edit

clippy_lints/src/methods/str_splitn.rs

@@ -254,21 +254,25 @@ pub(super) fn check_needless_splitn(
     }
     let ctxt = expr.span.ctxt();
     let mut app = Applicability::MachineApplicable;
+    let (reverse, message) = if method_name == "splitn" {
+        (false, "unnecessary use of `splitn`")
+    } else {
+        (true, "unnecessary use of `rsplitn`")
+    };
     if_chain! {
-        if method_name == "splitn";
         if count >= 2;
         if check_iter(cx, ctxt, cx.tcx.hir().parent_iter(expr.hir_id), count);
         then {
             span_lint_and_sugg(
                 cx,
                 NEEDLESS_SPLITN,
                 expr.span,
-                "unnecessary use of `splitn`",
+                message,
                 "try this",
                 format!(
                     "{}.{}({})",
                     snippet_with_context(cx, self_arg.span, ctxt, "..", &mut app).0,
-                    "split",
+                    if reverse {"rsplit"} else {"split"},
                     snippet_with_context(cx, pat_arg.span, ctxt, "..", &mut app).0
                 ),
                 app,

tests/ui/needless_splitn.fixed

@@ -18,4 +18,10 @@ fn main() {
     let (_, _) = str.splitn(2, '=').next_tuple().unwrap();
     let (_, _) = str.split('=').next_tuple().unwrap();
     let _: Vec<&str> = str.splitn(3, '=').collect();
+
+    let _ = str.rsplit('=').next();
+    let _ = str.rsplit('=').nth(0);
+    let _ = str.rsplitn(2, '=').nth(1);
+    let (_, _) = str.rsplitn(2, '=').next_tuple().unwrap();
+    let (_, _) = str.rsplit('=').next_tuple().unwrap();
 }

tests/ui/needless_splitn.rs

@@ -18,4 +18,10 @@ fn main() {
     let (_, _) = str.splitn(2, '=').next_tuple().unwrap();
     let (_, _) = str.splitn(3, '=').next_tuple().unwrap();
     let _: Vec<&str> = str.splitn(3, '=').collect();
+
+    let _ = str.rsplitn(2, '=').next();
+    let _ = str.rsplitn(2, '=').nth(0);
+    let _ = str.rsplitn(2, '=').nth(1);
+    let (_, _) = str.rsplitn(2, '=').next_tuple().unwrap();
+    let (_, _) = str.rsplitn(3, '=').next_tuple().unwrap();
 }

tests/ui/needless_splitn.stderr

@@ -18,5 +18,23 @@ error: unnecessary use of `splitn`
 LL |     let (_, _) = str.splitn(3, '=').next_tuple().unwrap();
    |                  ^^^^^^^^^^^^^^^^^^ help: try this: `str.split('=')`
 
-error: aborting due to 3 previous errors
+error: unnecessary use of `rsplitn`
+  --> $DIR/needless_splitn.rs:22:13
+   |
+LL |     let _ = str.rsplitn(2, '=').next();
+   |             ^^^^^^^^^^^^^^^^^^^ help: try this: `str.rsplit('=')`
+
+error: unnecessary use of `rsplitn`
+  --> $DIR/needless_splitn.rs:23:13
+   |
+LL |     let _ = str.rsplitn(2, '=').nth(0);
+   |             ^^^^^^^^^^^^^^^^^^^ help: try this: `str.rsplit('=')`
+
+error: unnecessary use of `rsplitn`
+  --> $DIR/needless_splitn.rs:26:18
+   |
+LL |     let (_, _) = str.rsplitn(3, '=').next_tuple().unwrap();
+   |                  ^^^^^^^^^^^^^^^^^^^ help: try this: `str.rsplit('=')`
+
+error: aborting due to 6 previous errors