clippy_lints/src/bytecount.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::ty;
   4 use syntax::ast::{Name, UintTy};
   5 use utils::{contains_name, get_pat_name, match_type, paths, single_segment_path,
   6             snippet, span_lint_and_sugg, walk_ptrs_ty};
   7
   8 /// **What it does:** Checks for naive byte counts
   9 ///
  10 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  11 /// crate has methods to count your bytes faster, especially for large slices.
  12 ///
  13 /// **Known problems:** If you have predominantly small slices, the
  14 /// `bytecount::count(..)` method may actually be slower. However, if you can
  15 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  16 /// faster in those cases.
  17 ///
  18 /// **Example:**
  19 ///
  20 /// ```rust
  21 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  22 /// ```
  23 declare_lint! {
  24     pub NAIVE_BYTECOUNT,
  25     Warn,
  26     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  27 }
  28
  29 #[derive(Copy, Clone)]
  30 pub struct ByteCount;
  31
  32 impl LintPass for ByteCount {
  33     fn get_lints(&self) -> LintArray {
  34         lint_array!(NAIVE_BYTECOUNT)
  35     }
  36 }
  37
  38 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  39     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
  40         if_let_chain!([
  41             let ExprMethodCall(ref count, _, ref count_args) = expr.node,
  42             count.name == "count",
  43             count_args.len() == 1,
  44             let ExprMethodCall(ref filter, _, ref filter_args) = count_args[0].node,
  45             filter.name == "filter",
  46             filter_args.len() == 2,
  47             let ExprClosure(_, _, body_id, _, _) = filter_args[1].node,
  48         ], {
  49             let body = cx.tcx.hir.body(body_id);
  50             if_let_chain!([
  51                 body.arguments.len() == 1,
  52                 let Some(argname) = get_pat_name(&body.arguments[0].pat),
  53                 let ExprBinary(ref op, ref l, ref r) = body.value.node,
  54                 op.node == BiEq,
  55                 match_type(cx,
  56                            walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  57                            &paths::SLICE_ITER),
  58             ], {
  59                 let needle = match get_path_name(l) {
  60                     Some(name) if check_arg(name, argname, r) => r,
  61                     _ => match get_path_name(r) {
  62                         Some(name) if check_arg(name, argname, l) => l,
  63                         _ => { return; }
  64                     }
  65                 };
  66                 if ty::TyUint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  67                     return;
  68                 }
  69                 let haystack = if let ExprMethodCall(ref path, _, ref args) =
  70                         filter_args[0].node {
  71                     let p = path.name;
  72                     if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  73                         &args[0]
  74                     } else {
  75                         &filter_args[0]
  76                     }
  77                 } else {
  78                     &filter_args[0]
  79                 };
  80                 span_lint_and_sugg(cx,
  81                                    NAIVE_BYTECOUNT,
  82                                    expr.span,
  83                                    "You appear to be counting bytes the naive way",
  84                                    "Consider using the bytecount crate",
  85                                    format!("bytecount::count({}, {})",
  86                                             snippet(cx, haystack.span, ".."),
  87                                             snippet(cx, needle.span, "..")));
  88             });
  89         });
  90     }
  91 }
  92
  93 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
  94     name == arg && !contains_name(name, needle)
  95 }
  96
  97 fn get_path_name(expr: &Expr) -> Option<Name> {
  98     match expr.node {
  99         ExprBox(ref e) | ExprAddrOf(_, ref e) | ExprUnary(UnOp::UnDeref, ref e) => get_path_name(e),
 100         ExprBlock(ref b) => if b.stmts.is_empty() {
 101             b.expr.as_ref().and_then(|p| get_path_name(p))
 102         } else {
 103             None
 104         },
 105         ExprPath(ref qpath) => single_segment_path(qpath).map(|ps| ps.name),
 106         _ => None,
 107     }
 108 }