src/tools/clippy/clippy_lints/src/trait_bounds.rs

   1 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
   2 use clippy_utils::source::{snippet, snippet_opt, snippet_with_applicability};
   3 use clippy_utils::{SpanlessEq, SpanlessHash};
   4 use core::hash::{Hash, Hasher};
   5 use if_chain::if_chain;
   6 use itertools::Itertools;
   7 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   8 use rustc_data_structures::unhash::UnhashMap;
   9 use rustc_errors::Applicability;
  10 use rustc_hir::def::Res;
  11 use rustc_hir::{
  12     GenericArg, GenericBound, Generics, Item, ItemKind, Node, Path, PathSegment, PredicateOrigin, QPath,
  13     TraitBoundModifier, TraitItem, TraitRef, Ty, TyKind, WherePredicate,
  14 };
  15 use rustc_lint::{LateContext, LateLintPass};
  16 use rustc_session::{declare_tool_lint, impl_lint_pass};
  17 use rustc_span::{BytePos, Span};
  18 use std::collections::hash_map::Entry;
  19
  20 declare_clippy_lint! {
  21     /// ### What it does
  22     /// This lint warns about unnecessary type repetitions in trait bounds
  23     ///
  24     /// ### Why is this bad?
  25     /// Repeating the type for every bound makes the code
  26     /// less readable than combining the bounds
  27     ///
  28     /// ### Example
  29     /// ```rust
  30     /// pub fn foo<T>(t: T) where T: Copy, T: Clone {}
  31     /// ```
  32     ///
  33     /// Use instead:
  34     /// ```rust
  35     /// pub fn foo<T>(t: T) where T: Copy + Clone {}
  36     /// ```
  37     #[clippy::version = "1.38.0"]
  38     pub TYPE_REPETITION_IN_BOUNDS,
  39     nursery,
  40     "types are repeated unnecessary in trait bounds use `+` instead of using `T: _, T: _`"
  41 }
  42
  43 declare_clippy_lint! {
  44     /// ### What it does
  45     /// Checks for cases where generics are being used and multiple
  46     /// syntax specifications for trait bounds are used simultaneously.
  47     ///
  48     /// ### Why is this bad?
  49     /// Duplicate bounds makes the code
  50     /// less readable than specifying them only once.
  51     ///
  52     /// ### Example
  53     /// ```rust
  54     /// fn func<T: Clone + Default>(arg: T) where T: Clone + Default {}
  55     /// ```
  56     ///
  57     /// Use instead:
  58     /// ```rust
  59     /// # mod hidden {
  60     /// fn func<T: Clone + Default>(arg: T) {}
  61     /// # }
  62     ///
  63     /// // or
  64     ///
  65     /// fn func<T>(arg: T) where T: Clone + Default {}
  66     /// ```
  67     ///
  68     /// ```rust
  69     /// fn foo<T: Default + Default>(bar: T) {}
  70     /// ```
  71     /// Use instead:
  72     /// ```rust
  73     /// fn foo<T: Default>(bar: T) {}
  74     /// ```
  75     ///
  76     /// ```rust
  77     /// fn foo<T>(bar: T) where T: Default + Default {}
  78     /// ```
  79     /// Use instead:
  80     /// ```rust
  81     /// fn foo<T>(bar: T) where T: Default {}
  82     /// ```
  83     #[clippy::version = "1.47.0"]
  84     pub TRAIT_DUPLICATION_IN_BOUNDS,
  85     nursery,
  86     "check if the same trait bounds are specified more than once during a generic declaration"
  87 }
  88
  89 #[derive(Copy, Clone)]
  90 pub struct TraitBounds {
  91     max_trait_bounds: u64,
  92 }
  93
  94 impl TraitBounds {
  95     #[must_use]
  96     pub fn new(max_trait_bounds: u64) -> Self {
  97         Self { max_trait_bounds }
  98     }
  99 }
 100
 101 impl_lint_pass!(TraitBounds => [TYPE_REPETITION_IN_BOUNDS, TRAIT_DUPLICATION_IN_BOUNDS]);
 102
 103 impl<'tcx> LateLintPass<'tcx> for TraitBounds {
 104     fn check_generics(&mut self, cx: &LateContext<'tcx>, gen: &'tcx Generics<'_>) {
 105         self.check_type_repetition(cx, gen);
 106         check_trait_bound_duplication(cx, gen);
 107     }
 108
 109     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
 110         // special handling for self trait bounds as these are not considered generics
 111         // ie. trait Foo: Display {}
 112         if let Item {
 113             kind: ItemKind::Trait(_, _, _, bounds, ..),
 114             ..
