clippy_lints/src/invalid_upcast_comparisons.rs

   1 use std::cmp::Ordering;
   2
   3 use rustc_hir::{Expr, ExprKind};
   4 use rustc_lint::{LateContext, LateLintPass};
   5 use rustc_middle::ty::{self, IntTy, UintTy};
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7 use rustc_span::Span;
   8 use rustc_target::abi::LayoutOf;
   9
  10 use clippy_utils::comparisons::Rel;
  11 use clippy_utils::consts::{constant, Constant};
  12 use clippy_utils::diagnostics::span_lint;
  13 use clippy_utils::source::snippet;
  14 use clippy_utils::{comparisons, sext};
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for comparisons where the relation is always either
  18     /// true or false, but where one side has been upcast so that the comparison is
  19     /// necessary. Only integer types are checked.
  20     ///
  21     /// **Why is this bad?** An expression like `let x : u8 = ...; (x as u32) > 300`
  22     /// will mistakenly imply that it is possible for `x` to be outside the range of
  23     /// `u8`.
  24     ///
  25     /// **Known problems:**
  26     /// https://github.com/rust-lang/rust-clippy/issues/886
  27     ///
  28     /// **Example:**
  29     /// ```rust
  30     /// let x: u8 = 1;
  31     /// (x as u32) > 300;
  32     /// ```
  33     pub INVALID_UPCAST_COMPARISONS,
  34     pedantic,
  35     "a comparison involving an upcast which is always true or false"
  36 }
  37
  38 declare_lint_pass!(InvalidUpcastComparisons => [INVALID_UPCAST_COMPARISONS]);
  39
  40 #[derive(Copy, Clone, Debug, Eq)]
  41 enum FullInt {
  42     S(i128),
  43     U(u128),
  44 }
  45
  46 impl FullInt {
  47     #[allow(clippy::cast_sign_loss)]
  48     #[must_use]
  49     fn cmp_s_u(s: i128, u: u128) -> Ordering {
  50         if s < 0 {
  51             Ordering::Less
  52         } else if u > (i128::MAX as u128) {
  53             Ordering::Greater
  54         } else {
  55             (s as u128).cmp(&u)
  56         }
  57     }
  58 }
  59
  60 impl PartialEq for FullInt {
  61     #[must_use]
  62     fn eq(&self, other: &Self) -> bool {
  63         self.partial_cmp(other).expect("`partial_cmp` only returns `Some(_)`") == Ordering::Equal
  64     }
  65 }
  66
  67 impl PartialOrd for FullInt {
  68     #[must_use]
  69     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
  70         Some(match (self, other) {
  71             (&Self::S(s), &Self::S(o)) => s.cmp(&o),
  72             (&Self::U(s), &Self::U(o)) => s.cmp(&o),
  73             (&Self::S(s), &Self::U(o)) => Self::cmp_s_u(s, o),
  74             (&Self::U(s), &Self::S(o)) => Self::cmp_s_u(o, s).reverse(),
  75         })
  76     }
  77 }
  78
  79 impl Ord for FullInt {
  80     #[must_use]
  81     fn cmp(&self, other: &Self) -> Ordering {
  82         self.partial_cmp(other)
  83             .expect("`partial_cmp` for FullInt can never return `None`")
  84     }
  85 }
  86
  87 fn numeric_cast_precast_bounds<'a>(cx: &LateContext<'_>, expr: &'a Expr<'_>) -> Option<(FullInt, FullInt)> {
  88     if let ExprKind::Cast(cast_exp, _) = expr.kind {
  89         let pre_cast_ty = cx.typeck_results().expr_ty(cast_exp);
  90         let cast_ty = cx.typeck_results().expr_ty(expr);
  91         // if it's a cast from i32 to u32 wrapping will invalidate all these checks
  92         if cx.layout_of(pre_cast_ty).ok().map(|l| l.size) == cx.layout_of(cast_ty).ok().map(|l| l.size) {
  93             return None;
  94         }
  95         match pre_cast_ty.kind() {
  96             ty::Int(int_ty) => Some(match int_ty {
  97                 IntTy::I8 => (FullInt::S(i128::from(i8::MIN)), FullInt::S(i128::from(i8::MAX))),
  98                 IntTy::I16 => (FullInt::S(i128::from(i16::MIN)), FullInt::S(i128::from(i16::MAX))),
  99                 IntTy::I32 => (FullInt::S(i128::from(i32::MIN)), FullInt::S(i128::from(i32::MAX))),
 100                 IntTy::I64 => (FullInt::S(i128::from(i64::MIN)), FullInt::S(i128::from(i64::MAX))),
 101                 IntTy::I128 => (FullInt::S(i128::MIN), FullInt::S(i128::MAX)),
 102                 IntTy::Isize => (FullInt::S(isize::MIN as i128), FullInt::S(isize::MAX as i128)),
 103             }),
 104             ty::Uint(uint_ty) => Some(match uint_ty {
 105                 UintTy::U8 => (FullInt::U(u128::from(u8::MIN)), FullInt::U(u128::from(u8::MAX))),
 106                 UintTy::U16 => (FullInt::U(u128::from(u16::MIN)), FullInt::U(u128::from(u16::MAX))),
 107                 UintTy::U32 => (FullInt::U(u128::from(u32::MIN)), FullInt::U(u128::from(u32::MAX))),
