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 crate::utils::sym;
   6 use if_chain::if_chain;
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::ty;
  10 use rustc::{declare_lint_pass, declare_tool_lint};
  11 use rustc_errors::Applicability;
  12 use syntax::ast::{Name, UintTy};
  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     /// &my_data.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.node;
  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].node;
  44             if filter.ident.name == *sym::filter;
  45             if filter_args.len() == 2;
  46             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  47             then {
  48                 let body = cx.tcx.hir().body(body_id);
  49                 if_chain! {
  50                     if body.arguments.len() == 1;
  51                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  52                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  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)).sty {
  66                             return;
  67                         }
  68                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  69                                 filter_args[0].node {
  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.node {
 104         ExprKind::Box(ref e) | ExprKind::AddrOf(_, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => get_path_name(e),
 105         ExprKind::Block(ref b, _) => {
 106             if b.stmts.is_empty() {
 107                 b.expr.as_ref().and_then(|p| get_path_name(p))
 108             } else {
 109                 None
 110             }
 111         },
 112         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 113         _ => None,
 114     }
 115 }