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