clippy_lints/src/bytecount.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::{declare_lint, lint_array};
   4 use if_chain::if_chain;
   5 use rustc::ty;
   6 use syntax::ast::{Name, UintTy};
   7 use crate::utils::{contains_name, get_pat_name, match_type, paths, single_segment_path, snippet, span_lint_and_sugg,
   8             walk_ptrs_ty};
   9
  10 /// **What it does:** Checks for naive byte counts
  11 ///
  12 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  13 /// crate has methods to count your bytes faster, especially for large slices.
  14 ///
  15 /// **Known problems:** If you have predominantly small slices, the
  16 /// `bytecount::count(..)` method may actually be slower. However, if you can
  17 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  18 /// faster in those cases.
  19 ///
  20 /// **Example:**
  21 ///
  22 /// ```rust
  23 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  24 /// ```
  25 declare_clippy_lint! {
  26     pub NAIVE_BYTECOUNT,
  27     perf,
  28     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  29 }
  30
  31 #[derive(Copy, Clone)]
  32 pub struct ByteCount;
  33
  34 impl LintPass for ByteCount {
  35     fn get_lints(&self) -> LintArray {
  36         lint_array!(NAIVE_BYTECOUNT)
  37     }
  38 }
  39
  40 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  41     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
  42         if_chain! {
  43             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.node;
  44             if count.ident.name == "count";
  45             if count_args.len() == 1;
  46             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].node;
  47             if filter.ident.name == "filter";
  48             if filter_args.len() == 2;
  49             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  50             then {
  51                 let body = cx.tcx.hir.body(body_id);
  52                 if_chain! {
  53                     if body.arguments.len() == 1;
  54                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  55                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  56                     if op.node == BinOpKind::Eq;
  57                     if match_type(cx,
  58                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  59                                &paths::SLICE_ITER);
  60                     then {
  61                         let needle = match get_path_name(l) {
  62                             Some(name) if check_arg(name, argname, r) => r,
  63                             _ => match get_path_name(r) {
  64                                 Some(name) if check_arg(name, argname, l) => l,
  65                                 _ => { return; }
  66                             }
  67                         };
  68                         if ty::TyUint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  69                             return;
  70                         }
  71                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  72                                 filter_args[0].node {
  73                             let p = path.ident.name;
  74                             if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  75                                 &args[0]
  76                             } else {
  77                                 &filter_args[0]
  78                             }
  79                         } else {
  80                             &filter_args[0]
  81                         };
  82                         span_lint_and_sugg(cx,
  83                                            NAIVE_BYTECOUNT,
  84                                            expr.span,
  85                                            "You appear to be counting bytes the naive way",
  86                                            "Consider using the bytecount crate",
  87                                            format!("bytecount::count({}, {})",
  88                                                     snippet(cx, haystack.span, ".."),
  89                                                     snippet(cx, needle.span, "..")));
  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, _) => if b.stmts.is_empty() {
 105             b.expr.as_ref().and_then(|p| get_path_name(p))
 106         } else {
 107             None
 108         },
 109         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 110         _ => None,
 111     }
 112 }