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, match_type, paths, single_segment_path, snippet, span_lint_and_sugg, walk_ptrs_ty};
   6
   7 /// **What it does:** Checks for naive byte counts
   8 ///
   9 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  10 /// crate has methods to count your bytes faster, especially for large slices.
  11 ///
  12 /// **Known problems:** If you have predominantly small slices, the
  13 /// `bytecount::count(..)` method may actually be slower. However, if you can
  14 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  15 /// faster in those cases.
  16 ///
  17 /// **Example:**
  18 ///
  19 /// ```rust
  20 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  21 /// ```
  22 declare_lint! {
  23     pub NAIVE_BYTECOUNT,
  24     Warn,
  25     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  26 }
  27
  28 #[derive(Copy, Clone)]
  29 pub struct ByteCount;
  30
  31 impl LintPass for ByteCount {
  32     fn get_lints(&self) -> LintArray {
  33         lint_array!(NAIVE_BYTECOUNT)
  34     }
  35 }
  36
  37 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  38     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
  39         if_let_chain!([
  40             let ExprMethodCall(ref count, _, ref count_args) = expr.node,
  41             count.name == "count",
  42             count_args.len() == 1,
  43             let ExprMethodCall(ref filter, _, ref filter_args) = count_args[0].node,
  44             filter.name == "filter",
  45             filter_args.len() == 2,
  46             let ExprClosure(_, _, body_id, _, _) = filter_args[1].node,
  47         ], {
  48             let body = cx.tcx.hir.body(body_id);
  49             if_let_chain!([
  50                 body.arguments.len() == 1,
  51                 let Some(argname) = get_pat_name(&body.arguments[0].pat),
  52                 let ExprBinary(ref op, ref l, ref r) = body.value.node,
  53                 op.node == BiEq,
  54                 match_type(cx,
  55                            walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  56                            &paths::SLICE_ITER),
  57             ], {
  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::TyUint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  66                     return;
  67                 }
  68                 let haystack = if let ExprMethodCall(ref path, _, ref args) =
  69                         filter_args[0].node {
  70                     let p = path.name;
  71                     if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  72                         &args[0]
  73                     } else {
  74                         &filter_args[0]
  75                     }
  76                 } else {
  77                     &filter_args[0]
  78                 };
  79                 span_lint_and_sugg(cx,
  80                                    NAIVE_BYTECOUNT,
  81                                    expr.span,
  82                                    "You appear to be counting bytes the naive way",
  83                                    "Consider using the bytecount crate",
  84                                    format!("bytecount::count({}, {})",
  85                                             snippet(cx, haystack.span, ".."),
  86                                             snippet(cx, needle.span, "..")));
  87             });
  88         });
  89     }
  90 }
  91
  92 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
  93     name == arg && !contains_name(name, needle)
  94 }
  95
  96 fn get_pat_name(pat: &Pat) -> Option<Name> {
  97     match pat.node {
  98         PatKind::Binding(_, _, ref spname, _) => Some(spname.node),
  99         PatKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.name),
 100         PatKind::Box(ref p) | PatKind::Ref(ref p, _) => get_pat_name(&*p),
 101         _ => None,
 102     }
 103 }
 104
 105 fn get_path_name(expr: &Expr) -> Option<Name> {
 106     match expr.node {
 107         ExprBox(ref e) | ExprAddrOf(_, ref e) | ExprUnary(UnOp::UnDeref, ref e) => get_path_name(e),
 108         ExprBlock(ref b) => if b.stmts.is_empty() {
 109             b.expr.as_ref().and_then(|p| get_path_name(p))
 110         } else {
 111             None
 112         },
 113         ExprPath(ref qpath) => single_segment_path(qpath).map(|ps| ps.name),
 114         _ => None,
 115     }
 116 }