1 use rustc_hir::{BinOpKind, Expr, ExprKind};
2 use rustc_lint::{LateContext, LateLintPass};
4 use rustc_session::{declare_lint_pass, declare_tool_lint};
6 use clippy_utils::comparisons::{normalize_comparison, Rel};
7 use clippy_utils::consts::{constant, Constant};
8 use clippy_utils::diagnostics::span_lint_and_help;
9 use clippy_utils::source::snippet;
10 use clippy_utils::ty::is_isize_or_usize;
11 use clippy_utils::{clip, int_bits, unsext};
13 declare_clippy_lint! {
14 /// **What it does:** Checks for comparisons where one side of the relation is
15 /// either the minimum or maximum value for its type and warns if it involves a
16 /// case that is always true or always false. Only integer and boolean types are
19 /// **Why is this bad?** An expression like `min <= x` may misleadingly imply
20 /// that it is possible for `x` to be less than the minimum. Expressions like
21 /// `max < x` are probably mistakes.
23 /// **Known problems:** For `usize` the size of the current compile target will
24 /// be assumed (e.g., 64 bits on 64 bit systems). This means code that uses such
25 /// a comparison to detect target pointer width will trigger this lint. One can
26 /// use `mem::sizeof` and compare its value or conditional compilation
28 /// like `#[cfg(target_pointer_width = "64")] ..` instead.
33 /// let vec: Vec<isize> = Vec::new();
34 /// if vec.len() <= 0 {}
35 /// if 100 > i32::MAX {}
37 pub ABSURD_EXTREME_COMPARISONS,
39 "a comparison with a maximum or minimum value that is always true or false"
42 declare_lint_pass!(AbsurdExtremeComparisons => [ABSURD_EXTREME_COMPARISONS]);
44 impl<'tcx> LateLintPass<'tcx> for AbsurdExtremeComparisons {
45 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
46 if let ExprKind::Binary(ref cmp, lhs, rhs) = expr.kind {
47 if let Some((culprit, result)) = detect_absurd_comparison(cx, cmp.node, lhs, rhs) {
48 if !expr.span.from_expansion() {
49 let msg = "this comparison involving the minimum or maximum element for this \
50 type contains a case that is always true or always false";
52 let conclusion = match result {
53 AbsurdComparisonResult::AlwaysFalse => "this comparison is always false".to_owned(),
54 AbsurdComparisonResult::AlwaysTrue => "this comparison is always true".to_owned(),
55 AbsurdComparisonResult::InequalityImpossible => format!(
56 "the case where the two sides are not equal never occurs, consider using `{} == {}` \
58 snippet(cx, lhs.span, "lhs"),
59 snippet(cx, rhs.span, "rhs")
64 "because `{}` is the {} value for this type, {}",
65 snippet(cx, culprit.expr.span, "x"),
67 ExtremeType::Minimum => "minimum",
68 ExtremeType::Maximum => "maximum",
73 span_lint_and_help(cx, ABSURD_EXTREME_COMPARISONS, expr.span, msg, None, &help);
85 struct ExtremeExpr<'a> {
90 enum AbsurdComparisonResult {
96 fn is_cast_between_fixed_and_target<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
97 if let ExprKind::Cast(cast_exp, _) = expr.kind {
98 let precast_ty = cx.typeck_results().expr_ty(cast_exp);
99 let cast_ty = cx.typeck_results().expr_ty(expr);
101 return is_isize_or_usize(precast_ty) != is_isize_or_usize(cast_ty);
107 fn detect_absurd_comparison<'tcx>(
108 cx: &LateContext<'tcx>,
112 ) -> Option<(ExtremeExpr<'tcx>, AbsurdComparisonResult)> {
113 use AbsurdComparisonResult::{AlwaysFalse, AlwaysTrue, InequalityImpossible};
114 use ExtremeType::{Maximum, Minimum};
115 // absurd comparison only makes sense on primitive types
116 // primitive types don't implement comparison operators with each other
117 if cx.typeck_results().expr_ty(lhs) != cx.typeck_results().expr_ty(rhs) {
121 // comparisons between fix sized types and target sized types are considered unanalyzable
122 if is_cast_between_fixed_and_target(cx, lhs) || is_cast_between_fixed_and_target(cx, rhs) {
126 let (rel, normalized_lhs, normalized_rhs) = normalize_comparison(op, lhs, rhs)?;
128 let lx = detect_extreme_expr(cx, normalized_lhs);
129 let rx = detect_extreme_expr(cx, normalized_rhs);
134 (Some(l @ ExtremeExpr { which: Maximum, .. }), _) => (l, AlwaysFalse), // max < x
135 (_, Some(r @ ExtremeExpr { which: Minimum, .. })) => (r, AlwaysFalse), // x < min
141 (Some(l @ ExtremeExpr { which: Minimum, .. }), _) => (l, AlwaysTrue), // min <= x
142 (Some(l @ ExtremeExpr { which: Maximum, .. }), _) => (l, InequalityImpossible), // max <= x
143 (_, Some(r @ ExtremeExpr { which: Minimum, .. })) => (r, InequalityImpossible), // x <= min
144 (_, Some(r @ ExtremeExpr { which: Maximum, .. })) => (r, AlwaysTrue), // x <= max
148 Rel::Ne | Rel::Eq => return None,
152 fn detect_extreme_expr<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<ExtremeExpr<'tcx>> {
153 let ty = cx.typeck_results().expr_ty(expr);
155 let cv = constant(cx, cx.typeck_results(), expr)?.0;
157 let which = match (ty.kind(), cv) {
158 (&ty::Bool, Constant::Bool(false)) | (&ty::Uint(_), Constant::Int(0)) => ExtremeType::Minimum,
159 (&ty::Int(ity), Constant::Int(i)) if i == unsext(cx.tcx, i128::MIN >> (128 - int_bits(cx.tcx, ity)), ity) => {
163 (&ty::Bool, Constant::Bool(true)) => ExtremeType::Maximum,
164 (&ty::Int(ity), Constant::Int(i)) if i == unsext(cx.tcx, i128::MAX >> (128 - int_bits(cx.tcx, ity)), ity) => {
167 (&ty::Uint(uty), Constant::Int(i)) if clip(cx.tcx, u128::MAX, uty) == i => ExtremeType::Maximum,
171 Some(ExtremeExpr { which, expr })