clippy_lints/src/bytecount.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::{declare_lint, lint_array};
   4 use rustc::ty;
   5 use syntax::ast::{Name, UintTy};
   6 use crate::utils::{contains_name, get_pat_name, match_type, paths, single_segment_path, snippet, span_lint_and_sugg,
   7             walk_ptrs_ty};
   8
   9 /// **What it does:** Checks for naive byte counts
  10 ///
  11 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  12 /// crate has methods to count your bytes faster, especially for large slices.
  13 ///
  14 /// **Known problems:** If you have predominantly small slices, the
  15 /// `bytecount::count(..)` method may actually be slower. However, if you can
  16 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  17 /// faster in those cases.
  18 ///
  19 /// **Example:**
  20 ///
  21 /// ```rust
  22 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  23 /// ```
  24 declare_clippy_lint! {
  25     pub NAIVE_BYTECOUNT,
  26     perf,
  27     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  28 }
  29
  30 #[derive(Copy, Clone)]
  31 pub struct ByteCount;
  32
  33 impl LintPass for ByteCount {
  34     fn get_lints(&self) -> LintArray {
  35         lint_array!(NAIVE_BYTECOUNT)
  36     }
  37 }
  38
  39 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  40     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
  41         if_chain! {
  42             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.node;
  43             if count.ident.name == "count";
  44             if count_args.len() == 1;
  45             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].node;
  46             if filter.ident.name == "filter";
  47             if filter_args.len() == 2;
  48             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  49             then {
  50                 let body = cx.tcx.hir.body(body_id);
  51                 if_chain! {
  52                     if body.arguments.len() == 1;
  53                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  54                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  55                     if op.node == BinOpKind::Eq;
  56                     if match_type(cx,
  57                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  58                                &paths::SLICE_ITER);
  59                     then {
  60                         let needle = match get_path_name(l) {
  61                             Some(name) if check_arg(name, argname, r) => r,
  62                             _ => match get_path_name(r) {
  63                                 Some(name) if check_arg(name, argname, l) => l,
  64                                 _ => { return; }
  65                             }
  66                         };
  67                         if ty::TyUint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  68                             return;
  69                         }
  70                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  71                                 filter_args[0].node {
  72                             let p = path.ident.name;
  73                             if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  74                                 &args[0]
  75                             } else {
  76                                 &filter_args[0]
  77                             }
  78                         } else {
  79                             &filter_args[0]
  80                         };
  81                         span_lint_and_sugg(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(cx, haystack.span, ".."),
  88                                                     snippet(cx, needle.span, "..")));
  89                     }
  90                 };
  91             }
  92         };
  93     }
  94 }
  95
  96 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
  97     name == arg && !contains_name(name, needle)
  98 }
  99
 100 fn get_path_name(expr: &Expr) -> Option<Name> {
 101     match expr.node {
 102         ExprKind::Box(ref e) | ExprKind::AddrOf(_, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => get_path_name(e),
 103         ExprKind::Block(ref b, _) => if b.stmts.is_empty() {
 104             b.expr.as_ref().and_then(|p| get_path_name(p))
 105         } else {
 106             None
 107         },
 108         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 109         _ => None,
 110     }
 111 }