rust/clippy_lints/src/instant_subtraction.rs

use clippy_utils::{
    diagnostics::{self, span_lint_and_sugg},
    meets_msrv, msrvs, source, ty,
};
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_semver::RustcVersion;
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::{source_map::Spanned, sym};

declare_clippy_lint! {
    /// ### What it does
    /// Lints subtraction between `Instant::now()` and another `Instant`.
    ///
    /// ### Why is this bad?
    /// It is easy to accidentally write `prev_instant - Instant::now()`, which will always be 0ns
    /// as `Instant` subtraction saturates.
    ///
    /// `prev_instant.elapsed()` also more clearly signals intention.
    ///
    /// ### Example
    /// ```rust
    /// use std::time::Instant;
    /// let prev_instant = Instant::now();
    /// let duration = Instant::now() - prev_instant;
    /// ```
    /// Use instead:
    /// ```rust
    /// use std::time::Instant;
    /// let prev_instant = Instant::now();
    /// let duration = prev_instant.elapsed();
    /// ```
    #[clippy::version = "1.64.0"]
    pub MANUAL_INSTANT_ELAPSED,
    pedantic,
    "subtraction between `Instant::now()` and previous `Instant`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Finds patterns of unchecked subtraction of [`Duration`] from [`Instant::now()`].
    ///
    /// ### Why is this bad?
    /// Unchecked subtraction could cause underflow on certain platforms, leading to
    /// unintentional panics.
    ///
    /// ### Example
    /// ```rust
    /// let time_passed = Instant::now() - Duration::from_secs(5);
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// let time_passed = Instant::now().checked_sub(Duration::from_secs(5));
    /// ```
    ///
    /// [`Duration`]: std::time::Duration
    /// [`Instant::now()`]: std::time::Instant::now;
    #[clippy::version = "1.65.0"]
    pub UNCHECKED_DURATION_SUBTRACTION,
    suspicious,
    "finds unchecked subtraction of a 'Duration' from an 'Instant'"
}

pub struct InstantSubtraction {
    msrv: Option<RustcVersion>,
}

impl InstantSubtraction {
    #[must_use]
    pub fn new(msrv: Option<RustcVersion>) -> Self {
        Self { msrv }
    }
}

impl_lint_pass!(InstantSubtraction => [MANUAL_INSTANT_ELAPSED, UNCHECKED_DURATION_SUBTRACTION]);

impl LateLintPass<'_> for InstantSubtraction {
    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
        if let ExprKind::Binary(Spanned {node: BinOpKind::Sub, ..}, lhs, rhs) = expr.kind
            && check_instant_now_call(cx, lhs)
            && let ty_resolved = cx.typeck_results().expr_ty(rhs)
            && let rustc_middle::ty::Adt(def, _) = ty_resolved.kind()
            && clippy_utils::match_def_path(cx, def.did(), &clippy_utils::paths::INSTANT)
            && let Some(sugg) = clippy_utils::sugg::Sugg::hir_opt(cx, rhs)
        {
            span_lint_and_sugg(
                cx,
                MANUAL_INSTANT_ELAPSED,
                expr.span,
                "manual implementation of `Instant::elapsed`",
                "try",
                format!("{}.elapsed()", sugg.maybe_par()),
                Applicability::MachineApplicable,
            );
        }

        if expr.span.from_expansion() || !meets_msrv(self.msrv, msrvs::TRY_FROM) {
            return;
        }

        if_chain! {
            if let ExprKind::Binary(op, lhs, rhs) = expr.kind;

            if let BinOpKind::Sub = op.node;

            // get types of left and right side
            if is_an_instant(cx, lhs);
            if is_a_duration(cx, rhs);

            then {
                print_lint_and_sugg(cx, lhs, rhs, expr)
            }
        }
    }
}

fn check_instant_now_call(cx: &LateContext<'_>, expr_block: &'_ Expr<'_>) -> bool {
    if let ExprKind::Call(fn_expr, []) = expr_block.kind
        && let Some(fn_id) = clippy_utils::path_def_id(cx, fn_expr)
        && clippy_utils::match_def_path(cx, fn_id, &clippy_utils::paths::INSTANT_NOW)
    {
        true
    } else {
        false
    }
}

fn is_an_instant(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    let expr_ty = cx.typeck_results().expr_ty(expr);

    match expr_ty.kind() {
        rustc_middle::ty::Adt(def, _) => clippy_utils::match_def_path(cx, def.did(), &clippy_utils::paths::INSTANT),
        _ => false,
    }
}

fn is_a_duration(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    let expr_ty = cx.typeck_results().expr_ty(expr);
    ty::is_type_diagnostic_item(cx, expr_ty, sym::Duration)
}

fn print_lint_and_sugg(cx: &LateContext<'_>, left_expr: &Expr<'_>, right_expr: &Expr<'_>, expr: &Expr<'_>) {
    let mut applicability = Applicability::MachineApplicable;

    let left_expr =
        source::snippet_with_applicability(cx, left_expr.span, "std::time::Instant::now()", &mut applicability);
    let right_expr = source::snippet_with_applicability(
        cx,
        right_expr.span,
        "std::time::Duration::from_secs(1)",
        &mut applicability,
    );

    diagnostics::span_lint_and_sugg(
        cx,
        UNCHECKED_DURATION_SUBTRACTION,
        expr.span,
        "unchecked subtraction of a 'Duration' from an 'Instant'",
        "try",
        format!("{}.checked_sub({}).unwrap();", left_expr, right_expr),
        applicability,
    );
}