1 use clippy_utils::diagnostics::span_lint;
2 use clippy_utils::trait_ref_of_method;
3 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
4 use rustc_hir::intravisit::{
5 walk_fn_decl, walk_generic_param, walk_generics, walk_item, walk_param_bound, walk_poly_trait_ref, walk_ty, Visitor,
7 use rustc_hir::FnRetTy::Return;
9 BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, ImplItem,
10 ImplItemKind, Item, ItemKind, LangItem, Lifetime, LifetimeName, ParamName, PolyTraitRef, TraitBoundModifier,
11 TraitFn, TraitItem, TraitItemKind, Ty, TyKind, WhereClause, WherePredicate,
13 use rustc_lint::{LateContext, LateLintPass};
14 use rustc_session::{declare_lint_pass, declare_tool_lint};
15 use rustc_span::source_map::Span;
16 use rustc_span::symbol::{kw, Symbol};
18 declare_clippy_lint! {
20 /// Checks for lifetime annotations which can be removed by
21 /// relying on lifetime elision.
23 /// ### Why is this bad?
24 /// The additional lifetimes make the code look more
25 /// complicated, while there is nothing out of the ordinary going on. Removing
26 /// them leads to more readable code.
28 /// ### Known problems
29 /// - We bail out if the function has a `where` clause where lifetimes
30 /// are mentioned due to potential false positives.
31 /// - Lifetime bounds such as `impl Foo + 'a` and `T: 'a` must be elided with the
32 /// placeholder notation `'_` because the fully elided notation leaves the type bound to `'static`.
36 /// // Bad: unnecessary lifetime annotations
37 /// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
42 /// fn elided(x: &u8, y: u8) -> &u8 {
46 #[clippy::version = "pre 1.29.0"]
47 pub NEEDLESS_LIFETIMES,
49 "using explicit lifetimes for references in function arguments when elision rules \
50 would allow omitting them"
53 declare_clippy_lint! {
55 /// Checks for lifetimes in generics that are never used
58 /// ### Why is this bad?
59 /// The additional lifetimes make the code look more
60 /// complicated, while there is nothing out of the ordinary going on. Removing
61 /// them leads to more readable code.
65 /// // Bad: unnecessary lifetimes
66 /// fn unused_lifetime<'a>(x: u8) {
71 /// fn no_lifetime(x: u8) {
75 #[clippy::version = "pre 1.29.0"]
76 pub EXTRA_UNUSED_LIFETIMES,
78 "unused lifetimes in function definitions"
81 declare_lint_pass!(Lifetimes => [NEEDLESS_LIFETIMES, EXTRA_UNUSED_LIFETIMES]);
83 impl<'tcx> LateLintPass<'tcx> for Lifetimes {
84 fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
85 if let ItemKind::Fn(ref sig, ref generics, id) = item.kind {
86 check_fn_inner(cx, sig.decl, Some(id), generics, item.span, true);
90 fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
91 if let ImplItemKind::Fn(ref sig, id) = item.kind {
92 let report_extra_lifetimes = trait_ref_of_method(cx, item.def_id).is_none();
99 report_extra_lifetimes,
104 fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'_>) {
105 if let TraitItemKind::Fn(ref sig, ref body) = item.kind {
106 let body = match *body {
107 TraitFn::Required(_) => None,
108 TraitFn::Provided(id) => Some(id),
110 check_fn_inner(cx, sig.decl, body, &item.generics, item.span, true);
115 /// The lifetime of a &-reference.
116 #[derive(PartialEq, Eq, Hash, Debug, Clone)]
123 fn check_fn_inner<'tcx>(
124 cx: &LateContext<'tcx>,
125 decl: &'tcx FnDecl<'_>,
126 body: Option<BodyId>,
127 generics: &'tcx Generics<'_>,
129 report_extra_lifetimes: bool,
131 if span.from_expansion() || has_where_lifetimes(cx, &generics.where_clause) {
138 .filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
140 for bound in typ.bounds {
141 let mut visitor = RefVisitor::new(cx);
142 walk_param_bound(&mut visitor, bound);
143 if visitor.lts.iter().any(|lt| matches!(lt, RefLt::Named(_))) {
146 if let GenericBound::Trait(ref trait_ref, _) = *bound {
147 let params = &trait_ref
152 .expect("a path must have at least one segment")
154 if let Some(params) = *params {
155 let lifetimes = params.args.iter().filter_map(|arg| match arg {
156 GenericArg::Lifetime(lt) => Some(lt),
159 for bound in lifetimes {
160 if bound.name != LifetimeName::Static && !bound.is_elided() {
168 if could_use_elision(cx, decl, body, generics.params) {
172 span.with_hi(decl.output.span().hi()),
173 "explicit lifetimes given in parameter types where they could be elided \
174 (or replaced with `'_` if needed by type declaration)",
177 if report_extra_lifetimes {
178 self::report_extra_lifetimes(cx, decl, generics);
182 fn could_use_elision<'tcx>(
183 cx: &LateContext<'tcx>,
184 func: &'tcx FnDecl<'_>,
185 body: Option<BodyId>,
186 named_generics: &'tcx [GenericParam<'_>],
188 // There are two scenarios where elision works:
189 // * no output references, all input references have different LT
190 // * output references, exactly one input reference with same LT
191 // All lifetimes must be unnamed, 'static or defined without bounds on the
192 // level of the current item.
