clippy_lints/src/indexing_slicing.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 //! lint on indexing and slicing operations
  12
  13 use crate::consts::{constant, Constant};
  14 use crate::utils;
  15 use crate::utils::higher;
  16 use crate::utils::higher::Range;
  17 use crate::rustc::hir::*;
  18 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  19 use crate::rustc::{declare_tool_lint, lint_array};
  20 use crate::rustc::ty;
  21 use crate::syntax::ast::RangeLimits;
  22
  23 /// **What it does:** Checks for out of bounds array indexing with a constant
  24 /// index.
  25 ///
  26 /// **Why is this bad?** This will always panic at runtime.
  27 ///
  28 /// **Known problems:** Hopefully none.
  29 ///
  30 /// **Example:**
  31 /// ```rust
  32 /// let x = [1,2,3,4];
  33 ///
  34 /// // Bad
  35 /// x[9];
  36 /// &x[2..9];
  37 ///
  38 /// // Good
  39 /// x[0];
  40 /// x[3];
  41 /// ```
  42 declare_clippy_lint! {
  43     pub OUT_OF_BOUNDS_INDEXING,
  44     correctness,
  45     "out of bounds constant indexing"
  46 }
  47
  48 /// **What it does:** Checks for usage of indexing or slicing. Arrays are special cased, this lint
  49 /// does report on arrays if we can tell that slicing operations are in bounds and does not
  50 /// lint on constant `usize` indexing on arrays because that is handled by rustc's `const_err` lint.
  51 ///
  52 /// **Why is this bad?** Indexing and slicing can panic at runtime and there are
  53 /// safe alternatives.
  54 ///
  55 /// **Known problems:** Hopefully none.
  56 ///
  57 /// **Example:**
  58 /// ```rust
  59 /// // Vector
  60 /// let x = vec![0; 5];
  61 ///
  62 /// // Bad
  63 /// x[2];
  64 /// &x[2..100];
  65 /// &x[2..];
  66 /// &x[..100];
  67 ///
  68 /// // Good
  69 /// x.get(2);
  70 /// x.get(2..100);
  71 /// x.get(2..);
  72 /// x.get(..100);
  73 ///
  74 /// // Array
  75 /// let y = [0, 1, 2, 3];
  76 ///
  77 /// // Bad
  78 /// &y[10..100];
  79 /// &y[10..];
  80 /// &y[..100];
  81 ///
  82 /// // Good
  83 /// &y[2..];
  84 /// &y[..2];
  85 /// &y[0..3];
  86 /// y.get(10);
  87 /// y.get(10..100);
  88 /// y.get(10..);
  89 /// y.get(..100);
  90 /// ```
  91 declare_clippy_lint! {
  92     pub INDEXING_SLICING,
  93     restriction,
  94     "indexing/slicing usage"
  95 }
  96
  97 #[derive(Copy, Clone)]
  98 pub struct IndexingSlicing;
  99
 100 impl LintPass for IndexingSlicing {
 101     fn get_lints(&self) -> LintArray {
 102         lint_array!(INDEXING_SLICING, OUT_OF_BOUNDS_INDEXING)
 103     }
 104 }
 105
 106 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IndexingSlicing {
 107     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
 108         if let ExprKind::Index(ref array, ref index) = &expr.node {
 109             let ty = cx.tables.expr_ty(array);
 110             if let Some(range) = higher::range(cx, index) {
 111                 // Ranged indexes, i.e. &x[n..m], &x[n..], &x[..n] and &x[..]
 112                 if let ty::Array(_, s) = ty.sty {
 113                     let size: u128 = s.assert_usize(cx.tcx).unwrap().into();
 114
 115                     match to_const_range(cx, range, size) {
 116                         (None, None) => {},
 117                         (Some(start), None) => {
 118                             if start > size {
 119                                 utils::span_lint(
 120                                     cx,
 121                                     OUT_OF_BOUNDS_INDEXING,
 122                                     expr.span,
 123                                     "range is out of bounds",
 124                                 );
 125                                 return;
 126                             }
 127                         },
 128                         (None, Some(end)) => {
 129                             if end > size {
 130                                 utils::span_lint(
 131                                     cx,
 132                                     OUT_OF_BOUNDS_INDEXING,
 133                                     expr.span,
 134                                     "range is out of bounds",
 135                                 );
 136                                 return;
 137                             }
 138                         },
 139                         (Some(start), Some(end)) => {
 140                             if start > size || end > size {
 141                                 utils::span_lint(
 142                                     cx,
 143                                     OUT_OF_BOUNDS_INDEXING,
 144                                     expr.span,
 145                                     "range is out of bounds",
 146                                 );
 147                             }
 148                             // early return because both start and end are constant
 149                             return;
 150                         },
 151                     }
 152                 }
 153
 154                 let help_msg = match (range.start, range.end) {
 155                     (None, Some(_)) => "Consider using `.get(..n)`or `.get_mut(..n)` instead",
 156                     (Some(_), None) => "Consider using `.get(n..)` or .get_mut(n..)` instead",
 157                     (Some(_), Some(_)) => "Consider using `.get(n..m)` or `.get_mut(n..m)` instead",
 158                     (None, None) => return, // [..] is ok.
 159                 };
 160
 161                 utils::span_help_and_lint(
 162                     cx,
 163                     INDEXING_SLICING,
 164                     expr.span,
 165                     "slicing may panic.",
 166                     help_msg,
 167                 );
 168             } else {
 169                 // Catchall non-range index, i.e. [n] or [n << m]
 170                 if let ty::Array(..) = ty.sty {
 171                     // Index is a constant uint.
 172                     if let Some(..) = constant(cx, cx.tables, index) {
 173                         // Let rustc's `const_err` lint handle constant `usize` indexing on arrays.
 174                         return;
 175                     }
 176                 }
 177
 178                 utils::span_help_and_lint(
 179                     cx,
 180                     INDEXING_SLICING,
 181                     expr.span,
 182                     "indexing may panic.",
 183                     "Consider using `.get(n)` or `.get_mut(n)` instead",
 184                 );
 185             }
 186         }
 187     }
 188 }
 189
 190 /// Returns a tuple of options with the start and end (exclusive) values of
 191 /// the range. If the start or end is not constant, None is returned.
 192 fn to_const_range<'a, 'tcx>(
 193     cx: &LateContext<'a, 'tcx>,
 194     range: Range<'_>,
 195     array_size: u128,
 196 ) -> (Option<u128>, Option<u128>) {
 197     let s = range
 198         .start
 199         .map(|expr| constant(cx, cx.tables, expr).map(|(c, _)| c));
 200     let start = match s {
 201         Some(Some(Constant::Int(x))) => Some(x),
 202         Some(_) => None,
 203         None => Some(0),
 204     };
 205
 206     let e = range
 207         .end
 208         .map(|expr| constant(cx, cx.tables, expr).map(|(c, _)| c));
 209     let end = match e {
 210         Some(Some(Constant::Int(x))) => if range.limits == RangeLimits::Closed {
 211             Some(x + 1)
 212         } else {
 213             Some(x)
 214         },
 215         Some(_) => None,
 216         None => Some(array_size),
 217     };
 218
 219     (start, end)
 220 }