clippy_lints/src/bytecount.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::utils::{
  11     contains_name, get_pat_name, match_type, paths, single_segment_path, snippet_with_applicability,
  12     span_lint_and_sugg, walk_ptrs_ty,
  13 };
  14 use if_chain::if_chain;
  15 use rustc::hir::*;
  16 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  17 use rustc::ty;
  18 use rustc::{declare_tool_lint, lint_array};
  19 use rustc_errors::Applicability;
  20 use syntax::ast::{Name, UintTy};
  21
  22 /// **What it does:** Checks for naive byte counts
  23 ///
  24 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  25 /// crate has methods to count your bytes faster, especially for large slices.
  26 ///
  27 /// **Known problems:** If you have predominantly small slices, the
  28 /// `bytecount::count(..)` method may actually be slower. However, if you can
  29 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  30 /// faster in those cases.
  31 ///
  32 /// **Example:**
  33 ///
  34 /// ```rust
  35 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  36 /// ```
  37 declare_clippy_lint! {
  38     pub NAIVE_BYTECOUNT,
  39     perf,
  40     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  41 }
  42
  43 #[derive(Copy, Clone)]
  44 pub struct ByteCount;
  45
  46 impl LintPass for ByteCount {
  47     fn get_lints(&self) -> LintArray {
  48         lint_array!(NAIVE_BYTECOUNT)
  49     }
  50 }
  51
  52 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  53     fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr) {
  54         if_chain! {
  55             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.node;
  56             if count.ident.name == "count";
  57             if count_args.len() == 1;
  58             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].node;
  59             if filter.ident.name == "filter";
  60             if filter_args.len() == 2;
  61             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  62             then {
  63                 let body = cx.tcx.hir().body(body_id);
  64                 if_chain! {
  65                     if body.arguments.len() == 1;
  66                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  67                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  68                     if op.node == BinOpKind::Eq;
  69                     if match_type(cx,
  70                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  71                                &paths::SLICE_ITER);
  72                     then {
  73                         let needle = match get_path_name(l) {
  74                             Some(name) if check_arg(name, argname, r) => r,
  75                             _ => match get_path_name(r) {
  76                                 Some(name) if check_arg(name, argname, l) => l,
  77                                 _ => { return; }
  78                             }
  79                         };
  80                         if ty::Uint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  81                             return;
  82                         }
  83                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  84                                 filter_args[0].node {
  85                             let p = path.ident.name;
  86                             if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  87                                 &args[0]
  88                             } else {
  89                                 &filter_args[0]
  90                             }
  91                         } else {
  92                             &filter_args[0]
  93                         };
  94                         let mut applicability = Applicability::MachineApplicable;
  95                         span_lint_and_sugg(
  96                             cx,
  97                             NAIVE_BYTECOUNT,
  98                             expr.span,
  99                             "You appear to be counting bytes the naive way",
 100                             "Consider using the bytecount crate",
 101                             format!("bytecount::count({}, {})",
 102                                     snippet_with_applicability(cx, haystack.span, "..", &mut applicability),
 103                                     snippet_with_applicability(cx, needle.span, "..", &mut applicability)),
 104                             applicability,
 105                         );
 106                     }
 107                 };
 108             }
 109         };
 110     }
 111 }
 112
 113 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
 114     name == arg && !contains_name(name, needle)
 115 }
 116
 117 fn get_path_name(expr: &Expr) -> Option<Name> {
 118     match expr.node {
 119         ExprKind::Box(ref e) | ExprKind::AddrOf(_, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => get_path_name(e),
 120         ExprKind::Block(ref b, _) => {
 121             if b.stmts.is_empty() {
 122                 b.expr.as_ref().and_then(|p| get_path_name(p))
 123             } else {
 124                 None
 125             }
 126         },
 127         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 128         _ => None,
 129     }
 130 }