clippy_lints/src/bytecount.rs

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