::take(&mut self.ret_vars)
470 self.ret_vars.append(&mut vars);
473 let mut lhs_vars = Vec::new();
474 pat.each_binding(|_, id, _, _| lhs_vars.push((id, false)));
475 self.visit_expr(expr);
476 let rhs_vars = std::mem::take(&mut self.ret_vars);
477 self.connect_assign(&lhs_vars, &rhs_vars, connect_self);
478 self.ret_vars = rhs_vars;
483 fn visit_fn(&mut self, callee: &'tcx Expr<'tcx>, args: &'tcx [Expr<'tcx>]) {
484 self.visit_expr(callee);
485 let mut ret_vars = std::mem::take(&mut self.ret_vars);
486 self.add_side_effect(ret_vars.clone());
488 let mut is_recursive = false;
492 if let ExprKind::Path(QPath::Resolved(_, path)) = callee.kind;
493 if let Res::Def(DefKind::Fn, def_id) = path.res;
494 if self.fn_def_id == def_id;
501 if !self.has_self && self.is_method;
502 if let ExprKind::Path(QPath::TypeRelative(ty, segment)) = callee.kind;
503 if segment.ident == self.fn_ident;
504 if let TyKind::Path(QPath::Resolved(_, path)) = ty.kind;
505 if let Res::SelfTy{ .. } = path.res;
512 izip!(self.params.clone(), args).for_each(|(pat, expr)| {
513 self.visit_pat_expr(pat, expr, true);
514 self.ret_vars.clear();
517 // This would set arguments used in closure that does not have side-effect.
518 // Closure itself can be detected whether there is a side-effect, but the
519 // value of variable that is holding closure can change.
520 // So, we just check the variables.
524 self.visit_expr(expr);
525 std::mem::take(&mut self.ret_vars)
530 self.has_side_effect.insert(id.0);
534 self.contains_side_effect = true;
537 self.ret_vars.append(&mut ret_vars);
540 fn visit_method_call(&mut self, path: &'tcx PathSegment<'tcx>, args: &'tcx [Expr<'tcx>]) {
543 if path.ident == self.fn_ident;
544 if let ExprKind::Path(QPath::Resolved(_, path)) = args.first().unwrap().kind;
545 if let Res::Local(..) = path.res;
546 let ident = path.segments.last().unwrap().ident;
547 if ident.name == kw::SelfLower;
549 izip!(self.params.clone(), args.iter())
550 .for_each(|(pat, expr)| {
551 self.visit_pat_expr(pat, expr, true);
552 self.ret_vars.clear();
558 self.visit_expr(expr);
559 std::mem::take(&mut self.ret_vars)
564 self.has_side_effect.insert(a.0);
568 self.contains_side_effect = true;
573 fn visit_if(&mut self, bind: &'tcx Expr<'tcx>, then_expr: &'tcx Expr<'tcx>, else_expr: Option<&'tcx Expr<'tcx>>) {
574 let contains_side_effect = self.contains_side_effect;
575 self.contains_side_effect = false;
576 self.visit_expr(bind);
577 let mut vars = std::mem::take(&mut self.ret_vars);
578 self.visit_expr(then_expr);
579 let mut then_vars = std::mem::take(&mut self.ret_vars);
580 walk_list!(self, visit_expr, else_expr);
581 if self.contains_side_effect {
582 self.add_side_effect(vars.clone());
584 self.contains_side_effect |= contains_side_effect;
585 self.ret_vars.append(&mut vars);
586 self.ret_vars.append(&mut then_vars);
589 fn visit_match(&mut self, expr: &'tcx Expr<'tcx>, arms: &'tcx [Arm<'tcx>]) {
590 self.visit_expr(expr);
591 let mut expr_vars = std::mem::take(&mut self.ret_vars);
595 let contains_side_effect = self.contains_side_effect;
596 self.contains_side_effect = false;
597 // this would visit `expr` multiple times
598 // but couldn't think of a better way
599 self.visit_pat_expr(arm.pat, expr, false);
600 let mut vars = std::mem::take(&mut self.ret_vars);
601 let _ = arm.guard.as_ref().map(|guard| {
602 self.visit_expr(match guard {
603 Guard::If(expr) | Guard::IfLet(_, expr) => expr,
605 vars.append(&mut self.ret_vars);
607 self.visit_expr(arm.body);
608 if self.contains_side_effect {
609 self.add_side_effect(vars.clone());
610 self.add_side_effect(expr_vars.clone());
612 self.contains_side_effect |= contains_side_effect;
613 vars.append(&mut self.ret_vars);
617 self.ret_vars.append(&mut expr_vars);
620 fn connect_assign(&mut self, lhs: &[(HirId, bool)], rhs: &[(HirId, bool)], connect_self: bool) {
621 // if mutable dereference is on assignment it can have side-effect
622 // (this can lead to parameter mutable dereference and change the original value)
623 // too hard to detect whether this value is from parameter, so this would all
624 // check mutable dereference assignment to side effect
625 lhs.iter().filter(|(_, b)| *b).for_each(|(id, _)| {
626 self.has_side_effect.insert(*id);
627 self.contains_side_effect = true;
630 // there is no connection
631 if lhs.is_empty() || rhs.is_empty() {
635 // by connected rhs in cycle, the connections would decrease
636 // from `n * m` to `n + m`
637 // where `n` and `m` are length of `lhs` and `rhs`.
639 // unwrap is possible since rhs is not empty
640 let rhs_first = rhs.first().unwrap();
641 for (id, _) in lhs.iter() {
642 if connect_self || *id != rhs_first.0 {
645 .or_insert_with(FxHashSet::default)
646 .insert(rhs_first.0);
650 let rhs = rhs.iter();
651 izip!(rhs.clone().cycle().skip(1), rhs).for_each(|(from, to)| {
652 if connect_self || from.0 != to.0 {
653 self.graph.entry(from.0).or_insert_with(FxHashSet::default).insert(to.0);
658 fn add_side_effect(&mut self, v: Vec<(HirId, bool)>) {
660 self.has_side_effect.insert(id);
661 self.contains_side_effect = true;