clippy_lints/src/bytecount.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet_with_applicability;
   3 use clippy_utils::ty::match_type;
   4 use clippy_utils::visitors::is_local_used;
   5 use clippy_utils::{path_to_local_id, paths, peel_blocks, peel_ref_operators, strip_pat_refs};
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BinOpKind, Expr, ExprKind, PatKind};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_middle::ty::{self, UintTy};
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use rustc_span::sym;
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for naive byte counts
  17     ///
  18     /// ### Why is this bad?
  19     /// The [`bytecount`](https://crates.io/crates/bytecount)
  20     /// crate has methods to count your bytes faster, especially for large slices.
  21     ///
  22     /// ### Known problems
  23     /// If you have predominantly small slices, the
  24     /// `bytecount::count(..)` method may actually be slower. However, if you can
  25     /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  26     /// faster in those cases.
  27     ///
  28     /// ### Example
  29     /// ```rust
  30     /// # let vec = vec![1_u8];
  31     /// let count = vec.iter().filter(|x| **x == 0u8).count();
  32     /// ```
  33     ///
  34     /// Use instead:
  35     /// ```rust,ignore
  36     /// # let vec = vec![1_u8];
  37     /// let count = bytecount::count(&vec, 0u8);
  38     /// ```
  39     #[clippy::version = "pre 1.29.0"]
  40     pub NAIVE_BYTECOUNT,
  41     pedantic,
  42     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  43 }
  44
  45 declare_lint_pass!(ByteCount => [NAIVE_BYTECOUNT]);
  46
  47 impl<'tcx> LateLintPass<'tcx> for ByteCount {
  48     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
  49         if_chain! {
  50             if let ExprKind::MethodCall(count, [count_recv], _) = expr.kind;
  51             if count.ident.name == sym::count;
  52             if let ExprKind::MethodCall(filter, [filter_recv, filter_arg], _) = count_recv.kind;
  53             if filter.ident.name == sym!(filter);
  54             if let ExprKind::Closure(_, _, body_id, _, _) = filter_arg.kind;
  55             let body = cx.tcx.hir().body(body_id);
  56             if let [param] = body.params;
  57             if let PatKind::Binding(_, arg_id, _, _) = strip_pat_refs(param.pat).kind;
  58             if let ExprKind::Binary(ref op, l, r) = body.value.kind;
  59             if op.node == BinOpKind::Eq;
  60             if match_type(cx,
  61                        cx.typeck_results().expr_ty(filter_recv).peel_refs(),
  62                        &paths::SLICE_ITER);
  63             let operand_is_arg = |expr| {
  64                 let expr = peel_ref_operators(cx, peel_blocks(expr));
  65                 path_to_local_id(expr, arg_id)
  66             };
  67             let needle = if operand_is_arg(l) {
  68                 r
  69             } else if operand_is_arg(r) {
  70                 l
  71             } else {
  72                 return;
  73             };
  74             if ty::Uint(UintTy::U8) == *cx.typeck_results().expr_ty(needle).peel_refs().kind();
  75             if !is_local_used(cx, needle, arg_id);
  76             then {
  77                 let haystack = if let ExprKind::MethodCall(path, args, _) =
  78                         filter_recv.kind {
  79                     let p = path.ident.name;
  80                     if (p == sym::iter || p == sym!(iter_mut)) && args.len() == 1 {
  81                         &args[0]
  82                     } else {
  83                         filter_recv
  84                     }
  85                 } else {
  86                     filter_recv
  87                 };
  88                 let mut applicability = Applicability::MaybeIncorrect;
  89                 span_lint_and_sugg(
  90                     cx,
  91                     NAIVE_BYTECOUNT,
  92                     expr.span,
  93                     "you appear to be counting bytes the naive way",
  94                     "consider using the bytecount crate",
  95                     format!("bytecount::count({}, {})",
  96                             snippet_with_applicability(cx, haystack.span, "..", &mut applicability),
  97                             snippet_with_applicability(cx, needle.span, "..", &mut applicability)),
  98                     applicability,
  99                 );
 100             }
 101         };
 102     }
 103 }