 115         } = item
 116         {
 117             rollup_traits(cx, bounds, "these bounds contain repeated elements");
 118         }
 119     }
 120
 121     fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'tcx>) {
 122         let mut self_bounds_map = FxHashMap::default();
 123
 124         for predicate in item.generics.predicates {
 125             if_chain! {
 126                 if let WherePredicate::BoundPredicate(ref bound_predicate) = predicate;
 127                 if bound_predicate.origin != PredicateOrigin::ImplTrait;
 128                 if !bound_predicate.span.from_expansion();
 129                 if let TyKind::Path(QPath::Resolved(_, Path { segments, .. })) = bound_predicate.bounded_ty.kind;
 130                 if let Some(PathSegment {
 131                     res: Res::SelfTyParam { trait_: def_id }, ..
 132                 }) = segments.first();
 133                 if let Some(
 134                     Node::Item(
 135                         Item {
 136                             kind: ItemKind::Trait(_, _, _, self_bounds, _),
 137                             .. }
 138                         )
 139                     ) = cx.tcx.hir().get_if_local(*def_id);
 140                 then {
 141                     if self_bounds_map.is_empty() {
 142                         for bound in self_bounds.iter() {
 143                             let Some((self_res, self_segments, _)) = get_trait_info_from_bound(bound) else { continue };
 144                             self_bounds_map.insert(self_res, self_segments);
 145                         }
 146                     }
 147
 148                     bound_predicate
 149                         .bounds
 150                         .iter()
 151                         .filter_map(get_trait_info_from_bound)
 152                         .for_each(|(trait_item_res, trait_item_segments, span)| {
 153                             if let Some(self_segments) = self_bounds_map.get(&trait_item_res) {
 154                                 if SpanlessEq::new(cx).eq_path_segments(self_segments, trait_item_segments) {
 155                                     span_lint_and_help(
 156                                         cx,
 157                                         TRAIT_DUPLICATION_IN_BOUNDS,
 158                                         span,
 159                                         "this trait bound is already specified in trait declaration",
 160                                         None,
 161                                         "consider removing this trait bound",
 162                                     );
 163                                 }
 164                             }
 165                         });
 166                 }
 167             }
 168         }
 169     }
 170 }
 171
 172 impl TraitBounds {
 173     fn check_type_repetition<'tcx>(self, cx: &LateContext<'tcx>, gen: &'tcx Generics<'_>) {
 174         struct SpanlessTy<'cx, 'tcx> {
 175             ty: &'tcx Ty<'tcx>,
 176             cx: &'cx LateContext<'tcx>,
 177         }
 178         impl PartialEq for SpanlessTy<'_, '_> {
 179             fn eq(&self, other: &Self) -> bool {
 180                 let mut eq = SpanlessEq::new(self.cx);
 181                 eq.inter_expr().eq_ty(self.ty, other.ty)
 182             }
 183         }
 184         impl Hash for SpanlessTy<'_, '_> {
 185             fn hash<H: Hasher>(&self, h: &mut H) {
 186                 let mut t = SpanlessHash::new(self.cx);
