clippy_lints/src/bytecount.rs

   1 use crate::utils::{
   2     contains_name, get_pat_name, match_type, paths, single_segment_path, snippet_with_applicability,
   3     span_lint_and_sugg, walk_ptrs_ty,
   4 };
   5 use if_chain::if_chain;
   6 use rustc::lint::{LateContext, LateLintPass};
   7 use rustc::ty;
   8 use rustc_errors::Applicability;
   9 use rustc_hir::*;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use syntax::ast::{Name, UintTy};
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for naive byte counts
  15     ///
  16     /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  17     /// crate has methods to count your bytes faster, especially for large slices.
  18     ///
  19     /// **Known problems:** If you have predominantly small slices, the
  20     /// `bytecount::count(..)` method may actually be slower. However, if you can
  21     /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  22     /// faster in those cases.
  23     ///
  24     /// **Example:**
  25     ///
  26     /// ```rust
  27     /// # let vec = vec![1_u8];
  28     /// &vec.iter().filter(|x| **x == 0u8).count(); // use bytecount::count instead
  29     /// ```
  30     pub NAIVE_BYTECOUNT,
  31     perf,
  32     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  33 }
  34
  35 declare_lint_pass!(ByteCount => [NAIVE_BYTECOUNT]);
  36
  37 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  38     fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
  39         if_chain! {
  40             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.kind;
  41             if count.ident.name == sym!(count);
  42             if count_args.len() == 1;
  43             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].kind;
  44             if filter.ident.name == sym!(filter);
  45             if filter_args.len() == 2;
  46             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].kind;
  47             then {
  48                 let body = cx.tcx.hir().body(body_id);
  49                 if_chain! {
  50                     if body.params.len() == 1;
  51                     if let Some(argname) = get_pat_name(&body.params[0].pat);
  52                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.kind;
  53                     if op.node == BinOpKind::Eq;
  54                     if match_type(cx,
  55                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  56                                &paths::SLICE_ITER);
  57                     then {
  58                         let needle = match get_path_name(l) {
  59                             Some(name) if check_arg(name, argname, r) => r,
  60                             _ => match get_path_name(r) {
  61                                 Some(name) if check_arg(name, argname, l) => l,
  62                                 _ => { return; }
  63                             }
  64                         };
  65                         if ty::Uint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).kind {
  66                             return;
  67                         }
  68                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  69                                 filter_args[0].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_args[0]
  75                             }
  76                         } else {
  77                             &filter_args[0]
  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         };
  95     }
  96 }
  97
  98 fn check_arg(name: Name, arg: Name, needle: &Expr<'_>) -> bool {
  99     name == arg && !contains_name(name, needle)
 100 }
 101
 102 fn get_path_name(expr: &Expr<'_>) -> Option<Name> {
 103     match expr.kind {
 104         ExprKind::Box(ref e) | ExprKind::AddrOf(BorrowKind::Ref, _, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => {
 105             get_path_name(e)
 106         },
 107         ExprKind::Block(ref b, _) => {
 108             if b.stmts.is_empty() {
 109                 b.expr.as_ref().and_then(|p| get_path_name(p))
 110             } else {
 111                 None
 112             }
 113         },
 114         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 115         _ => None,
 116     }
 117 }