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, Ident, 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), None, 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();
100 report_extra_lifetimes,
105 fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'_>) {
106 if let TraitItemKind::Fn(ref sig, ref body) = item.kind {
107 let (body, trait_sig) = match *body {
108 TraitFn::Required(sig) => (None, Some(sig)),
109 TraitFn::Provided(id) => (Some(id), None),
111 check_fn_inner(cx, sig.decl, body, trait_sig, &item.generics, item.span, true);
116 /// The lifetime of a &-reference.
117 #[derive(PartialEq, Eq, Hash, Debug, Clone)]
124 fn check_fn_inner<'tcx>(
125 cx: &LateContext<'tcx>,
126 decl: &'tcx FnDecl<'_>,
127 body: Option<BodyId>,
128 trait_sig: Option<&[Ident]>,
129 generics: &'tcx Generics<'_>,
131 report_extra_lifetimes: bool,
133 if span.from_expansion() || has_where_lifetimes(cx, &generics.where_clause) {
140 .filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
142 for bound in typ.bounds {
143 let mut visitor = RefVisitor::new(cx);
144 walk_param_bound(&mut visitor, bound);
145 if visitor.lts.iter().any(|lt| matches!(lt, RefLt::Named(_))) {
148 if let GenericBound::Trait(ref trait_ref, _) = *bound {
149 let params = &trait_ref
154 .expect("a path must have at least one segment")
156 if let Some(params) = *params {
157 let lifetimes = params.args.iter().filter_map(|arg| match arg {
158 GenericArg::Lifetime(lt) => Some(lt),
161 for bound in lifetimes {
162 if bound.name != LifetimeName::Static && !bound.is_elided() {
170 if could_use_elision(cx, decl, body, trait_sig, generics.params) {
174 span.with_hi(decl.output.span().hi()),
175 "explicit lifetimes given in parameter types where they could be elided \
176 (or replaced with `'_` if needed by type declaration)",
179 if report_extra_lifetimes {
180 self::report_extra_lifetimes(cx, decl, generics);
184 // elision doesn't work for explicit self types, see rust-lang/rust#69064
185 fn explicit_self_type<'tcx>(cx: &LateContext<'tcx>, func: &FnDecl<'tcx>, ident: Option<Ident>) -> bool {
187 if let Some(ident) = ident;
188 if ident.name == kw::SelfLower;
189 if !func.implicit_self.has_implicit_self();
191 if let Some(self_ty) = func.inputs.first();
193 let mut visitor = RefVisitor::new(cx);
194 visitor.visit_ty(self_ty);
196 !visitor.all_lts().is_empty()
204 fn could_use_elision<'tcx>(
205 cx: &LateContext<'tcx>,
206 func: &'tcx FnDecl<'_>,
207 body: Option<BodyId>,
208 trait_sig: Option<&[Ident]>,
209 named_generics: &'tcx [GenericParam<'_>],
211 // There are two scenarios where elision works:
212 // * no output references, all input references have different LT
213 // * output references, exactly one input reference with same LT
214 // All lifetimes must be unnamed, 'static or defined without bounds on the
215 // level of the current item.
218 let allowed_lts = allowed_lts_from(named_generics);
220 // these will collect all the lifetimes for references in arg/return types
221 let mut input_visitor = RefVisitor::new(cx);
222 let mut output_visitor = RefVisitor::new(cx);
224 // extract lifetimes in input argument types
225 for arg in func.inputs {
226 input_visitor.visit_ty(arg);
228 // extract lifetimes in output type
229 if let Return(ty) = func.output {
230 output_visitor.visit_ty(ty);
232 for lt in named_generics {
233 input_visitor.visit_generic_param(lt);
236 if input_visitor.abort() || output_visitor.abort() {
243 .nested_elision_site_lts
245 .chain(output_visitor.nested_elision_site_lts.iter())
247 .filter(|v| matches!(v, RefLt::Named(_)))
256 let input_lts = input_visitor.lts;
257 let output_lts = output_visitor.lts;
259 if let Some(trait_sig) = trait_sig {
260 if explicit_self_type(cx, func, trait_sig.first().copied()) {
265 if let Some(body_id) = body {
266 let body = cx.tcx.hir().body(body_id);
268 let first_ident = body.params.first().and_then(|param| param.pat.simple_ident());
269 if explicit_self_type(cx, func, first_ident) {
273 let mut checker = BodyLifetimeChecker {
274 lifetimes_used_in_body: false,
276 checker.visit_expr(&body.value);
277 if checker.lifetimes_used_in_body {
282 // check for lifetimes from higher scopes
283 for lt in input_lts.iter().chain(output_lts.iter()) {
284 if !allowed_lts.contains(lt) {
289 // no input lifetimes? easy case!
