1 files changed, 69 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/manual_instant_elapsed.rs b/src/tools/clippy/clippy_lints/src/manual_instant_elapsed.rs
new file mode 100644
index 000000000..331cda1db
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/manual_instant_elapsed.rs
@@ -0,0 +1,69 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use rustc_errors::Applicability;
+use rustc_hir::{BinOpKind, Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Spanned;
+
+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_lint_pass!(ManualInstantElapsed => [MANUAL_INSTANT_ELAPSED]);
+
+impl LateLintPass<'_> for ManualInstantElapsed {
+    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,
+            );
+        }
+    }
+}
+
+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
+    }
+}