src/tools/clippy/clippy_lints/src/lifetimes.rs

   1 use reexport::*;
   2 use rustc::lint::*;
   3 use rustc::hir::def::Def;
   4 use rustc::hir::*;
   5 use rustc::hir::intravisit::*;
   6 use std::collections::{HashMap, HashSet};
   7 use syntax::codemap::Span;
   8 use utils::{in_external_macro, last_path_segment, span_lint};
   9 use syntax::symbol::keywords;
  10
  11 /// **What it does:** Checks for lifetime annotations which can be removed by
  12 /// relying on lifetime elision.
  13 ///
  14 /// **Why is this bad?** The additional lifetimes make the code look more
  15 /// complicated, while there is nothing out of the ordinary going on. Removing
  16 /// them leads to more readable code.
  17 ///
  18 /// **Known problems:** Potential false negatives: we bail out if the function
  19 /// has a `where` clause where lifetimes are mentioned.
  20 ///
  21 /// **Example:**
  22 /// ```rust
  23 /// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 { x }
  24 /// ```
  25 declare_clippy_lint! {
  26     pub NEEDLESS_LIFETIMES,
  27     complexity,
  28     "using explicit lifetimes for references in function arguments when elision rules \
  29      would allow omitting them"
  30 }
  31
  32 /// **What it does:** Checks for lifetimes in generics that are never used
  33 /// anywhere else.
  34 ///
  35 /// **Why is this bad?** The additional lifetimes make the code look more
  36 /// complicated, while there is nothing out of the ordinary going on. Removing
  37 /// them leads to more readable code.
  38 ///
  39 /// **Known problems:** None.
  40 ///
  41 /// **Example:**
  42 /// ```rust
  43 /// fn unused_lifetime<'a>(x: u8) { .. }
  44 /// ```
  45 declare_clippy_lint! {
  46     pub UNUSED_LIFETIMES,
  47     complexity,
  48     "unused lifetimes in function definitions"
  49 }
  50
  51 #[derive(Copy, Clone)]
  52 pub struct LifetimePass;
  53
  54 impl LintPass for LifetimePass {
  55     fn get_lints(&self) -> LintArray {
  56         lint_array!(NEEDLESS_LIFETIMES, UNUSED_LIFETIMES)
  57     }
  58 }
  59
  60 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LifetimePass {
  61     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
  62         if let ItemFn(ref decl, _, _, _, ref generics, id) = item.node {
  63             check_fn_inner(cx, decl, Some(id), generics, item.span);
  64         }
  65     }
  66
  67     fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx ImplItem) {
  68         if let ImplItemKind::Method(ref sig, id) = item.node {
  69             check_fn_inner(cx, &sig.decl, Some(id), &item.generics, item.span);
  70         }
  71     }
  72
  73     fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx TraitItem) {
  74         if let TraitItemKind::Method(ref sig, ref body) = item.node {
  75             let body = match *body {
  76                 TraitMethod::Required(_) => None,
  77                 TraitMethod::Provided(id) => Some(id),
  78             };
  79             check_fn_inner(cx, &sig.decl, body, &item.generics, item.span);
  80         }
  81     }
  82 }
  83
  84 /// The lifetime of a &-reference.
