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_ref_operators, remove_blocks, 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     /// &vec.iter().filter(|x| **x == 0u8).count(); // use bytecount::count instead
  32     /// ```
  33     #[clippy::version = "pre 1.29.0"]
  34     pub NAIVE_BYTECOUNT,
  35     pedantic,
  36     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  37 }
  38
  39 declare_lint_pass!(ByteCount => [NAIVE_BYTECOUNT]);
  40
  41 impl<'tcx> LateLintPass<'tcx> for ByteCount {
  42     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
  43         if_chain! {
  44             if let ExprKind::MethodCall(count, _, [count_recv], _) = expr.kind;
  45             if count.ident.name == sym!(count);
  46             if let ExprKind::MethodCall(filter, _, [filter_recv, filter_arg], _) = count_recv.kind;
  47             if filter.ident.name == sym!(filter);
  48             if let ExprKind::Closure(_, _, body_id, _, _) = filter_arg.kind;
  49             let body = cx.tcx.hir().body(body_id);
  50             if let [param] = body.params;
  51             if let PatKind::Binding(_, arg_id, _, _) = strip_pat_refs(param.pat).kind;
  52             if let ExprKind::Binary(ref op, l, r) = body.value.kind;
  53             if op.node == BinOpKind::Eq;
  54             if match_type(cx,
  55                        cx.typeck_results().expr_ty(filter_recv).peel_refs(),
  56                        &paths::SLICE_ITER);
  57             let operand_is_arg = |expr| {
  58                 let expr = peel_ref_operators(cx, remove_blocks(expr));
  59                 path_to_local_id(expr, arg_id)
  60             };
  61             let needle = if operand_is_arg(l) {
  62                 r
  63             } else if operand_is_arg(r) {
  64                 l
  65             } else {
  66                 return;
  67             };
  68             if ty::Uint(UintTy::U8) == *cx.typeck_results().expr_ty(needle).peel_refs().kind();
  69             if !is_local_used(cx, needle, arg_id);
  70             then {
  71                 let haystack = if let ExprKind::MethodCall(path, _, args, _) =
  72                         filter_recv.kind {
  73                     let p = path.ident.name;
  74                     if (p == sym::iter || p == sym!(iter_mut)) && args.len() == 1 {
  75                         &args[0]
  76                     } else {
  77                         filter_recv
  78                     }
  79                 } else {
  80                     filter_recv
  81                 };
  82                 let mut applicability = Applicability::MaybeIncorrect;
  83                 span_lint_and_sugg(
  84                     cx,
  85                     NAIVE_BYTECOUNT,
  86                     expr.span,
  87                     "you appear to be counting bytes the naive way",
  88                     "consider using the bytecount crate",
  89                     format!("bytecount::count({}, {})",
  90                             snippet_with_applicability(cx, haystack.span, "..", &mut applicability),
  91                             snippet_with_applicability(cx, needle.span, "..", &mut applicability)),
  92                     applicability,
  93                 );
  94             }
  95         };
  96     }
  97 }