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

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