  85 #[derive(PartialEq, Eq, Hash, Debug)]
  86 enum RefLt {
  87     Unnamed,
  88     Static,
  89     Named(Name),
  90 }
  91
  92 fn check_fn_inner<'a, 'tcx>(
  93     cx: &LateContext<'a, 'tcx>,
  94     decl: &'tcx FnDecl,
  95     body: Option<BodyId>,
  96     generics: &'tcx Generics,
  97     span: Span,
  98 ) {
  99     if in_external_macro(cx, span) || has_where_lifetimes(cx, &generics.where_clause) {
 100         return;
 101     }
 102
 103     let mut bounds_lts = Vec::new();
 104     for typ in generics.ty_params() {
 105         for bound in &typ.bounds {
 106             if let TraitTyParamBound(ref trait_ref, _) = *bound {
 107                 let params = &trait_ref
 108                     .trait_ref
 109                     .path
 110                     .segments
 111                     .last()
 112                     .expect("a path must have at least one segment")
 113                     .parameters;
 114                 if let Some(ref params) = *params {
 115                     for bound in &params.lifetimes {
 116                         if bound.name.name() != "'static" && !bound.is_elided() {
 117                             return;
 118                         }
 119                         bounds_lts.push(bound);
 120                     }
 121                 }
 122             }
 123         }
 124     }
 125     if could_use_elision(cx, decl, body, &generics.params, bounds_lts) {
 126         span_lint(
 127             cx,
 128             NEEDLESS_LIFETIMES,
 129             span,
 130             "explicit lifetimes given in parameter types where they could be elided",
 131         );
 132     }
 133     report_extra_lifetimes(cx, decl, generics);
 134 }
 135
 136 fn could_use_elision<'a, 'tcx: 'a>(
 137     cx: &LateContext<'a, 'tcx>,
 138     func: &'tcx FnDecl,
 139     body: Option<BodyId>,
 140     named_generics: &'tcx [GenericParam],
 141     bounds_lts: Vec<&'tcx Lifetime>,
 142 ) -> bool {
 143     // There are two scenarios where elision works:
 144     // * no output references, all input references have different LT
 145     // * output references, exactly one input reference with same LT
 146     // All lifetimes must be unnamed, 'static or defined without bounds on the
 147     // level of the current item.
 148
 149     // check named LTs
 150     let allowed_lts = allowed_lts_from(named_generics);
 151
 152     // these will collect all the lifetimes for references in arg/return types
 153     let mut input_visitor = RefVisitor::new(cx);
 154     let mut output_visitor = RefVisitor::new(cx);
 155
 156     // extract lifetimes in input argument types
 157     for arg in &func.inputs {
 158         input_visitor.visit_ty(arg);
 159     }
 160     // extract lifetimes in output type
 161     if let Return(ref ty) = func.output {
 162         output_visitor.visit_ty(ty);
 163     }
 164
 165     let input_lts = match input_visitor.into_vec() {
 166         Some(lts) => lts_from_bounds(lts, bounds_lts.into_iter()),
 167         None => return false,
 168     };
 169     let output_lts = match output_visitor.into_vec() {
 170         Some(val) => val,
 171         None => return false,
 172     };
 173
 174     if let Some(body_id) = body {
 175         let mut checker = BodyLifetimeChecker {
 176             lifetimes_used_in_body: false,
 177         };
 178         checker.visit_expr(&cx.tcx.hir.body(body_id).value);
 179         if checker.lifetimes_used_in_body {
 180             return false;
 181         }
 182     }
 183
 184     // check for lifetimes from higher scopes
 185     for lt in input_lts.iter().chain(output_lts.iter()) {
 186         if !allowed_lts.contains(lt) {
 187             return false;
 188         }
 189     }
 190
 191     // no input lifetimes? easy case!
 192     if input_lts.is_empty() {
 193         false
 194     } else if output_lts.is_empty() {
 195         // no output lifetimes, check distinctness of input lifetimes
 196
 197         // only unnamed and static, ok
 198         let unnamed_and_static = input_lts
 199             .iter()
 200             .all(|lt| *lt == RefLt::Unnamed || *lt == RefLt::Static);
 201         if unnamed_and_static {
 202             return false;
 203         }
 204         // we have no output reference, so we only need all distinct lifetimes
 205         input_lts.len() == unique_lifetimes(&input_lts)
 206     } else {
 207         // we have output references, so we need one input reference,
 208         // and all output lifetimes must be the same
 209         if unique_lifetimes(&output_lts) > 1 {
 210             return false;
 211         }
 212         if input_lts.len() == 1 {
 213             match (&input_lts[0], &output_lts[0]) {
 214                 (&RefLt::Named(n1), &RefLt::Named(n2)) if n1 == n2 => true,
 215                 (&RefLt::Named(_), &RefLt::Unnamed) => true,
 216                 _ => false, /* already elided, different named lifetimes
 217                              * or something static going on */
 218             }
 219         } else {
 220             false
 221         }
 222     }
 223 }
 224
 225 fn allowed_lts_from(named_generics: &[GenericParam]) -> HashSet<RefLt> {
 226     let mut allowed_lts = HashSet::new();
 227     for par in named_generics.iter() {
 228         if let GenericParam::Lifetime(ref lt) = *par {
 229             if lt.bounds.is_empty() {
 230                 allowed_lts.insert(RefLt::Named(lt.lifetime.name.name()));
 231             }
 232         }
 233     }
 234     allowed_lts.insert(RefLt::Unnamed);
 235     allowed_lts.insert(RefLt::Static);
 236     allowed_lts
 237 }
 238
 239 fn lts_from_bounds<'a, T: Iterator<Item = &'a Lifetime>>(mut vec: Vec<RefLt>, bounds_lts: T) -> Vec<RefLt> {
 240     for lt in bounds_lts {
 241         if lt.name.name() != "'static" {
 242             vec.push(RefLt::Named(lt.name.name()));
 243         }
 244     }
 245
 246     vec
 247 }
 248
 249 /// Number of unique lifetimes in the given vector.
