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::hir::*;
   7 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   8 use rustc::ty;
   9 use rustc::{declare_lint_pass, declare_tool_lint};
  10 use rustc_errors::Applicability;
  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     /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  28     /// ```
  29     pub NAIVE_BYTECOUNT,
  30     perf,
  31     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  32 }
  33
  34 declare_lint_pass!(ByteCount => [NAIVE_BYTECOUNT]);
  35
  36 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  37     fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr) {
  38         if_chain! {
  39             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.node;
  40             if count.ident.name == sym!(count);
  41             if count_args.len() == 1;
  42             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].node;
  43             if filter.ident.name == sym!(filter);
  44             if filter_args.len() == 2;
  45             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  46             then {
  47                 let body = cx.tcx.hir().body(body_id);
  48                 if_chain! {
  49                     if body.arguments.len() == 1;
  50                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  51                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  52                     if op.node == BinOpKind::Eq;
  53                     if match_type(cx,
  54                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  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) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  65                             return;
  66                         }
  67                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  68                                 filter_args[0].node {
  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 }
  96
  97 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
  98     name == arg && !contains_name(name, needle)
  99 }
 100
 101 fn get_path_name(expr: &Expr) -> Option<Name> {
 102     match expr.node {
 103         ExprKind::Box(ref e) | ExprKind::AddrOf(_, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => get_path_name(e),
 104         ExprKind::Block(ref 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 }