src/tools/clippy/clippy_lints/src/indexing_slicing.rs

   1 //! lint on indexing and slicing operations
   2
   3 use clippy_utils::consts::{constant, Constant};
   4 use clippy_utils::diagnostics::{span_lint, span_lint_and_help};
   5 use clippy_utils::higher;
   6 use rustc_ast::ast::RangeLimits;
   7 use rustc_hir::{Expr, ExprKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11
  12 declare_clippy_lint! {
  13     /// ### What it does
  14     /// Checks for out of bounds array indexing with a constant
  15     /// index.
  16     ///
  17     /// ### Why is this bad?
  18     /// This will always panic at runtime.
  19     ///
  20     /// ### Example
  21     /// ```rust,no_run
  22     /// let x = [1, 2, 3, 4];
  23     ///
  24     /// x[9];
  25     /// &x[2..9];
  26     /// ```
  27     ///
  28     /// Use instead:
  29     /// ```rust
  30     /// # let x = [1, 2, 3, 4];
  31     /// // Index within bounds
  32     ///
  33     /// x[0];
  34     /// x[3];
  35     /// ```
  36     #[clippy::version = "pre 1.29.0"]
  37     pub OUT_OF_BOUNDS_INDEXING,
  38     correctness,
  39     "out of bounds constant indexing"
  40 }
  41
  42 declare_clippy_lint! {
  43     /// ### What it does
  44     /// Checks for usage of indexing or slicing. Arrays are special cases, this lint
  45     /// does report on arrays if we can tell that slicing operations are in bounds and does not
  46     /// lint on constant `usize` indexing on arrays because that is handled by rustc's `const_err` lint.
  47     ///
  48     /// ### Why is this bad?
  49     /// Indexing and slicing can panic at runtime and there are
  50     /// safe alternatives.
  51     ///
  52     /// ### Example
  53     /// ```rust,no_run
  54     /// // Vector
  55     /// let x = vec![0; 5];
  56     ///
  57     /// x[2];
  58     /// &x[2..100];
  59     ///
  60     /// // Array
  61     /// let y = [0, 1, 2, 3];
  62     ///
  63     /// &y[10..100];
  64     /// &y[10..];
  65     /// ```
  66     ///
  67     /// Use instead:
  68     /// ```rust
  69     /// # #![allow(unused)]
  70     ///
  71     /// # let x = vec![0; 5];
  72     /// # let y = [0, 1, 2, 3];
  73     /// x.get(2);
  74     /// x.get(2..100);
  75     ///
  76     /// y.get(10);
  77     /// y.get(10..100);
  78     /// ```
  79     #[clippy::version = "pre 1.29.0"]
  80     pub INDEXING_SLICING,
  81     restriction,
  82     "indexing/slicing usage"
  83 }
  84
  85 declare_lint_pass!(IndexingSlicing => [INDEXING_SLICING, OUT_OF_BOUNDS_INDEXING]);
  86
  87 impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
  88     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  89         if cx.tcx.hir().is_inside_const_context(expr.hir_id) {
  90             return;
  91         }
  92
  93         if let ExprKind::Index(array, index) = &expr.kind {
  94             let ty = cx.typeck_results().expr_ty(array).peel_refs();
  95             if let Some(range) = higher::Range::hir(index) {
  96                 // Ranged indexes, i.e., &x[n..m], &x[n..], &x[..n] and &x[..]
  97                 if let ty::Array(_, s) = ty.kind() {
  98                     let size: u128 = if let Some(size) = s.try_eval_usize(cx.tcx, cx.param_env) {
  99                         size.into()
 100                     } else {
 101                         return;
 102                     };
 103
 104                     let const_range = to_const_range(cx, range, size);
 105
 106                     if let (Some(start), _) = const_range {
 107                         if start > size {
 108                             span_lint(
 109                                 cx,
 110                                 OUT_OF_BOUNDS_INDEXING,
 111                                 range.start.map_or(expr.span, |start| start.span),
 112                                 "range is out of bounds",
 113                             );
 114                             return;
 115                         }
 116                     }
 117
 118                     if let (_, Some(end)) = const_range {
 119                         if end > size {
 120                             span_lint(
 121                                 cx,
 122                                 OUT_OF_BOUNDS_INDEXING,
 123                                 range.end.map_or(expr.span, |end| end.span),
 124                                 "range is out of bounds",
 125                             );
 126                             return;
 127                         }
 128                     }
 129
 130                     if let (Some(_), Some(_)) = const_range {
 131                         // early return because both start and end are constants
 132                         // and we have proven above that they are in bounds
 133                         return;
 134                     }
 135                 }
 136
 137                 let help_msg = match (range.start, range.end) {
 138                     (None, Some(_)) => "consider using `.get(..n)`or `.get_mut(..n)` instead",
 139                     (Some(_), None) => "consider using `.get(n..)` or .get_mut(n..)` instead",
 140                     (Some(_), Some(_)) => "consider using `.get(n..m)` or `.get_mut(n..m)` instead",
 141                     (None, None) => return, // [..] is ok.
 142                 };
 143
 144                 span_lint_and_help(cx, INDEXING_SLICING, expr.span, "slicing may panic", None, help_msg);
 145             } else {
 146                 // Catchall non-range index, i.e., [n] or [n << m]
 147                 if let ty::Array(..) = ty.kind() {
 148                     // Index is a const block.
 149                     if let ExprKind::ConstBlock(..) = index.kind {
 150                         return;
 151                     }
 152                     // Index is a constant uint.
 153                     if let Some(..) = constant(cx, cx.typeck_results(), index) {
 154                         // Let rustc's `const_err` lint handle constant `usize` indexing on arrays.
 155                         return;
 156                     }
 157                 }
 158
 159                 span_lint_and_help(
 160                     cx,
 161                     INDEXING_SLICING,
 162                     expr.span,
 163                     "indexing may panic",
 164                     None,
 165                     "consider using `.get(n)` or `.get_mut(n)` instead",
 166                 );
 167             }
 168         }
 169     }
 170 }
 171
 172 /// Returns a tuple of options with the start and end (exclusive) values of
 173 /// the range. If the start or end is not constant, None is returned.
 174 fn to_const_range<'tcx>(
 175     cx: &LateContext<'tcx>,
 176     range: higher::Range<'_>,
 177     array_size: u128,
 178 ) -> (Option<u128>, Option<u128>) {
 179     let s = range
 180         .start
 181         .map(|expr| constant(cx, cx.typeck_results(), expr).map(|(c, _)| c));
 182     let start = match s {
 183         Some(Some(Constant::Int(x))) => Some(x),
 184         Some(_) => None,
 185         None => Some(0),
 186     };
 187
 188     let e = range
 189         .end
 190         .map(|expr| constant(cx, cx.typeck_results(), expr).map(|(c, _)| c));
 191     let end = match e {
 192         Some(Some(Constant::Int(x))) => {
 193             if range.limits == RangeLimits::Closed {
 194                 Some(x + 1)
 195             } else {
 196                 Some(x)
 197             }
 198         },
 199         Some(_) => None,
 200         None => Some(array_size),
 201     };
 202
 203     (start, end)
 204 }