 250 fn unique_lifetimes(lts: &[RefLt]) -> usize {
 251     lts.iter().collect::<HashSet<_>>().len()
 252 }
 253
 254 /// A visitor usable for `rustc_front::visit::walk_ty()`.
 255 struct RefVisitor<'a, 'tcx: 'a> {
 256     cx: &'a LateContext<'a, 'tcx>,
 257     lts: Vec<RefLt>,
 258     abort: bool,
 259 }
 260
 261 impl<'v, 't> RefVisitor<'v, 't> {
 262     fn new(cx: &'v LateContext<'v, 't>) -> Self {
 263         Self {
 264             cx,
 265             lts: Vec::new(),
 266             abort: false,
 267         }
 268     }
 269
 270     fn record(&mut self, lifetime: &Option<Lifetime>) {
 271         if let Some(ref lt) = *lifetime {
 272             if lt.name.name() == "'static" {
 273                 self.lts.push(RefLt::Static);
 274             } else if lt.is_elided() {
 275                 self.lts.push(RefLt::Unnamed);
 276             } else {
 277                 self.lts.push(RefLt::Named(lt.name.name()));
 278             }
 279         } else {
 280             self.lts.push(RefLt::Unnamed);
 281         }
 282     }
 283
 284     fn into_vec(self) -> Option<Vec<RefLt>> {
 285         if self.abort {
 286             None
 287         } else {
 288             Some(self.lts)
 289         }
 290     }
 291
 292     fn collect_anonymous_lifetimes(&mut self, qpath: &QPath, ty: &Ty) {
 293         if let Some(ref last_path_segment) = last_path_segment(qpath).parameters {
 294             if !last_path_segment.parenthesized && last_path_segment.lifetimes.is_empty() {
 295                 let hir_id = self.cx.tcx.hir.node_to_hir_id(ty.id);
 296                 match self.cx.tables.qpath_def(qpath, hir_id) {
 297                     Def::TyAlias(def_id) | Def::Struct(def_id) => {
 298                         let generics = self.cx.tcx.generics_of(def_id);
 299                         for _ in generics.params.as_slice() {
 300                             self.record(&None);
 301                         }
 302                     },
 303                     Def::Trait(def_id) => {
 304                         let trait_def = self.cx.tcx.trait_def(def_id);
 305                         for _ in &self.cx.tcx.generics_of(trait_def.def_id).params {
 306                             self.record(&None);
 307                         }
 308                     },
 309                     _ => (),
 310                 }
 311             }
 312         }
 313     }
 314 }
 315
 316 impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
 317     // for lifetimes as parameters of generics
 318     fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
 319         self.record(&Some(*lifetime));
 320     }
 321
 322     fn visit_ty(&mut self, ty: &'tcx Ty) {
 323         match ty.node {
 324             TyRptr(ref lt, _) if lt.is_elided() => {
 325                 self.record(&None);
 326             },
 327             TyPath(ref path) => {
 328                 self.collect_anonymous_lifetimes(path, ty);
 329             },
 330             TyImplTraitExistential(ref exist_ty, _) => {
 331                 for bound in &exist_ty.bounds {
 332                     if let RegionTyParamBound(_) = *bound {
 333                         self.record(&None);
 334                     }
 335                 }
 336             }
 337             TyTraitObject(ref bounds, ref lt) => {
 338                 if !lt.is_elided() {
 339                     self.abort = true;
 340                 }
 341                 for bound in bounds {
 342                     self.visit_poly_trait_ref(bound, TraitBoundModifier::None);
 343                 }
 344                 return;
 345             },
 346             _ => (),
 347         }
 348         walk_ty(self, ty);
 349     }
 350     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 351         NestedVisitorMap::None
 352     }
 353 }
 354
 355 /// Are any lifetimes mentioned in the `where` clause? If yes, we don't try to
 356 /// reason about elision.