 108                 UintTy::U64 => (FullInt::U(u128::from(u64::MIN)), FullInt::U(u128::from(u64::MAX))),
 109                 UintTy::U128 => (FullInt::U(u128::MIN), FullInt::U(u128::MAX)),
 110                 UintTy::Usize => (FullInt::U(usize::MIN as u128), FullInt::U(usize::MAX as u128)),
 111             }),
 112             _ => None,
 113         }
 114     } else {
 115         None
 116     }
 117 }
 118
 119 fn node_as_const_fullint<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<FullInt> {
 120     let val = constant(cx, cx.typeck_results(), expr)?.0;
 121     if let Constant::Int(const_int) = val {
 122         match *cx.typeck_results().expr_ty(expr).kind() {
 123             ty::Int(ity) => Some(FullInt::S(sext(cx.tcx, const_int, ity))),
 124             ty::Uint(_) => Some(FullInt::U(const_int)),
 125             _ => None,
 126         }
 127     } else {
 128         None
 129     }
 130 }
 131
 132 fn err_upcast_comparison(cx: &LateContext<'_>, span: Span, expr: &Expr<'_>, always: bool) {
 133     if let ExprKind::Cast(cast_val, _) = expr.kind {
 134         span_lint(
 135             cx,
 136             INVALID_UPCAST_COMPARISONS,
 137             span,
 138             &format!(
 139                 "because of the numeric bounds on `{}` prior to casting, this expression is always {}",
 140                 snippet(cx, cast_val.span, "the expression"),
 141                 if always { "true" } else { "false" },
 142             ),
 143         );
 144     }
 145 }
 146
 147 fn upcast_comparison_bounds_err<'tcx>(
 148     cx: &LateContext<'tcx>,
 149     span: Span,
 150     rel: comparisons::Rel,
 151     lhs_bounds: Option<(FullInt, FullInt)>,
 152     lhs: &'tcx Expr<'_>,
 153     rhs: &'tcx Expr<'_>,
 154     invert: bool,
 155 ) {
 156     if let Some((lb, ub)) = lhs_bounds {
 157         if let Some(norm_rhs_val) = node_as_const_fullint(cx, rhs) {
 158             if rel == Rel::Eq || rel == Rel::Ne {
 159                 if norm_rhs_val < lb || norm_rhs_val > ub {
 160                     err_upcast_comparison(cx, span, lhs, rel == Rel::Ne);
 161                 }
 162             } else if match rel {
 163                 Rel::Lt => {
 164                     if invert {
 165                         norm_rhs_val < lb
 166                     } else {
 167                         ub < norm_rhs_val
 168                     }
 169                 },
 170                 Rel::Le => {
 171                     if invert {
 172                         norm_rhs_val <= lb
 173                     } else {
 174                         ub <= norm_rhs_val
 175                     }
 176                 },
 177                 Rel::Eq | Rel::Ne => unreachable!(),
 178             } {
 179                 err_upcast_comparison(cx, span, lhs, true);
 180             } else if match rel {
 181                 Rel::Lt => {
 182                     if invert {
 183                         norm_rhs_val >= ub
 184                     } else {
 185                         lb >= norm_rhs_val
 186                     }
 187                 },
 188                 Rel::Le => {
 189                     if invert {
 190                         norm_rhs_val > ub
 191                     } else {
 192                         lb > norm_rhs_val
 193                     }
 194                 },
 195                 Rel::Eq | Rel::Ne => unreachable!(),
 196             } {
 197                 err_upcast_comparison(cx, span, lhs, false);
 198             }
 199         }
 200     }
 201 }
 202
 203 impl<'tcx> LateLintPass<'tcx> for InvalidUpcastComparisons {
 204     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 205         if let ExprKind::Binary(ref cmp, lhs, rhs) = expr.kind {
 206             let normalized = comparisons::normalize_comparison(cmp.node, lhs, rhs);
 207             let (rel, normalized_lhs, normalized_rhs) = if let Some(val) = normalized {
 208                 val
 209             } else {
 210                 return;
 211             };
 212
 213             let lhs_bounds = numeric_cast_precast_bounds(cx, normalized_lhs);
 214             let rhs_bounds = numeric_cast_precast_bounds(cx, normalized_rhs);
 215
 216             upcast_comparison_bounds_err(cx, expr.span, rel, lhs_bounds, normalized_lhs, normalized_rhs, false);
 217             upcast_comparison_bounds_err(cx, expr.span, rel, rhs_bounds, normalized_rhs, normalized_lhs, true);
 218         }
 219     }
 220 }