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_tool_lint, lint_array};
  10 use rustc_errors::Applicability;
  11 use syntax::ast::{Name, UintTy};
  12
  13 /// **What it does:** Checks for naive byte counts
  14 ///
  15 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  16 /// crate has methods to count your bytes faster, especially for large slices.
  17 ///
  18 /// **Known problems:** If you have predominantly small slices, the
  19 /// `bytecount::count(..)` method may actually be slower. However, if you can
  20 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  21 /// faster in those cases.
  22 ///
  23 /// **Example:**
  24 ///
  25 /// ```rust
  26 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  27 /// ```
  28 declare_clippy_lint! {
  29     pub NAIVE_BYTECOUNT,
  30     perf,
  31     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  32 }
  33
  34 #[derive(Copy, Clone)]
  35 pub struct ByteCount;
  36
  37 impl LintPass for ByteCount {
  38     fn get_lints(&self) -> LintArray {
  39         lint_array!(NAIVE_BYTECOUNT)
  40     }
  41
  42     fn name(&self) -> &'static str {
  43         "ByteCount"
  44     }
  45 }
  46
  47 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  48     fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr) {
  49         if_chain! {
  50             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.node;
  51             if count.ident.name == "count";
  52             if count_args.len() == 1;
  53             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].node;
  54             if filter.ident.name == "filter";
  55             if filter_args.len() == 2;
  56             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  57             then {
  58                 let body = cx.tcx.hir().body(body_id);
  59                 if_chain! {
  60                     if body.arguments.len() == 1;
  61                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  62                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  63                     if op.node == BinOpKind::Eq;
  64                     if match_type(cx,
  65                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  66                                &paths::SLICE_ITER);
  67                     then {
  68                         let needle = match get_path_name(l) {
  69                             Some(name) if check_arg(name, argname, r) => r,
  70                             _ => match get_path_name(r) {
  71                                 Some(name) if check_arg(name, argname, l) => l,
  72                                 _ => { return; }
  73                             }
  74                         };
  75                         if ty::Uint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  76                             return;
  77                         }
  78                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  79                                 filter_args[0].node {
  80                             let p = path.ident.name;
  81                             if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  82                                 &args[0]
  83                             } else {
  84                                 &filter_args[0]
  85                             }
  86                         } else {
  87                             &filter_args[0]
  88                         };
  89                         let mut applicability = Applicability::MachineApplicable;
  90                         span_lint_and_sugg(
  91                             cx,
  92                             NAIVE_BYTECOUNT,
  93                             expr.span,
  94                             "You appear to be counting bytes the naive way",
  95                             "Consider using the bytecount crate",
  96                             format!("bytecount::count({}, {})",
  97                                     snippet_with_applicability(cx, haystack.span, "..", &mut applicability),
  98                                     snippet_with_applicability(cx, needle.span, "..", &mut applicability)),
  99                             applicability,
 100                         );
 101                     }
 102                 };
 103             }
 104         };
 105     }
 106 }
 107
 108 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
 109     name == arg && !contains_name(name, needle)
 110 }
 111
 112 fn get_path_name(expr: &Expr) -> Option<Name> {
 113     match expr.node {
 114         ExprKind::Box(ref e) | ExprKind::AddrOf(_, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => get_path_name(e),
 115         ExprKind::Block(ref b, _) => {
 116             if b.stmts.is_empty() {
 117                 b.expr.as_ref().and_then(|p| get_path_name(p))
 118             } else {
 119                 None
 120             }
 121         },
 122         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 123         _ => None,
 124     }
 125 }