src/tools/clippy/clippy_lints/src/instant_subtraction.rs

   1 use clippy_utils::diagnostics::{self, span_lint_and_sugg};
   2 use clippy_utils::msrvs::{self, Msrv};
   3 use clippy_utils::source;
   4 use clippy_utils::sugg::Sugg;
   5 use clippy_utils::ty;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BinOpKind, Expr, ExprKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_tool_lint, impl_lint_pass};
  10 use rustc_span::{source_map::Spanned, sym};
  11
  12 declare_clippy_lint! {
  13     /// ### What it does
  14     /// Lints subtraction between `Instant::now()` and another `Instant`.
  15     ///
  16     /// ### Why is this bad?
  17     /// It is easy to accidentally write `prev_instant - Instant::now()`, which will always be 0ns
  18     /// as `Instant` subtraction saturates.
  19     ///
  20     /// `prev_instant.elapsed()` also more clearly signals intention.
  21     ///
  22     /// ### Example
  23     /// ```rust
  24     /// use std::time::Instant;
  25     /// let prev_instant = Instant::now();
  26     /// let duration = Instant::now() - prev_instant;
  27     /// ```
  28     /// Use instead:
  29     /// ```rust
  30     /// use std::time::Instant;
  31     /// let prev_instant = Instant::now();
  32     /// let duration = prev_instant.elapsed();
  33     /// ```
  34     #[clippy::version = "1.65.0"]
  35     pub MANUAL_INSTANT_ELAPSED,
  36     pedantic,
  37     "subtraction between `Instant::now()` and previous `Instant`"
  38 }
  39
  40 declare_clippy_lint! {
  41     /// ### What it does
  42     /// Lints subtraction between an [`Instant`] and a [`Duration`].
  43     ///
  44     /// ### Why is this bad?
  45     /// Unchecked subtraction could cause underflow on certain platforms, leading to
  46     /// unintentional panics.
  47     ///
  48     /// ### Example
  49     /// ```rust
  50     /// # use std::time::{Instant, Duration};
  51     /// let time_passed = Instant::now() - Duration::from_secs(5);
  52     /// ```
  53     ///
  54     /// Use instead:
  55     /// ```rust
  56     /// # use std::time::{Instant, Duration};
  57     /// let time_passed = Instant::now().checked_sub(Duration::from_secs(5));
  58     /// ```
  59     ///
  60     /// [`Duration`]: std::time::Duration
  61     /// [`Instant::now()`]: std::time::Instant::now;
  62     #[clippy::version = "1.65.0"]
  63     pub UNCHECKED_DURATION_SUBTRACTION,
  64     suspicious,
  65     "finds unchecked subtraction of a 'Duration' from an 'Instant'"
  66 }
  67
  68 pub struct InstantSubtraction {
  69     msrv: Msrv,
  70 }
  71
  72 impl InstantSubtraction {
  73     #[must_use]
  74     pub fn new(msrv: Msrv) -> Self {
  75         Self { msrv }
  76     }
  77 }
  78
  79 impl_lint_pass!(InstantSubtraction => [MANUAL_INSTANT_ELAPSED, UNCHECKED_DURATION_SUBTRACTION]);
  80
  81 impl LateLintPass<'_> for InstantSubtraction {
  82     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
  83         if let ExprKind::Binary(
  84             Spanned {
  85                 node: BinOpKind::Sub, ..
  86             },
  87             lhs,
  88             rhs,
  89         ) = expr.kind
  90         {
  91             if_chain! {
  92                 if is_instant_now_call(cx, lhs);
  93
  94                 if is_an_instant(cx, rhs);
  95                 if let Some(sugg) = Sugg::hir_opt(cx, rhs);
  96
  97                 then {
  98                     print_manual_instant_elapsed_sugg(cx, expr, sugg)
  99                 } else {
 100                     if_chain! {
 101                         if !expr.span.from_expansion();
 102                         if self.msrv.meets(msrvs::TRY_FROM);
 103
 104                         if is_an_instant(cx, lhs);
 105                         if is_a_duration(cx, rhs);
 106
 107                         then {
 108                             print_unchecked_duration_subtraction_sugg(cx, lhs, rhs, expr)
 109                         }
 110                     }
 111                 }
 112             }
 113         }
 114     }
 115
 116     extract_msrv_attr!(LateContext);
 117 }
 118
 119 fn is_instant_now_call(cx: &LateContext<'_>, expr_block: &'_ Expr<'_>) -> bool {
 120     if let ExprKind::Call(fn_expr, []) = expr_block.kind
 121         && let Some(fn_id) = clippy_utils::path_def_id(cx, fn_expr)
 122         && clippy_utils::match_def_path(cx, fn_id, &clippy_utils::paths::INSTANT_NOW)
 123     {
 124         true
 125     } else {
 126         false
 127     }
 128 }
 129
 130 fn is_an_instant(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 131     let expr_ty = cx.typeck_results().expr_ty(expr);
 132
 133     match expr_ty.kind() {
 134         rustc_middle::ty::Adt(def, _) => clippy_utils::match_def_path(cx, def.did(), &clippy_utils::paths::INSTANT),
 135         _ => false,
 136     }
 137 }
 138
 139 fn is_a_duration(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 140     let expr_ty = cx.typeck_results().expr_ty(expr);
 141     ty::is_type_diagnostic_item(cx, expr_ty, sym::Duration)
 142 }
 143
 144 fn print_manual_instant_elapsed_sugg(cx: &LateContext<'_>, expr: &Expr<'_>, sugg: Sugg<'_>) {
 145     span_lint_and_sugg(
 146         cx,
 147         MANUAL_INSTANT_ELAPSED,
 148         expr.span,
 149         "manual implementation of `Instant::elapsed`",
 150         "try",
 151         format!("{}.elapsed()", sugg.maybe_par()),
 152         Applicability::MachineApplicable,
 153     );
 154 }
 155
 156 fn print_unchecked_duration_subtraction_sugg(
 157     cx: &LateContext<'_>,
 158     left_expr: &Expr<'_>,
 159     right_expr: &Expr<'_>,
 160     expr: &Expr<'_>,
 161 ) {
 162     let mut applicability = Applicability::MachineApplicable;
 163
 164     let left_expr =
 165         source::snippet_with_applicability(cx, left_expr.span, "std::time::Instant::now()", &mut applicability);
 166     let right_expr = source::snippet_with_applicability(
 167         cx,
 168         right_expr.span,
 169         "std::time::Duration::from_secs(1)",
 170         &mut applicability,
 171     );
 172
 173     diagnostics::span_lint_and_sugg(
 174         cx,
 175         UNCHECKED_DURATION_SUBTRACTION,
 176         expr.span,
 177         "unchecked subtraction of a 'Duration' from an 'Instant'",
 178         "try",
 179         format!("{left_expr}.checked_sub({right_expr}).unwrap()"),
 180         applicability,
 181     );
 182 }