195 let allowed_lts = allowed_lts_from(named_generics);
197 // these will collect all the lifetimes for references in arg/return types
198 let mut input_visitor = RefVisitor::new(cx);
199 let mut output_visitor = RefVisitor::new(cx);
201 // extract lifetimes in input argument types
202 for arg in func.inputs {
203 input_visitor.visit_ty(arg);
205 // extract lifetimes in output type
206 if let Return(ty) = func.output {
207 output_visitor.visit_ty(ty);
209 for lt in named_generics {
210 input_visitor.visit_generic_param(lt);
213 if input_visitor.abort() || output_visitor.abort() {
220 .nested_elision_site_lts
222 .chain(output_visitor.nested_elision_site_lts.iter())
224 .filter(|v| matches!(v, RefLt::Named(_)))
233 let input_lts = input_visitor.lts;
234 let output_lts = output_visitor.lts;
236 if let Some(body_id) = body {
237 let mut checker = BodyLifetimeChecker {
238 lifetimes_used_in_body: false,
240 checker.visit_expr(&cx.tcx.hir().body(body_id).value);
241 if checker.lifetimes_used_in_body {
246 // check for lifetimes from higher scopes
247 for lt in input_lts.iter().chain(output_lts.iter()) {
248 if !allowed_lts.contains(lt) {
253 // no input lifetimes? easy case!
254 if input_lts.is_empty() {
256 } else if output_lts.is_empty() {
257 // no output lifetimes, check distinctness of input lifetimes
259 // only unnamed and static, ok
260 let unnamed_and_static = input_lts.iter().all(|lt| *lt == RefLt::Unnamed || *lt == RefLt::Static);
261 if unnamed_and_static {
264 // we have no output reference, so we only need all distinct lifetimes
265 input_lts.len() == unique_lifetimes(&input_lts)
267 // we have output references, so we need one input reference,
268 // and all output lifetimes must be the same
269 if unique_lifetimes(&output_lts) > 1 {
272 if input_lts.len() == 1 {
273 match (&input_lts[0], &output_lts[0]) {
274 (&RefLt::Named(n1), &RefLt::Named(n2)) if n1 == n2 => true,
275 (&RefLt::Named(_), &RefLt::Unnamed) => true,
276 _ => false, /* already elided, different named lifetimes
277 * or something static going on */
285 fn allowed_lts_from(named_generics: &[GenericParam<'_>]) -> FxHashSet<RefLt> {
286 let mut allowed_lts = FxHashSet::default();
287 for par in named_generics.iter() {
288 if let GenericParamKind::Lifetime { .. } = par.kind {
289 if par.bounds.is_empty() {
290 allowed_lts.insert(RefLt::Named(par.name.ident().name));
294 allowed_lts.insert(RefLt::Unnamed);
295 allowed_lts.insert(RefLt::Static);
299 /// Number of unique lifetimes in the given vector.
301 fn unique_lifetimes(lts: &[RefLt]) -> usize {
302 lts.iter().collect::<FxHashSet<_>>().len()
305 const CLOSURE_TRAIT_BOUNDS: [LangItem; 3] = [LangItem::Fn, LangItem::FnMut, LangItem::FnOnce];
307 /// A visitor usable for `rustc_front::visit::walk_ty()`.
308 struct RefVisitor<'a, 'tcx> {
309 cx: &'a LateContext<'tcx>,
311 nested_elision_site_lts: Vec<RefLt>,
312 unelided_trait_object_lifetime: bool,
315 impl<'a, 'tcx> RefVisitor<'a, 'tcx> {
316 fn new(cx: &'a LateContext<'tcx>) -> Self {
320 nested_elision_site_lts: Vec::new(),
321 unelided_trait_object_lifetime: false,
325 fn record(&mut self, lifetime: &Option<Lifetime>) {
326 if let Some(ref lt) = *lifetime {
327 if lt.name == LifetimeName::Static {
328 self.lts.push(RefLt::Static);
329 } else if let LifetimeName::Param(ParamName::Fresh(_)) = lt.name {
330 // Fresh lifetimes generated should be ignored.
