[Clippy] Swap instant_subtraction to use diagnostic item instead of path

compiler/rustc_span/src/symbol.rs

@@ -1061,6 +1061,7 @@
         inline_const,
         inline_const_pat,
         inout,
+        instant_now,
         instruction_set,
         integer_: "integer", // underscore to avoid clashing with the function `sym::integer` below
         integral,

library/std/src/time.rs

@@ -280,6 +280,7 @@ impl Instant {
     /// ```
     #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "instant_now")]
     pub fn now() -> Instant {
         Instant(time::Instant::now())
     }

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

@@ -112,7 +112,7 @@ fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
 fn is_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)
+        && cx.tcx.is_diagnostic_item(sym::instant_now, fn_id)
     {
         true
     } else {

src/tools/clippy/clippy_utils/src/paths.rs

@@ -72,4 +72,3 @@
 pub const TOKIO_IO_OPEN_OPTIONS: [&str; 4] = ["tokio", "fs", "open_options", "OpenOptions"];
 #[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const TOKIO_IO_OPEN_OPTIONS_NEW: [&str; 5] = ["tokio", "fs", "open_options", "OpenOptions", "new"];
-pub const INSTANT_NOW: [&str; 4] = ["std", "time", "Instant", "now"];