clippy_lints/src/bytecount.rs

   1 use consts::{constant, Constant};
   2 use rustc_const_math::ConstInt;
   3 use rustc::hir::*;
   4 use rustc::lint::*;
   5 use utils::{match_type, paths, snippet, span_lint_and_sugg, walk_ptrs_ty};
   6
   7 /// **What it does:** Checks for naive byte counts
   8 ///
   9 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  10 /// crate has methods to count your bytes faster, especially for large slices.
  11 ///
  12 /// **Known problems:** If you have predominantly small slices, the
  13 /// `bytecount::count(..)` method may actually be slower. However, if you can
  14 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  15 /// faster in those cases.
  16 ///
  17 /// **Example:**
  18 ///
  19 /// ```rust
  20 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  21 /// ```
  22 declare_lint! {
  23     pub NAIVE_BYTECOUNT,
  24     Warn,
  25     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  26 }
  27
  28 #[derive(Copy, Clone)]
  29 pub struct ByteCount;
  30
  31 impl LintPass for ByteCount {
  32     fn get_lints(&self) -> LintArray {
  33         lint_array!(NAIVE_BYTECOUNT)
  34     }
  35 }
  36
  37 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  38     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
  39         if_let_chain!([
  40             let ExprMethodCall(ref count, _, ref count_args) = expr.node,
  41             count.name == "count",
  42             count_args.len() == 1,
  43             let ExprMethodCall(ref filter, _, ref filter_args) = count_args[0].node,
  44             filter.name == "filter",
  45             filter_args.len() == 2,
  46             let ExprClosure(_, _, body_id, _) = filter_args[1].node,
  47         ], {
  48             let body = cx.tcx.hir.body(body_id);
  49             if_let_chain!([
  50                 let ExprBinary(ref op, ref l, ref r) = body.value.node,
  51                 op.node == BiEq,
  52                 match_type(cx,
  53                            walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  54                            &paths::SLICE_ITER),
  55                 let Some((Constant::Int(ConstInt::U8(needle)), _)) =
  56                         constant(cx, l).or_else(|| constant(cx, r))
  57             ], {
  58                 let haystack = if let ExprMethodCall(ref path, _, ref args) =
  59                         filter_args[0].node {
  60                     let p = path.name;
  61                     if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  62                         &args[0]
  63                     } else {
  64                         &filter_args[0]
  65                     }
  66                 } else {
  67                     &filter_args[0]
  68                 };
  69                 span_lint_and_sugg(cx,
  70                                    NAIVE_BYTECOUNT,
  71                                    expr.span,
  72                                    "You appear to be counting bytes the naive way",
  73                                    "Consider using the bytecount crate",
  74                                    format!("bytecount::count({}, {})",
  75                                             snippet(cx, haystack.span, ".."),
  76                                             needle));
  77             });
  78         });
  79     }
  80 }