clippy_lints/src/trait_bounds.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::source::{snippet, snippet_with_applicability};
   3 use clippy_utils::{in_macro, SpanlessHash};
   4 use if_chain::if_chain;
   5 use rustc_data_structures::fx::FxHashMap;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{def::Res, GenericBound, Generics, ParamName, Path, QPath, TyKind, WherePredicate};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_tool_lint, impl_lint_pass};
  10 use rustc_span::Span;
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** This lint warns about unnecessary type repetitions in trait bounds
  14     ///
  15     /// **Why is this bad?** Repeating the type for every bound makes the code
  16     /// less readable than combining the bounds
  17     ///
  18     /// **Known problems:** None.
  19     ///
  20     /// **Example:**
  21     /// ```rust
  22     /// pub fn foo<T>(t: T) where T: Copy, T: Clone {}
  23     /// ```
  24     ///
  25     /// Could be written as:
  26     ///
  27     /// ```rust
  28     /// pub fn foo<T>(t: T) where T: Copy + Clone {}
  29     /// ```
  30     pub TYPE_REPETITION_IN_BOUNDS,
  31     pedantic,
  32     "Types are repeated unnecessary in trait bounds use `+` instead of using `T: _, T: _`"
  33 }
  34
  35 declare_clippy_lint! {
  36     /// **What it does:** Checks for cases where generics are being used and multiple
  37     /// syntax specifications for trait bounds are used simultaneously.
  38     ///
  39     /// **Why is this bad?** Duplicate bounds makes the code
  40     /// less readable than specifing them only once.
  41     ///
  42     /// **Known problems:** None.
  43     ///
  44     /// **Example:**
  45     /// ```rust
  46     /// fn func<T: Clone + Default>(arg: T) where T: Clone + Default {}
  47     /// ```
  48     ///
  49     /// Could be written as:
  50     ///
  51     /// ```rust
  52     /// fn func<T: Clone + Default>(arg: T) {}
  53     /// ```
  54     /// or
  55     ///
  56     /// ```rust
  57     /// fn func<T>(arg: T) where T: Clone + Default {}
  58     /// ```
  59     pub TRAIT_DUPLICATION_IN_BOUNDS,
  60     pedantic,
  61     "Check if the same trait bounds are specified twice during a function declaration"
  62 }
  63
  64 #[derive(Copy, Clone)]
  65 pub struct TraitBounds {
  66     max_trait_bounds: u64,
  67 }
  68
  69 impl TraitBounds {
  70     #[must_use]
  71     pub fn new(max_trait_bounds: u64) -> Self {
  72         Self { max_trait_bounds }
  73     }
  74 }
  75
  76 impl_lint_pass!(TraitBounds => [TYPE_REPETITION_IN_BOUNDS, TRAIT_DUPLICATION_IN_BOUNDS]);
  77
  78 impl<'tcx> LateLintPass<'tcx> for TraitBounds {
  79     fn check_generics(&mut self, cx: &LateContext<'tcx>, gen: &'tcx Generics<'_>) {
  80         self.check_type_repetition(cx, gen);
  81         check_trait_bound_duplication(cx, gen);
  82     }
  83 }
  84
  85 fn get_trait_res_span_from_bound(bound: &GenericBound<'_>) -> Option<(Res, Span)> {
  86     if let GenericBound::Trait(t, _) = bound {
  87         Some((t.trait_ref.path.res, t.span))
  88     } else {
  89         None
  90     }
  91 }
  92
  93 impl TraitBounds {
  94     fn check_type_repetition(self, cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
  95         if in_macro(gen.span) {
  96             return;
  97         }
  98         let hash = |ty| -> u64 {
  99             let mut hasher = SpanlessHash::new(cx);
 100             hasher.hash_ty(ty);
 101             hasher.finish()
 102         };
 103         let mut map = FxHashMap::default();
 104         let mut applicability = Applicability::MaybeIncorrect;
 105         for bound in gen.where_clause.predicates {
 106             if_chain! {
 107                 if let WherePredicate::BoundPredicate(ref p) = bound;
 108                 if p.bounds.len() as u64 <= self.max_trait_bounds;
 109                 if !in_macro(p.span);
 110                 let h = hash(p.bounded_ty);
 111                 if let Some(ref v) = map.insert(h, p.bounds.iter().collect::<Vec<_>>());
 112
 113                 then {
 114                     let mut hint_string = format!(
 115                         "consider combining the bounds: `{}:",
 116                         snippet(cx, p.bounded_ty.span, "_")
 117                     );
 118                     for b in v.iter() {
 119                         if let GenericBound::Trait(ref poly_trait_ref, _) = b {
 120                             let path = &poly_trait_ref.trait_ref.path;
 121                             hint_string.push_str(&format!(
 122                                 " {} +",
 123                                 snippet_with_applicability(cx, path.span, "..", &mut applicability)
 124                             ));
 125                         }
 126                     }
 127                     for b in p.bounds.iter() {
 128                         if let GenericBound::Trait(ref poly_trait_ref, _) = b {
 129                             let path = &poly_trait_ref.trait_ref.path;
 130                             hint_string.push_str(&format!(
 131                                 " {} +",
 132                                 snippet_with_applicability(cx, path.span, "..", &mut applicability)
 133                             ));
 134                         }
 135                     }
 136                     hint_string.truncate(hint_string.len() - 2);
 137                     hint_string.push('`');
 138                     span_lint_and_help(
 139                         cx,
 140                         TYPE_REPETITION_IN_BOUNDS,
 141                         p.span,
 142                         "this type has already been used as a bound predicate",
 143                         None,
 144                         &hint_string,
 145                     );
 146                 }
 147             }
 148         }
 149     }
 150 }
 151
 152 fn check_trait_bound_duplication(cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
 153     if in_macro(gen.span) || gen.params.is_empty() || gen.where_clause.predicates.is_empty() {
 154         return;
 155     }
 156
 157     let mut map = FxHashMap::default();
 158     for param in gen.params {
 159         if let ParamName::Plain(ref ident) = param.name {
 160             let res = param
 161                 .bounds
 162                 .iter()
 163                 .filter_map(get_trait_res_span_from_bound)
 164                 .collect::<Vec<_>>();
 165             map.insert(*ident, res);
 166         }
 167     }
 168
 169     for predicate in gen.where_clause.predicates {
 170         if_chain! {
 171             if let WherePredicate::BoundPredicate(ref bound_predicate) = predicate;
 172             if !in_macro(bound_predicate.span);
 173             if let TyKind::Path(QPath::Resolved(_, Path { segments, .. })) = bound_predicate.bounded_ty.kind;
 174             if let Some(segment) = segments.first();
 175             if let Some(trait_resolutions_direct) = map.get(&segment.ident);
 176             then {
 177                 for (res_where, _) in bound_predicate.bounds.iter().filter_map(get_trait_res_span_from_bound) {
 178                     if let Some((_, span_direct)) = trait_resolutions_direct
 179                                                 .iter()
 180                                                 .find(|(res_direct, _)| *res_direct == res_where) {
 181                         span_lint_and_help(
 182                             cx,
 183                             TRAIT_DUPLICATION_IN_BOUNDS,
 184                             *span_direct,
 185                             "this trait bound is already specified in the where clause",
 186                             None,
 187                             "consider removing this trait bound",
 188                         );
 189                     }
 190                 }
 191             }
 192         }
 193     }
 194 }