instant_subtraction: Reduce redundant work.

Author: Jarcho

clippy_lints/src/instant_subtraction.rs

@@ -85,16 +85,19 @@ impl LateLintPass<'_> for InstantSubtraction {
             lhs,
             rhs,
         ) = expr.kind
+            && let typeck = cx.typeck_results()
+            && ty::is_type_diagnostic_item(cx, typeck.expr_ty(lhs), sym::Instant)
         {
+            let rhs_ty = typeck.expr_ty(rhs);
+
             if is_instant_now_call(cx, lhs)
-                && is_an_instant(cx, rhs)
+                && ty::is_type_diagnostic_item(cx, rhs_ty, sym::Instant)
                 && let Some(sugg) = Sugg::hir_opt(cx, rhs)
             {
                 print_manual_instant_elapsed_sugg(cx, expr, sugg);
-            } else if !expr.span.from_expansion()
+            } else if ty::is_type_diagnostic_item(cx, rhs_ty, sym::Duration)
+                && !expr.span.from_expansion()
                 && self.msrv.meets(msrvs::TRY_FROM)
-                && is_an_instant(cx, lhs)
-                && is_a_duration(cx, rhs)
             {
                 print_unchecked_duration_subtraction_sugg(cx, lhs, rhs, expr);
             }
@@ -115,16 +118,6 @@ fn is_instant_now_call(cx: &LateContext<'_>, expr_block: &'_ Expr<'_>) -> bool {
     }
 }
 
-fn is_an_instant(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
-    let expr_ty = cx.typeck_results().expr_ty(expr);
-    ty::is_type_diagnostic_item(cx, expr_ty, sym::Instant)
-}
-
-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_manual_instant_elapsed_sugg(cx: &LateContext<'_>, expr: &Expr<'_>, sugg: Sugg<'_>) {
     span_lint_and_sugg(
         cx,