Add new test case

Author: Yusuf Raji

clippy_lints/src/single_option_map.rs

@@ -16,7 +16,7 @@ declare_clippy_lint! {
     ///
     /// ### Why is this bad?
     /// Taking and returning an `Option<T>` may require additional
-    /// `Some(_)` and `unwrap` if all you have is a `T`
+    /// `Some(_)` and `unwrap` if all you have is a `T`.
     ///
     /// ### Example
     /// ```no_run

tests/ui/single_option_map.rs

@@ -15,6 +15,15 @@ fn j(arg: Option<u64>) -> Option<u64> {
     arg.map(|x| x * 2)
 }
 
+fn mul_args(a: String, b: u64) -> String {
+    a
+}
+
+fn mul_args_opt(a: Option<String>, b: u64) -> Option<String> {
+    //~^ ERROR: `fn` that only maps over argument
+    a.map(|val| mul_args(val, b + 1))
+}
+
 // No lint: no `Option` argument argument
 fn maps_static_option() -> Option<usize> {
     MAYBE_ATOMIC.map(|a| a.load(Ordering::Relaxed))

tests/ui/single_option_map.stderr

@@ -22,5 +22,16 @@ LL | | }
    |
    = help: move the `.map` to the caller or to an `_opt` function
 
-error: aborting due to 2 previous errors
+error: `fn` that only maps over argument
+  --> tests/ui/single_option_map.rs:23:1
+   |
+LL | / fn mul_args_opt(a: Option<String>, b: u64) -> Option<String> {
+LL | |
+LL | |     a.map(|val| mul_args(val, b + 1))
+LL | | }
+   | |_^
+   |
+   = help: move the `.map` to the caller or to an `_opt` function
+
+error: aborting due to 3 previous errors