 187                 t.hash_ty(self.ty);
 188                 h.write_u64(t.finish());
 189             }
 190         }
 191         impl Eq for SpanlessTy<'_, '_> {}
 192
 193         if gen.span.from_expansion() {
 194             return;
 195         }
 196         let mut map: UnhashMap<SpanlessTy<'_, '_>, Vec<&GenericBound<'_>>> = UnhashMap::default();
 197         let mut applicability = Applicability::MaybeIncorrect;
 198         for bound in gen.predicates {
 199             if_chain! {
 200                 if let WherePredicate::BoundPredicate(ref p) = bound;
 201                 if p.origin != PredicateOrigin::ImplTrait;
 202                 if p.bounds.len() as u64 <= self.max_trait_bounds;
 203                 if !p.span.from_expansion();
 204                 if let Some(ref v) = map.insert(
 205                     SpanlessTy { ty: p.bounded_ty, cx },
 206                     p.bounds.iter().collect::<Vec<_>>()
 207                 );
 208
 209                 then {
 210                     let trait_bounds = v
 211                         .iter()
 212                         .copied()
 213                         .chain(p.bounds.iter())
 214                         .filter_map(get_trait_info_from_bound)
 215                         .map(|(_, _, span)| snippet_with_applicability(cx, span, "..", &mut applicability))
 216                         .join(" + ");
 217                     let hint_string = format!(
 218                         "consider combining the bounds: `{}: {trait_bounds}`",
 219                         snippet(cx, p.bounded_ty.span, "_"),
 220                     );
 221                     span_lint_and_help(
 222                         cx,
 223                         TYPE_REPETITION_IN_BOUNDS,
 224                         p.span,
 225                         "this type has already been used as a bound predicate",
 226                         None,
 227                         &hint_string,
 228                     );
 229                 }
 230             }
 231         }
 232     }
 233 }
 234
 235 fn check_trait_bound_duplication(cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
 236     if gen.span.from_expansion() {
 237         return;
 238     }
 239
 240     // Explanation:
 241     // fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z)
 242     // where T: Clone + Default, { unimplemented!(); }
 243     //       ^^^^^^^^^^^^^^^^^^
 244     //       |
 245     // collects each of these where clauses into a set keyed by generic name and comparable trait
 246     // eg. (T, Clone)
 247     let where_predicates = gen
 248         .predicates
 249         .iter()
 250         .filter_map(|pred| {
 251             if_chain! {
 252                 if pred.in_where_clause();
 253                 if let WherePredicate::BoundPredicate(bound_predicate) = pred;
 254                 if let TyKind::Path(QPath::Resolved(_, path)) =  bound_predicate.bounded_ty.kind;
 255                 then {
 256                     return Some(
 257                         rollup_traits(cx, bound_predicate.bounds, "these where clauses contain repeated elements")
 258                         .into_iter().map(|(trait_ref, _)| (path.res, trait_ref)))
 259                 }
 260             }
 261             None
 262         })
 263         .flatten()
 264         .collect::<FxHashSet<_>>();
 265
 266     // Explanation:
 267     // fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z) ...