290 if input_lts.is_empty() {
292 } else if output_lts.is_empty() {
293 // no output lifetimes, check distinctness of input lifetimes
295 // only unnamed and static, ok
296 let unnamed_and_static = input_lts.iter().all(|lt| *lt == RefLt::Unnamed || *lt == RefLt::Static);
297 if unnamed_and_static {
300 // we have no output reference, so we only need all distinct lifetimes
301 input_lts.len() == unique_lifetimes(&input_lts)
303 // we have output references, so we need one input reference,
304 // and all output lifetimes must be the same
305 if unique_lifetimes(&output_lts) > 1 {
308 if input_lts.len() == 1 {
309 match (&input_lts[0], &output_lts[0]) {
310 (&RefLt::Named(n1), &RefLt::Named(n2)) if n1 == n2 => true,
311 (&RefLt::Named(_), &RefLt::Unnamed) => true,
312 _ => false, /* already elided, different named lifetimes
313 * or something static going on */
321 fn allowed_lts_from(named_generics: &[GenericParam<'_>]) -> FxHashSet<RefLt> {
322 let mut allowed_lts = FxHashSet::default();
323 for par in named_generics.iter() {
324 if let GenericParamKind::Lifetime { .. } = par.kind {
325 if par.bounds.is_empty() {
326 allowed_lts.insert(RefLt::Named(par.name.ident().name));
330 allowed_lts.insert(RefLt::Unnamed);
331 allowed_lts.insert(RefLt::Static);
335 /// Number of unique lifetimes in the given vector.
337 fn unique_lifetimes(lts: &[RefLt]) -> usize {
338 lts.iter().collect::<FxHashSet<_>>().len()
341 const CLOSURE_TRAIT_BOUNDS: [LangItem; 3] = [LangItem::Fn, LangItem::FnMut, LangItem::FnOnce];
343 /// A visitor usable for `rustc_front::visit::walk_ty()`.
344 struct RefVisitor<'a, 'tcx> {
345 cx: &'a LateContext<'tcx>,
347 nested_elision_site_lts: Vec<RefLt>,
348 unelided_trait_object_lifetime: bool,
351 impl<'a, 'tcx> RefVisitor<'a, 'tcx> {
352 fn new(cx: &'a LateContext<'tcx>) -> Self {
356 nested_elision_site_lts: Vec::new(),
357 unelided_trait_object_lifetime: false,
361 fn record(&mut self, lifetime: &Option<Lifetime>) {
362 if let Some(ref lt) = *lifetime {
363 if lt.name == LifetimeName::Static {
364 self.lts.push(RefLt::Static);
365 } else if let LifetimeName::Param(ParamName::Fresh(_)) = lt.name {
366 // Fresh lifetimes generated should be ignored.