 357 fn has_where_lifetimes<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, where_clause: &'tcx WhereClause) -> bool {
 358     for predicate in &where_clause.predicates {
 359         match *predicate {
 360             WherePredicate::RegionPredicate(..) => return true,
 361             WherePredicate::BoundPredicate(ref pred) => {
 362                 // a predicate like F: Trait or F: for<'a> Trait<'a>
 363                 let mut visitor = RefVisitor::new(cx);
 364                 // walk the type F, it may not contain LT refs
 365                 walk_ty(&mut visitor, &pred.bounded_ty);
 366                 if !visitor.lts.is_empty() {
 367                     return true;
 368                 }
 369                 // if the bounds define new lifetimes, they are fine to occur
 370                 let allowed_lts = allowed_lts_from(&pred.bound_generic_params);
 371                 // now walk the bounds
 372                 for bound in pred.bounds.iter() {
 373                     walk_ty_param_bound(&mut visitor, bound);
 374                 }
 375                 // and check that all lifetimes are allowed
 376                 match visitor.into_vec() {
 377                     None => return false,
 378                     Some(lts) => for lt in lts {
 379                         if !allowed_lts.contains(&lt) {
 380                             return true;
 381                         }
 382                     },
 383                 }
 384             },
 385             WherePredicate::EqPredicate(ref pred) => {
 386                 let mut visitor = RefVisitor::new(cx);
 387                 walk_ty(&mut visitor, &pred.lhs_ty);
 388                 walk_ty(&mut visitor, &pred.rhs_ty);
 389                 if !visitor.lts.is_empty() {
 390                     return true;
 391                 }
 392             },
 393         }
 394     }
 395     false
 396 }
 397
 398 struct LifetimeChecker {
 399     map: HashMap<Name, Span>,
 400 }
 401
 402 impl<'tcx> Visitor<'tcx> for LifetimeChecker {
 403     // for lifetimes as parameters of generics
 404     fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
 405         self.map.remove(&lifetime.name.name());
 406     }
 407
 408     fn visit_generic_param(&mut self, param: &'tcx GenericParam) {
 409         // don't actually visit `<'a>` or `<'a: 'b>`
 410         // we've already visited the `'a` declarations and
 411         // don't want to spuriously remove them
 412         // `'b` in `'a: 'b` is useless unless used elsewhere in
 413         // a non-lifetime bound
 414         if param.is_type_param() {
 415             walk_generic_param(self, param)
 416         }
 417     }
 418     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 419         NestedVisitorMap::None
 420     }
 421 }
 422
 423 fn report_extra_lifetimes<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, func: &'tcx FnDecl, generics: &'tcx Generics) {
 424     let hs = generics
 425         .lifetimes()
 426         .map(|lt| (lt.lifetime.name.name(), lt.lifetime.span))
 427         .collect();
 428     let mut checker = LifetimeChecker { map: hs };
 429
 430     walk_generics(&mut checker, generics);
 431     walk_fn_decl(&mut checker, func);
 432
 433     for &v in checker.map.values() {
 434         span_lint(cx, UNUSED_LIFETIMES, v, "this lifetime isn't used in the function definition");
 435     }
 436 }
 437
 438 struct BodyLifetimeChecker {
 439     lifetimes_used_in_body: bool,
 440 }
 441
 442 impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
 443     // for lifetimes as parameters of generics
 444     fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
 445         if lifetime.name.name() != keywords::Invalid.name() && lifetime.name.name() != "'static" {
 446             self.lifetimes_used_in_body = true;
 447         }
 448     }
 449
 450     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 451         NestedVisitorMap::None
 452     }
 453 }