 268     //            ^^^^^^^^^^^^^^^^^^  ^^^^^^^
 269     //            |
 270     // compare trait bounds keyed by generic name and comparable trait to collected where
 271     // predicates eg. (T, Clone)
 272     for predicate in gen.predicates.iter().filter(|pred| !pred.in_where_clause()) {
 273         if_chain! {
 274             if let WherePredicate::BoundPredicate(bound_predicate) = predicate;
 275             if bound_predicate.origin != PredicateOrigin::ImplTrait;
 276             if !bound_predicate.span.from_expansion();
 277             if let TyKind::Path(QPath::Resolved(_, path)) =  bound_predicate.bounded_ty.kind;
 278             then {
 279                 let traits = rollup_traits(cx, bound_predicate.bounds, "these bounds contain repeated elements");
 280                 for (trait_ref, span) in traits {
 281                     let key = (path.res, trait_ref);
 282                     if where_predicates.contains(&key) {
 283                         span_lint_and_help(
 284                             cx,
 285                             TRAIT_DUPLICATION_IN_BOUNDS,
 286                             span,
 287                             "this trait bound is already specified in the where clause",
 288                             None,
 289                             "consider removing this trait bound",
 290                             );
 291                     }
 292                 }
 293             }
 294         }
 295     }
 296 }
 297
 298 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
 299 struct ComparableTraitRef(Res, Vec<Res>);
 300 impl Default for ComparableTraitRef {
 301     fn default() -> Self {
 302         Self(Res::Err, Vec::new())
 303     }
 304 }
 305
 306 fn get_trait_info_from_bound<'a>(bound: &'a GenericBound<'_>) -> Option<(Res, &'a [PathSegment<'a>], Span)> {
 307     if let GenericBound::Trait(t, tbm) = bound {
 308         let trait_path = t.trait_ref.path;
 309         let trait_span = {
 310             let path_span = trait_path.span;
 311             if let TraitBoundModifier::Maybe = tbm {
 312                 path_span.with_lo(path_span.lo() - BytePos(1)) // include the `?`
 313             } else {
 314                 path_span
 315             }
 316         };
 317         Some((trait_path.res, trait_path.segments, trait_span))
 318     } else {
 319         None
 320     }
 321 }
 322
 323 // FIXME: ComparableTraitRef does not support nested bounds needed for associated_type_bounds
 324 fn into_comparable_trait_ref(trait_ref: &TraitRef<'_>) -> ComparableTraitRef {
 325     ComparableTraitRef(
 326         trait_ref.path.res,
 327         trait_ref
 328             .path
 329             .segments
 330             .iter()
 331             .filter_map(|segment| {
 332                 // get trait bound type arguments
 333                 Some(segment.args?.args.iter().filter_map(|arg| {
 334                     if_chain! {
 335                         if let GenericArg::Type(ty) = arg;
 336                         if let TyKind::Path(QPath::Resolved(_, path)) = ty.kind;
 337                         then { return Some(path.res) }
 338                     }
 339                     None
 340                 }))
 341             })
 342             .flatten()
 343             .collect(),
 344     )
 345 }
 346
 347 fn rollup_traits(cx: &LateContext<'_>, bounds: &[GenericBound<'_>], msg: &str) -> Vec<(ComparableTraitRef, Span)> {
 348     let mut map = FxHashMap::default();
 349     let mut repeated_res = false;
 350
 351     let only_comparable_trait_refs = |bound: &GenericBound<'_>| {
 352         if let GenericBound::Trait(t, _) = bound {
 353             Some((into_comparable_trait_ref(&t.trait_ref), t.span))
 354         } else {
 355             None
 356         }
 357     };
 358
 359     let mut i = 0usize;
 360     for bound in bounds.iter().filter_map(only_comparable_trait_refs) {
 361         let (comparable_bound, span_direct) = bound;
 362         match map.entry(comparable_bound) {
 363             Entry::Occupied(_) => repeated_res = true,
 364             Entry::Vacant(e) => {
 365                 e.insert((span_direct, i));
 366                 i += 1;
 367             },
 368         }
 369     }
 370
 371     // Put bounds in source order
 372     let mut comparable_bounds = vec![Default::default(); map.len()];
 373     for (k, (v, i)) in map {
 374         comparable_bounds[i] = (k, v);
 375     }
 376
 377     if_chain! {
 378         if repeated_res;
 379         if let [first_trait, .., last_trait] = bounds;
 380         then {
 381             let all_trait_span = first_trait.span().to(last_trait.span());
 382
 383             let traits = comparable_bounds.iter()
 384                 .filter_map(|&(_, span)| snippet_opt(cx, span))
 385                 .collect::<Vec<_>>();
 386             let traits = traits.join(" + ");
 387
 388             span_lint_and_sugg(
 389                 cx,
 390                 TRAIT_DUPLICATION_IN_BOUNDS,
 391                 all_trait_span,
 392                 msg,
 393                 "try",
 394                 traits,
 395                 Applicability::MachineApplicable
 396             );
 397         }
 398     }
 399
 400     comparable_bounds
 401 }