367 } else if lt.is_elided() {
368 self.lts.push(RefLt::Unnamed);
370 self.lts.push(RefLt::Named(lt.name.ident().name));
373 self.lts.push(RefLt::Unnamed);
377 fn all_lts(&self) -> Vec<RefLt> {
380 .chain(self.nested_elision_site_lts.iter())
385 fn abort(&self) -> bool {
386 self.unelided_trait_object_lifetime
390 impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
391 // for lifetimes as parameters of generics
392 fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
393 self.record(&Some(*lifetime));
396 fn visit_poly_trait_ref(&mut self, poly_tref: &'tcx PolyTraitRef<'tcx>, tbm: TraitBoundModifier) {
397 let trait_ref = &poly_tref.trait_ref;
398 if CLOSURE_TRAIT_BOUNDS.iter().any(|&item| {
403 .map_or(false, |id| Some(id) == trait_ref.trait_def_id())
405 let mut sub_visitor = RefVisitor::new(self.cx);
406 sub_visitor.visit_trait_ref(trait_ref);
407 self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
409 walk_poly_trait_ref(self, poly_tref, tbm);
413 fn visit_ty(&mut self, ty: &'tcx Ty<'_>) {
415 TyKind::OpaqueDef(item, bounds) => {
416 let map = self.cx.tcx.hir();
417 let item = map.item(item);
418 walk_item(self, item);
420 self.lts.extend(bounds.iter().filter_map(|bound| match bound {
421 GenericArg::Lifetime(l) => Some(RefLt::Named(l.name.ident().name)),
425 TyKind::BareFn(&BareFnTy { decl, .. }) => {
426 let mut sub_visitor = RefVisitor::new(self.cx);
427 sub_visitor.visit_fn_decl(decl);
428 self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
431 TyKind::TraitObject(bounds, ref lt, _) => {
433 self.unelided_trait_object_lifetime = true;
435 for bound in bounds {
436 self.visit_poly_trait_ref(bound, TraitBoundModifier::None);
446 /// Are any lifetimes mentioned in the `where` clause? If so, we don't try to
447 /// reason about elision.
448 fn has_where_lifetimes<'tcx>(cx: &LateContext<'tcx>, where_clause: &'tcx WhereClause<'_>) -> bool {
449 for predicate in where_clause.predicates {
451 WherePredicate::RegionPredicate(..) => return true,
452 WherePredicate::BoundPredicate(ref pred) => {
453 // a predicate like F: Trait or F: for<'a> Trait<'a>
454 let mut visitor = RefVisitor::new(cx);
455 // walk the type F, it may not contain LT refs
456 walk_ty(&mut visitor, pred.bounded_ty);
457 if !visitor.all_lts().is_empty() {
460 // if the bounds define new lifetimes, they are fine to occur
461 let allowed_lts = allowed_lts_from(pred.bound_generic_params);
462 // now walk the bounds
463 for bound in pred.bounds.iter() {
464 walk_param_bound(&mut visitor, bound);
466 // and check that all lifetimes are allowed
467 if visitor.all_lts().iter().any(|it| !allowed_lts.contains(it)) {
471 WherePredicate::EqPredicate(ref pred) => {
472 let mut visitor = RefVisitor::new(cx);
473 walk_ty(&mut visitor, pred.lhs_ty);
474 walk_ty(&mut visitor, pred.rhs_ty);
475 if !visitor.lts.is_empty() {
484 struct LifetimeChecker {
485 map: FxHashMap<Symbol, Span>,
488 impl<'tcx> Visitor<'tcx> for LifetimeChecker {
489 // for lifetimes as parameters of generics
490 fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
491 self.map.remove(&lifetime.name.ident().name);
494 fn visit_generic_param(&mut self, param: &'tcx GenericParam<'_>) {
495 // don't actually visit `<'a>` or `<'a: 'b>`
496 // we've already visited the `'a` declarations and
497 // don't want to spuriously remove them
498 // `'b` in `'a: 'b` is useless unless used elsewhere in
499 // a non-lifetime bound
500 if let GenericParamKind::Type { .. } = param.kind {
501 walk_generic_param(self, param);
506 fn report_extra_lifetimes<'tcx>(cx: &LateContext<'tcx>, func: &'tcx FnDecl<'_>, generics: &'tcx Generics<'_>) {
510 .filter_map(|par| match par.kind {
511 GenericParamKind::Lifetime { .. } => Some((par.name.ident().name, par.span)),
515 let mut checker = LifetimeChecker { map: hs };
517 walk_generics(&mut checker, generics);
518 walk_fn_decl(&mut checker, func);
520 for &v in checker.map.values() {
523 EXTRA_UNUSED_LIFETIMES,
525 "this lifetime isn't used in the function definition",
530 struct BodyLifetimeChecker {
531 lifetimes_used_in_body: bool,
534 impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
535 // for lifetimes as parameters of generics
536 fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
537 if lifetime.name.ident().name != kw::Empty && lifetime.name.ident().name != kw::StaticLifetime {
538 self.lifetimes_used_in_body = true;