331 } else if lt.is_elided() {
332 self.lts.push(RefLt::Unnamed);
334 self.lts.push(RefLt::Named(lt.name.ident().name));
337 self.lts.push(RefLt::Unnamed);
341 fn all_lts(&self) -> Vec<RefLt> {
344 .chain(self.nested_elision_site_lts.iter())
349 fn abort(&self) -> bool {
350 self.unelided_trait_object_lifetime
354 impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
355 // for lifetimes as parameters of generics
356 fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
357 self.record(&Some(*lifetime));
360 fn visit_poly_trait_ref(&mut self, poly_tref: &'tcx PolyTraitRef<'tcx>, tbm: TraitBoundModifier) {
361 let trait_ref = &poly_tref.trait_ref;
362 if CLOSURE_TRAIT_BOUNDS.iter().any(|&item| {
367 .map_or(false, |id| Some(id) == trait_ref.trait_def_id())
369 let mut sub_visitor = RefVisitor::new(self.cx);
370 sub_visitor.visit_trait_ref(trait_ref);
371 self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
373 walk_poly_trait_ref(self, poly_tref, tbm);
377 fn visit_ty(&mut self, ty: &'tcx Ty<'_>) {
379 TyKind::OpaqueDef(item, bounds) => {
380 let map = self.cx.tcx.hir();
381 let item = map.item(item);
382 walk_item(self, item);
384 self.lts.extend(bounds.iter().filter_map(|bound| match bound {
385 GenericArg::Lifetime(l) => Some(RefLt::Named(l.name.ident().name)),
389 TyKind::BareFn(&BareFnTy { decl, .. }) => {
390 let mut sub_visitor = RefVisitor::new(self.cx);
391 sub_visitor.visit_fn_decl(decl);
392 self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
395 TyKind::TraitObject(bounds, ref lt, _) => {
397 self.unelided_trait_object_lifetime = true;
399 for bound in bounds {
400 self.visit_poly_trait_ref(bound, TraitBoundModifier::None);
410 /// Are any lifetimes mentioned in the `where` clause? If so, we don't try to
411 /// reason about elision.
412 fn has_where_lifetimes<'tcx>(cx: &LateContext<'tcx>, where_clause: &'tcx WhereClause<'_>) -> bool {
413 for predicate in where_clause.predicates {
415 WherePredicate::RegionPredicate(..) => return true,
416 WherePredicate::BoundPredicate(ref pred) => {
417 // a predicate like F: Trait or F: for<'a> Trait<'a>
418 let mut visitor = RefVisitor::new(cx);
419 // walk the type F, it may not contain LT refs
420 walk_ty(&mut visitor, pred.bounded_ty);
421 if !visitor.all_lts().is_empty() {
424 // if the bounds define new lifetimes, they are fine to occur
425 let allowed_lts = allowed_lts_from(pred.bound_generic_params);
426 // now walk the bounds
427 for bound in pred.bounds.iter() {
428 walk_param_bound(&mut visitor, bound);
430 // and check that all lifetimes are allowed
431 if visitor.all_lts().iter().any(|it| !allowed_lts.contains(it)) {
435 WherePredicate::EqPredicate(ref pred) => {
436 let mut visitor = RefVisitor::new(cx);
437 walk_ty(&mut visitor, pred.lhs_ty);
438 walk_ty(&mut visitor, pred.rhs_ty);
439 if !visitor.lts.is_empty() {
448 struct LifetimeChecker {
449 map: FxHashMap<Symbol, Span>,
452 impl<'tcx> Visitor<'tcx> for LifetimeChecker {
453 // for lifetimes as parameters of generics
454 fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
455 self.map.remove(&lifetime.name.ident().name);
458 fn visit_generic_param(&mut self, param: &'tcx GenericParam<'_>) {
459 // don't actually visit `<'a>` or `<'a: 'b>`
460 // we've already visited the `'a` declarations and
461 // don't want to spuriously remove them
462 // `'b` in `'a: 'b` is useless unless used elsewhere in
463 // a non-lifetime bound
464 if let GenericParamKind::Type { .. } = param.kind {
465 walk_generic_param(self, param);
470 fn report_extra_lifetimes<'tcx>(cx: &LateContext<'tcx>, func: &'tcx FnDecl<'_>, generics: &'tcx Generics<'_>) {
474 .filter_map(|par| match par.kind {
475 GenericParamKind::Lifetime { .. } => Some((par.name.ident().name, par.span)),
479 let mut checker = LifetimeChecker { map: hs };
481 walk_generics(&mut checker, generics);
482 walk_fn_decl(&mut checker, func);
484 for &v in checker.map.values() {
487 EXTRA_UNUSED_LIFETIMES,
489 "this lifetime isn't used in the function definition",
494 struct BodyLifetimeChecker {
495 lifetimes_used_in_body: bool,
498 impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
499 // for lifetimes as parameters of generics
500 fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
501 if lifetime.name.ident().name != kw::Empty && lifetime.name.ident().name != kw::StaticLifetime {
502 self.lifetimes_used_in_body = true;