src/tools/clippy/clippy_lints/src/operators/absurd_extreme_comparisons.rs

   1 use rustc_hir::{BinOpKind, Expr, ExprKind};
   2 use rustc_lint::LateContext;
   3 use rustc_middle::ty;
   4
   5 use clippy_utils::comparisons::{normalize_comparison, Rel};
   6 use clippy_utils::consts::{constant, Constant};
   7 use clippy_utils::diagnostics::span_lint_and_help;
   8 use clippy_utils::source::snippet;
   9 use clippy_utils::ty::is_isize_or_usize;
  10 use clippy_utils::{clip, int_bits, unsext};
  11
  12 use super::ABSURD_EXTREME_COMPARISONS;
  13
  14 pub(super) fn check<'tcx>(
  15     cx: &LateContext<'tcx>,
  16     expr: &'tcx Expr<'_>,
  17     op: BinOpKind,
  18     lhs: &'tcx Expr<'_>,
  19     rhs: &'tcx Expr<'_>,
  20 ) {
  21     if let Some((culprit, result)) = detect_absurd_comparison(cx, op, lhs, rhs) {
  22         let msg = "this comparison involving the minimum or maximum element for this \
  23                            type contains a case that is always true or always false";
  24
  25         let conclusion = match result {
  26             AbsurdComparisonResult::AlwaysFalse => "this comparison is always false".to_owned(),
  27             AbsurdComparisonResult::AlwaysTrue => "this comparison is always true".to_owned(),
  28             AbsurdComparisonResult::InequalityImpossible => format!(
  29                 "the case where the two sides are not equal never occurs, consider using `{} == {}` \
  30                          instead",
  31                 snippet(cx, lhs.span, "lhs"),
  32                 snippet(cx, rhs.span, "rhs")
  33             ),
  34         };
  35
  36         let help = format!(
  37             "because `{}` is the {} value for this type, {conclusion}",
  38             snippet(cx, culprit.expr.span, "x"),
  39             match culprit.which {
  40                 ExtremeType::Minimum => "minimum",
  41                 ExtremeType::Maximum => "maximum",
  42             }
  43         );
  44
  45         span_lint_and_help(cx, ABSURD_EXTREME_COMPARISONS, expr.span, msg, None, &help);
  46     }
  47 }
  48
  49 enum ExtremeType {
  50     Minimum,
  51     Maximum,
  52 }
  53
  54 struct ExtremeExpr<'a> {
  55     which: ExtremeType,
  56     expr: &'a Expr<'a>,
  57 }
  58
  59 enum AbsurdComparisonResult {
  60     AlwaysFalse,
  61     AlwaysTrue,
  62     InequalityImpossible,
  63 }
  64
  65 fn is_cast_between_fixed_and_target<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
  66     if let ExprKind::Cast(cast_exp, _) = expr.kind {
  67         let precast_ty = cx.typeck_results().expr_ty(cast_exp);
  68         let cast_ty = cx.typeck_results().expr_ty(expr);
  69
  70         return is_isize_or_usize(precast_ty) != is_isize_or_usize(cast_ty);
  71     }
  72
  73     false
  74 }
  75
  76 fn detect_absurd_comparison<'tcx>(
  77     cx: &LateContext<'tcx>,
  78     op: BinOpKind,
  79     lhs: &'tcx Expr<'_>,
  80     rhs: &'tcx Expr<'_>,
  81 ) -> Option<(ExtremeExpr<'tcx>, AbsurdComparisonResult)> {
  82     use AbsurdComparisonResult::{AlwaysFalse, AlwaysTrue, InequalityImpossible};
  83     use ExtremeType::{Maximum, Minimum};
  84     // absurd comparison only makes sense on primitive types
  85     // primitive types don't implement comparison operators with each other
  86     if cx.typeck_results().expr_ty(lhs) != cx.typeck_results().expr_ty(rhs) {
  87         return None;
  88     }
  89
  90     // comparisons between fix sized types and target sized types are considered unanalyzable
  91     if is_cast_between_fixed_and_target(cx, lhs) || is_cast_between_fixed_and_target(cx, rhs) {
  92         return None;
  93     }
  94
  95     let (rel, normalized_lhs, normalized_rhs) = normalize_comparison(op, lhs, rhs)?;
  96
  97     let lx = detect_extreme_expr(cx, normalized_lhs);
  98     let rx = detect_extreme_expr(cx, normalized_rhs);
  99
 100     Some(match rel {
 101         Rel::Lt => {
 102             match (lx, rx) {
 103                 (Some(l @ ExtremeExpr { which: Maximum, .. }), _) => (l, AlwaysFalse), // max < x
 104                 (_, Some(r @ ExtremeExpr { which: Minimum, .. })) => (r, AlwaysFalse), // x < min
 105                 _ => return None,
 106             }
 107         },
 108         Rel::Le => {
 109             match (lx, rx) {
 110                 (Some(l @ ExtremeExpr { which: Minimum, .. }), _) => (l, AlwaysTrue), // min <= x
 111                 (Some(l @ ExtremeExpr { which: Maximum, .. }), _) => (l, InequalityImpossible), // max <= x
 112                 (_, Some(r @ ExtremeExpr { which: Minimum, .. })) => (r, InequalityImpossible), // x <= min
 113                 (_, Some(r @ ExtremeExpr { which: Maximum, .. })) => (r, AlwaysTrue), // x <= max
 114                 _ => return None,
 115             }
 116         },
 117         Rel::Ne | Rel::Eq => return None,
 118     })
 119 }
 120
 121 fn detect_extreme_expr<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<ExtremeExpr<'tcx>> {
 122     let ty = cx.typeck_results().expr_ty(expr);
 123
 124     let cv = constant(cx, cx.typeck_results(), expr)?.0;
 125
 126     let which = match (ty.kind(), cv) {
 127         (&ty::Bool, Constant::Bool(false)) | (&ty::Uint(_), Constant::Int(0)) => ExtremeType::Minimum,
 128         (&ty::Int(ity), Constant::Int(i)) if i == unsext(cx.tcx, i128::MIN >> (128 - int_bits(cx.tcx, ity)), ity) => {
 129             ExtremeType::Minimum
 130         },
 131
 132         (&ty::Bool, Constant::Bool(true)) => ExtremeType::Maximum,
 133         (&ty::Int(ity), Constant::Int(i)) if i == unsext(cx.tcx, i128::MAX >> (128 - int_bits(cx.tcx, ity)), ity) => {
 134             ExtremeType::Maximum
 135         },
 136         (&ty::Uint(uty), Constant::Int(i)) if clip(cx.tcx, u128::MAX, uty) == i => ExtremeType::Maximum,
 137
 138         _ => return None,
 139     };
 140     Some(ExtremeExpr { which, expr })
 141 }