src/tools/clippy/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::{contains_name, get_pat_name, paths, single_segment_path};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, UnOp};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty::{self, UintTy};
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::sym;
  12 use rustc_span::Symbol;
  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_args, _) = expr.kind;
  42             if count.ident.name == sym!(count);
  43             if count_args.len() == 1;
  44             if let ExprKind::MethodCall(filter, _, filter_args, _) = count_args[0].kind;
  45             if filter.ident.name == sym!(filter);
  46             if filter_args.len() == 2;
  47             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].kind;
  48             let body = cx.tcx.hir().body(body_id);
  49             if body.params.len() == 1;
  50             if let Some(argname) = get_pat_name(body.params[0].pat);
  51             if let ExprKind::Binary(ref op, l, r) = body.value.kind;
  52             if op.node == BinOpKind::Eq;
  53             if match_type(cx,
  54                        cx.typeck_results().expr_ty(&filter_args[0]).peel_refs(),
  55                        &paths::SLICE_ITER);
  56             then {
  57                 let needle = match get_path_name(l) {
  58                     Some(name) if check_arg(name, argname, r) => r,
  59                     _ => match get_path_name(r) {
  60                         Some(name) if check_arg(name, argname, l) => l,
  61                         _ => { return; }
  62                     }
  63                 };
  64                 if ty::Uint(UintTy::U8) != *cx.typeck_results().expr_ty(needle).peel_refs().kind() {
  65                     return;
  66                 }
  67                 let haystack = if let ExprKind::MethodCall(path, _, args, _) =
  68                         filter_args[0].kind {
  69                     let p = path.ident.name;
  70                     if (p == sym::iter || p == sym!(iter_mut)) && args.len() == 1 {
  71                         &args[0]
  72                     } else {
  73                         &filter_args[0]
  74                     }
  75                 } else {
  76                     &filter_args[0]
  77                 };
  78                 let mut applicability = Applicability::MaybeIncorrect;
  79                 span_lint_and_sugg(
  80                     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_with_applicability(cx, haystack.span, "..", &mut applicability),
  87                             snippet_with_applicability(cx, needle.span, "..", &mut applicability)),
  88                     applicability,
  89                 );
  90             }
  91         };
  92     }
  93 }
  94
  95 fn check_arg(name: Symbol, arg: Symbol, needle: &Expr<'_>) -> bool {
  96     name == arg && !contains_name(name, needle)
  97 }
  98
  99 fn get_path_name(expr: &Expr<'_>) -> Option<Symbol> {
 100     match expr.kind {
 101         ExprKind::Box(e) | ExprKind::AddrOf(BorrowKind::Ref, _, e) | ExprKind::Unary(UnOp::Deref, e) => {
 102             get_path_name(e)
 103         },
 104         ExprKind::Block(b, _) => {
 105             if b.stmts.is_empty() {
 106                 b.expr.as_ref().and_then(|p| get_path_name(p))
 107             } else {
 108                 None
 109             }
 110         },
 111         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 112         _ => None,
 113     }
 114 }