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
  11 use crate::rustc::hir::*;
  12 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  13 use crate::rustc::{declare_tool_lint, lint_array};
  14 use if_chain::if_chain;
  15 use crate::rustc::ty;
  16 use crate::syntax::ast::{Name, UintTy};
  17 use crate::utils::{contains_name, get_pat_name, match_type, paths, single_segment_path, snippet, span_lint_and_sugg,
  18             walk_ptrs_ty};
  19
  20 /// **What it does:** Checks for naive byte counts
  21 ///
  22 /// **Why is this bad?** The [`bytecount`](https://crates.io/crates/bytecount)
  23 /// crate has methods to count your bytes faster, especially for large slices.
  24 ///
  25 /// **Known problems:** If you have predominantly small slices, the
  26 /// `bytecount::count(..)` method may actually be slower. However, if you can
  27 /// ensure that less than 2³²-1 matches arise, the `naive_count_32(..)` can be
  28 /// faster in those cases.
  29 ///
  30 /// **Example:**
  31 ///
  32 /// ```rust
  33 /// &my_data.filter(|&x| x == 0u8).count() // use bytecount::count instead
  34 /// ```
  35 declare_clippy_lint! {
  36     pub NAIVE_BYTECOUNT,
  37     perf,
  38     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
  39 }
  40
  41 #[derive(Copy, Clone)]
  42 pub struct ByteCount;
  43
  44 impl LintPass for ByteCount {
  45     fn get_lints(&self) -> LintArray {
  46         lint_array!(NAIVE_BYTECOUNT)
  47     }
  48 }
  49
  50 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ByteCount {
  51     fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr) {
  52         if_chain! {
  53             if let ExprKind::MethodCall(ref count, _, ref count_args) = expr.node;
  54             if count.ident.name == "count";
  55             if count_args.len() == 1;
  56             if let ExprKind::MethodCall(ref filter, _, ref filter_args) = count_args[0].node;
  57             if filter.ident.name == "filter";
  58             if filter_args.len() == 2;
  59             if let ExprKind::Closure(_, _, body_id, _, _) = filter_args[1].node;
  60             then {
  61                 let body = cx.tcx.hir.body(body_id);
  62                 if_chain! {
  63                     if body.arguments.len() == 1;
  64                     if let Some(argname) = get_pat_name(&body.arguments[0].pat);
  65                     if let ExprKind::Binary(ref op, ref l, ref r) = body.value.node;
  66                     if op.node == BinOpKind::Eq;
  67                     if match_type(cx,
  68                                walk_ptrs_ty(cx.tables.expr_ty(&filter_args[0])),
  69                                &paths::SLICE_ITER);
  70                     then {
  71                         let needle = match get_path_name(l) {
  72                             Some(name) if check_arg(name, argname, r) => r,
  73                             _ => match get_path_name(r) {
  74                                 Some(name) if check_arg(name, argname, l) => l,
  75                                 _ => { return; }
  76                             }
  77                         };
  78                         if ty::Uint(UintTy::U8) != walk_ptrs_ty(cx.tables.expr_ty(needle)).sty {
  79                             return;
  80                         }
  81                         let haystack = if let ExprKind::MethodCall(ref path, _, ref args) =
  82                                 filter_args[0].node {
  83                             let p = path.ident.name;
  84                             if (p == "iter" || p == "iter_mut") && args.len() == 1 {
  85                                 &args[0]
  86                             } else {
  87                                 &filter_args[0]
  88                             }
  89                         } else {
  90                             &filter_args[0]
  91                         };
  92                         span_lint_and_sugg(cx,
  93                                            NAIVE_BYTECOUNT,
  94                                            expr.span,
  95                                            "You appear to be counting bytes the naive way",
  96                                            "Consider using the bytecount crate",
  97                                            format!("bytecount::count({}, {})",
  98                                                     snippet(cx, haystack.span, ".."),
  99                                                     snippet(cx, needle.span, "..")));
 100                     }
 101                 };
 102             }
 103         };
 104     }
 105 }
 106
 107 fn check_arg(name: Name, arg: Name, needle: &Expr) -> bool {
 108     name == arg && !contains_name(name, needle)
 109 }
 110
 111 fn get_path_name(expr: &Expr) -> Option<Name> {
 112     match expr.node {
 113         ExprKind::Box(ref e) | ExprKind::AddrOf(_, ref e) | ExprKind::Unary(UnOp::UnDeref, ref e) => get_path_name(e),
 114         ExprKind::Block(ref b, _) => if b.stmts.is_empty() {
 115             b.expr.as_ref().and_then(|p| get_path_name(p))
 116         } else {
 117             None
 118         },
 119         ExprKind::Path(ref qpath) => single_segment_path(qpath).map(|ps| ps.ident.name),
 120         _ => None,
 121     }
 122 }