clippy_lints/src/trait_bounds.rs

   1 use crate::utils::{in_macro, snippet, snippet_with_applicability, span_lint_and_help, SpanlessHash};
   2 use rustc_data_structures::fx::FxHashMap;
   3 use rustc_errors::Applicability;
   4 use rustc_hir::{GenericBound, Generics, WherePredicate};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_session::{declare_tool_lint, impl_lint_pass};
   7
   8 #[derive(Copy, Clone)]
   9 pub struct TraitBounds;
  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     /// **Example:**
  18     /// ```rust
  19     /// pub fn foo<T>(t: T) where T: Copy, T: Clone {}
  20     /// ```
  21     ///
  22     /// Could be written as:
  23     ///
  24     /// ```rust
  25     /// pub fn foo<T>(t: T) where T: Copy + Clone {}
  26     /// ```
  27     pub TYPE_REPETITION_IN_BOUNDS,
  28     pedantic,
  29     "Types are repeated unnecessary in trait bounds use `+` instead of using `T: _, T: _`"
  30 }
  31
  32 impl_lint_pass!(TraitBounds => [TYPE_REPETITION_IN_BOUNDS]);
  33
  34 impl<'tcx> LateLintPass<'tcx> for TraitBounds {
  35     fn check_generics(&mut self, cx: &LateContext<'tcx>, gen: &'tcx Generics<'_>) {
  36         if in_macro(gen.span) {
  37             return;
  38         }
  39         let hash = |ty| -> u64 {
  40             let mut hasher = SpanlessHash::new(cx);
  41             hasher.hash_ty(ty);
  42             hasher.finish()
  43         };
  44         let mut map = FxHashMap::default();
  45         let mut applicability = Applicability::MaybeIncorrect;
  46         for bound in gen.where_clause.predicates {
  47             if let WherePredicate::BoundPredicate(ref p) = bound {
  48                 let h = hash(&p.bounded_ty);
  49                 if let Some(ref v) = map.insert(h, p.bounds.iter().collect::<Vec<_>>()) {
  50                     let mut hint_string = format!(
  51                         "consider combining the bounds: `{}:",
  52                         snippet(cx, p.bounded_ty.span, "_")
  53                     );
  54                     for b in v.iter() {
  55                         if let GenericBound::Trait(ref poly_trait_ref, _) = b {
  56                             let path = &poly_trait_ref.trait_ref.path;
  57                             hint_string.push_str(&format!(
  58                                 " {} +",
  59                                 snippet_with_applicability(cx, path.span, "..", &mut applicability)
  60                             ));
  61                         }
  62                     }
  63                     for b in p.bounds.iter() {
  64                         if let GenericBound::Trait(ref poly_trait_ref, _) = b {
  65                             let path = &poly_trait_ref.trait_ref.path;
  66                             hint_string.push_str(&format!(
  67                                 " {} +",
  68                                 snippet_with_applicability(cx, path.span, "..", &mut applicability)
  69                             ));
  70                         }
  71                     }
  72                     hint_string.truncate(hint_string.len() - 2);
  73                     hint_string.push('`');
  74                     span_lint_and_help(
  75                         cx,
  76                         TYPE_REPETITION_IN_BOUNDS,
  77                         p.span,
  78                         "this type has already been used as a bound predicate",
  79                         None,
  80                         &hint_string,
  81                     );
  82                 }
  83             }
  84         }
  85     }
  86 }