clippy_lints/src/trait_bounds.rs

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